Imaging The Sacrum and Coccyx
Imaging of the sacrum and coccyx is a common procedure in radiology. This article discusses various protocols for plain-film and CT imaging.
Author: Joseph, Nicholas
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Imaging The Sacrum and Coccyx
Written by Nicholas Joseph Jr. RT(R)(CT) ARRT, B.S., MHA
Upon completion, the reader should be able to:
- Discuss common reasons for imaging the sacrum and coccyx.
The sacrum is imaged in general radiology as well as in subspecialties such as computed tomography (CT), magnetic resonance (MR), nuclear medicine (NM), positron emission tomography (PET) and other modalities. Evaluating the sacrum is a component of pelvic imaging; however, specific imaging of the sacrum may not include the entire pelvis. Some common reasons to image the sacrum and sacroiliac (SI) joints include trauma, joint pain, and sacroiliitis. Other conditions such as gout, bone tumors, and Bechter’s syndrome are also common reasons for selective imaging of the sacrum or sacroiliac joints. Finding the best imaging modality when the clinical history is “low back or sacral pain” can be challenging since sacroiliac joint pain affects approximately 15 to 25% of patients presenting with low back pain. The source of pain that radiates to the lower back and SI joints can be challenging to diagnose for clinicians. Although physical examination and radiological studies help uncover the elusive etiology of SI joint pain, the most common method for diagnosing chronic low back pain originating from the SI joint is one’s response to a small-volume local anesthetic joint block (Stevenson, 2015). In other words, when anesthetic SI joint block successfully alleviates low back pain, it also proves diagnostic for the source of low back pain. Radiologic studies do confirm conditions like sacroiliitis and other inflammatory conditions. Correct diagnostic imaging and diagnosis are essential to early detection of ankylosing spondylitis. Some sacral lesions are difficult to visualize with plain film imaging and are best demonstrated using a cross-sectional modality such as CT or MRI. Imaging modalities that offer thin axial slices through the sacrum and ability to reformat image data into coronal and sagittal images provides the best diagnostic information. Magnetic resonance imaging is very useful for diagnosing congenital disorders like meningocele, sacral agenesis, and other congenital lesions. Selective imaging protocols display optimal sacral anatomy and pathology. The radiographer’s skills should include anatomy, pathology, and a basic understanding of mechanisms causing most sacral injuries. The goals of this self-learning article are to discuss these points and specific radiographic imaging protocol objectives when imaging the sacrum and SI joints.
Anatomy of the sacrum
Anatomically, the sacrum is part of the vertebral column. It articulates superiorly with the fifth lumbar vertebra at the lumbosacral junction. The sacrum and coccyx are the distal portions of the spinal column and are also structurally part of the pelvis. The sacrum and coccyx are classified as irregular bones and are also atypical vertebrae. A typical vertebra is a single bone having distinct parts like the spinous process, transverse processes, vertebral body and foramen, pedicles and lamina. The sacrum is a composite bone formed by the fusion of its five sacral vertebrae into a single bone at about age 20 years. This fusion presents distinct anterior and posterior sacral landscapes. On the whole, these fused sacral segments can still be identified on the adult sacrum allowing for accurate localization numbering. The main functions of the sacrum are to strengthen the pelvis, which stabilizes it, and transmitting the weight of the body to the pelvic girdle through the sacroiliac joints (SI joints). To perform these functions the sacrum forms five main articulations: 1-2) two with the inferior articular processes of the fifth lumbar vertebra, 3-4) bilaterally with the ilium to form right and left sacroiliac joints, and 5) inferiorly with the coccyx.
The anterior or pelvic surface of the sacrum is concave, which is a shape that adds depth to the pelvic cavity. This surface forms the posterior ring of the pelvis. Four pairs of foramina are on the anterior surface. They are the remnants of what would be intervertebral foramen of a typical vertebra seen in other parts of the spine. Structures seen on the anterior surface include prominent sacral wings (called the ala), superior articular processes, sacral promontory, transverse ridges (lines), vertebral bodies, and anterior (pelvic) sacral foramina. Sacral vertebrae are numbered from top to bottom so that the superior vertebra is the first sacral vertebra. It articulates with the 5th lumbar vertebra. Distally, the 5th sacral vertebra articulates with the 1st coccygeal segment. The 1st sacral vertebra articulates with the 5th lumbar vertebra through their articular processes forming bilateral zygoapophyseal joints at the lumbosacral junction. Thus, the lumbosacral junction occurs at the L5/S1 articulations.
These two 3D CT images show the relationship of the sacrum to the pelvis. The sacrum is a shovel-shaped structure with the apex pointing downward. Its anterior portion forms the posterior ring of the pelvis and plays a major role in its stability. The sacrum provides stability to the pelvis by forming articulations with the 5th lumbar vertebra above, coccyx below, and sacroiliac joints laterally. There are various ligament attachments of the sacrum to the hip bones to stabilize the pelvis further.
The anterior surface of the sacrum portion of the pelvic surface is concave, which is a shape that adds depth to the pelvic cavity. Four pairs of foramina are seen on the anterior sacral surface, which are the remnants of the intervertebral foramen viewed on other vertebrae of the spine. Structures seen on the anterior surface include the ala (wing), superior articular processes, sacral promontory, transverse ridges (lines), vertebral bodies, and anterior (pelvic) sacral foramina. The superior vertebra is the first sacral vertebra, which articulates with the 5th lumbar vertebra; the fifth sacral vertebra is most distal and articulates with the 1st coccygeal segment. The 1st sacral vertebra articulates with the 5th lumbar vertebra through their inferior and superior articular processes forming bilateral zygoapophyseal joints at the lumbosacral junction. Thus, the lumbosacral junction occurs at the L5/S1 articulations.
The body’s weight transmits from the trunk and upper extremities to the vertebral column and to the first sacral vertebra, which is large to receive these vectors. It bears the body’s weight on its large vertebral body supported by strong pillars called wings or ala. Sacral vertebrae become progressively smaller distally fusing into a single triangular bone in the adult. The smaller fifth sacral vertebra articulates with the first coccygeal vertebra and may also fuse with it. Four transverse lines representing the developmental separations of the five sacral vertebrae which are visible on the anterior surface. The body of an individual sacral vertebra is that portion between two transverse lines. Transverse lines give superior and inferior boundaries to the sacral body, which standardized the accurate counting of sacral vertebra. Four pairs of anterior sacral foramina (aka pelvic sacral foramina) align along the lateral anterior walls. Anterior sacral foramina are somewhat rounded modified remnants of intervertebral foramina; they project forward and laterally. Anterior sacral foramen serve as conduits through which anterior divisions of sacral nerves and blood vessels pass in route to structures in the pelvis and lower extremities. Sacral foramina have communications with the sacral canal, which is continuous with the longitudinal vertebral canal that runs the length of the spine. However, the vertebral canal does not extend into the coccyx.
The posterior surface of the sacrum is convex presenting a prominent bony midline ridge called the median sacral crest. It is a remnant of fused spinous processes of vertebrae 1 through 3. Only sacral vertebrae 1-3 have spinous processes, sacral vertebrae 4 and five do not. Instead, they form a U-shaped structure called the sacral hiatus from which two small bony projections called the sacral horns (cornua) protrude. Anesthetic agents are sometimes administered into the back through the sacral hiatus to relieve pain (called an epidural anesthetic). Two large wedged shaped surfaces called auricular surfaces (shaped like the auricle of the ear), form the lateral borders of the sacrum. Auricular borders of the sacrum articulate with corresponding auricular surfaces on the ilium to form the two sacroiliac joints (SI). Each SI joints open posteriorly and obliquely at an angle of about 30-degrees. Also presented on the posterior lateral surface are four corresponding pairs of irregularly shaped sacral foramina. Like the anterior sacral foramen, they transmit the posterior sacral nerves and blood vessels to the sacrum and pelvis.
These two 3D surface rendered CT images show the major features of the anterior sacrum. The large prominent wing or ala of the first sacral vertebra (A), anterior sacral foramina (SF), numbered are the individual partially fused sacral vertebra from proximal to distal (1-5), and the transverse lines (SL) that indicate the body of the individual vertebra.
These two 3D surface rendered CT images show the distinguishing features of the posterior sacrum: superior articular process (A) that articulate with the inferior articular processes of the 5th lumbar vertebra. Also demonstrated is the auricular surface (B), posterior sacral foramen (C), sacral cornua(D), sacral hiatus (E), and median sacral crest (F).
These two radiographs show the complex articulation of the sacrum with the ilium forming the paired sacroiliac joints. On the left is a 3D surface rendered CT image of the pelvis that is 30 degrees oblique to show an opened sacroiliac joint (A). A coronal 2D CT image on the right shows symmetrical sacroiliac joints; the right one is labeled (B). Sacroiliac joints stabilize the pelvis by the sacrum’s articulation with the ilium along with a complex arrangement of fortifying supporting ligaments.
When imaging the sacrum it is important to remember that the spinal cord is not the same length as the vertebral column, for instance, an infant’s spinal cord terminates at L2 or L3. However, because of linear growth, the adult spinal cord terminates at or near L1/L2 junction. Since the spinal cord does not extend the entire length of the spinal column, there is room below L2 for insertion of a spinal needle into the subarachnoid space (lumbar puncture). Needle puncture into the subarachnoid space in the lumbar region of the spinal cord to withdraw cerebrospinal fluid (CFS) or inject anesthetic drug is called a spinal tap, or lumbar puncture procedure. The risk of injuring the spinal cord itself from an approach below L2 is minimal. Also, the spinal cord is anatomically and functionally segmented. For example, the area of the spinal cord corresponding to L4 is not at the level of the fourth lumbar vertebra because the cord ends at L1/L2. Nerves for the lower lumbar segments, sacrum, and coccyx arise from their shorter cord segment to descend within the vertebral canal. They form a bundle of nerve roots in the distal subarachnoid space caudal to the termination of the spinal cord. Nerve roots extend distally from the cord at about L2 to form a tail-like collection called the caudal equina (L. horse’s tail). At termination, the spinal cord tapers to a cone shaped structure called the conus medullaris. Extending from the conus is a thin fibrous thread of meninges (mostly dura mater), the filum terminale, which anchors the cord to the dorsum of the coccyx.
These two CT myelogram images demonstrate the subarachnoid space within the vertebral canal. The coronal image on the left shows contrast material in the space that appears non-homogenous because of nerve roots that form the caudal equine. The sagittal image on the right demonstrates the relationship of the vertebral canal to the spinal cord. The spinal cord ends at about L1/L2 in the adult. Spinal fluid fills the distal portion of the sac.
These CT axial images are of the lumbar spine taken following a myelogram. The subarachnoid space and nerve roots within the sac are visible in both radiographs. The image on the right shows the exit site of the intervertebral foramen (white arrow) where spinal nerves exit the vertebral column. The distal nerve roots form the caudal equina (yellow arrow) as they descend to their respective vertebral level to exit intervertebral foramina.
Spinal nerves exit the vertebral canal through intervertebral and sacral foramina to innervate peripheral structures in the body. Spinal nerves are mixed nerves carrying both sensory and motor stimuli to and from the brain. Information from the body to the brain is sensory because these stimuli convey information about the environment. Examples of sensory information include sight, touch, proprioception (ability to know what your body is doing without looking at it, for, example, not looking at your hands when driving), two-point discrimination, pressure such as gripping the steering wheel, hearing, taste, balance, and others. Motor responses are those nerve impulses from the brain that coordinated muscle movements, like steering a car, accelerating or decelerating a vehicle, swinging a golf club, dancing movements, pulling one’s hand back from a hot stove, and the like. There are five pairs of sacral nerves and one coccygeal nerve that travel down the vertebral canal in the sacral canal to exit sacral foramina. Injury to sacral foramen can occur from trauma or degeneration, or be congenitally malformed. Therefore, the sacral canal and sacral foramina must be demonstrated on sectional images of the sacrum. It is especially important to do so when imaging the pelvis following high impact trauma with suspected pelvic instability.
These coronal CT images demonstrate the sacral canal and sacral foramina (yellow asterisks) through which the sacral nerves exit. The sacral foramina form by fusion of the sacral vertebra. Typically vertebrae do not fuse but do form intervertebral foramen between adjacent vertebrae. The intervertebral foramen is where spinal nerves exit the vertebral canal. The yellow arrow indicates the intervertebral foramen of L5/S1 on the coronal image on the left. Notice the shape of sacral foramina formed by sacral vertebrae that have fused (right CT image).
The coccyx, also called the tailbone marks the most distal portion of the vertebral column. It is highly regressed in humans and has no resemblance of a vertebra at all. It too is a composite bone formed by 3 to 5 fused coccygeal segments. The lateral sacrum/coccyx view demonstrates the dominant curve of the sacrum and forward projection of the coccyx. These curves are important when imaging the sacrum or coccyx because they determine how much to angle the central ray for a true anterior to posterior (AP) projection of these structures. The frontal plain film radiograph of the sacrum requires a cephalic tube angle, whereas the coccyx is best demonstrated using a caudal angle. One of the more understood roles of the coccyx is that it provides an attachment for the meninges; particularly the dura mater is anchored to it distally. By attaching to the coccyx, the meningeal sac suspends the spinal cord reducing unrestricted movement within the vertebral canal.
This sagittal CT view of the sacrum (left) shows its major structures. The sacral promontory labeled (A), and the concave curvature of the sacrum (B). The forward curvature of the coccyx (C ), the superior articular processes of the first sacral segment labeled D on the right CT image, and the sacral canal, which is continuous with the vertebral canal of the spine (yellow arrows).
This sagittal view of the sacrum/coccyx shows the distal spinal column. Shown is the 5th lumbar vertebra superiorly through the last coccygeal segment. The vertebrae of the sacrum are numbered beginning with L5 articulating with the first sacral vertebra. Sacral vertebrae are numbered from superior to inferior as are the coccygeal vertebrae. Numbering the coccygeal segments are from proximal to distal too, i.e. (Cx 1 through Cx4/Cx5).
A system of tightly woven muscles and ligaments provide support to the inherent anatomical weakness of the bony pelvis, stabilizing it. Strong ligaments arranged transversely, oblique, and horizontally resist forces that can externally rotate the pelvis, thereby opening it. Among these are the short posterior SI ligament, the anterior SI ligament, the iliolumbar ligament, sacrospinous ligaments and others. Their function is to counter opposing forces such as AP compression. These ligaments fail when forces exceed their ability causing compression type injury. Vertical stability is provided primarily by the short and long posterior SI ligaments. Other interosseous ligaments within the sacroiliac joints also provide additional vertical stability.
These stabilizing ligaments help perform one of the main functions of the pelvis, to transmit weight from the trunk and lumbar vertebrae to the lower extremity. Weight bearing forces from the body are transmitted primarily along vectors to the posterior pelvis, then to the sacrum and sacroiliac joints. During weight-bearing forces are conveyed to the acetabula for distribution to the femurs. During weight bearing movements of the body, the anterior pelvic arch functions like a strut maintaining the shape of the pelvic ring. During passive weight bearing such as sitting the vector is transmitted downward to the ischial tuberosities.
These three 3D volume rendered CT images depict some basic patterns of those dense, broad ligaments (blue lines) that support the bony pelvis. The CT image on the left demonstrate the arrangement of ligaments from the AP perspective; the middle CT image illustrates how these ligaments attach and support the posterior pelvis. The far right CT image demonstrates how these ligaments provide support laterally to stabilize the sacrum/coccyx and close the true pelvis inferiorly. Many ligaments and corresponding muscles support and provide short stabilizing struts for the pelvis. The greater sciatic notch forms an opening bridged by ligaments that form a soft tissue entrance into and out of the pelvis. The lesser sciatic notch provides an opening to the perineum. While it is not our purpose to name all of these ligaments it is important to understand the general distribution of ligaments and their role in stabilizing the pelvis. For example, the sacrotuberous ligament is a long band of fibrous tissue that extends from the posterior superior and posterior inferior iliac spines and the back and sides of the sacrum/coccyx down to the ischial tuberosities. Their role is to resist posterior rotation of the lower sacrum. We can see some of the muscles and their tendons and ligament that stabilize the pelvis on MR images below.
Some ligaments that stabilize the pelvis
These three coronal MR images show a portion of the extensive network of ligaments and muscles that stabilize the bony pelvis. They also can cause the pelvis to shift due to unopposed pull when high impact forces separate bone or joints causing pelvic opening. Pelvic compression can also separate bones and often results in rupture of the main blood vessels causing profound life-threatening intrapelvic hemorrhaging.
The schematic lines on the 3D CT image on the left (blue) represent some of the ligaments supporting and closing the true pelvis. Many ligaments of the pelvis are named according to their origins and insertions. Blue lines show some of the well-known ligaments that stabilize the pelvis. From superior to inferior is the posterior SI ligaments, sacrospinous ligaments, sacrotuberous ligaments, and sacrococcygeal ligaments. These ligaments also provide a soft tissue passageway for blood vessels and nerves to enter and exit the true pelvis and perineum. The greater sciatic notch (A) guards the entrance into and out of the true pelvis. The lesser sciatic notch (B) provides an opening into the perineum. The coronal MR image on the right shows some of the muscle and ligaments supporting the pelvis.
Accompanying pelvic fractures structural ligaments can be disrupted. Separation of the sacroiliac joints and severe fractures of the sacrum can cause instability of the pelvis. Crush injuries and rotational injuries that “open” the pelvis can have life-threatening consequences, in particular, uncontrolled hemorrhaging and shock. Of particular radiographic interest is the iliospinous ligaments that attach to the transverse processes of the fifth lumbar vertebra and the ilium. An avulsion type fracture of the fifth lumbar transverse process may be a single indicator of pelvic instability when seen on plain film x-ray. For this reason, the AP pelvis view should include the entire fifth lumbar vertebra and iliac crests.
These two CT images show a fracture of the transverse process of the fifth lumbar vertebra (arrows). Often because of fecal material in the colon and a filled bladder it is not easy to see the posterior elements of the pelvis on plain film. It is particularly the case of portable made radiographs of the pelvis with a history of trauma. A subtle fracture of the transverse process of the fifth lumbar may be the only indicator of pelvic injury and instability. Theses CT images show the fracture of the fifth lumbar vertebra and profound fractures of the ilium and sacrum. Less obvious fractures can be obscure but indicated by a transverse process fracture. The CT image demonstrates the posterior aspect of the sacrum and L5, which is an area that ‘s hard to show with the plain film AP x-ray view of the pelvis.
- The Tile classification system describes pelvic fractures that do not appreciably disrupt the pelvic ring (Tile type A).
- Type A, posterior pelvic brim arch is intact; however, stable fractures may be present. Stable fractures include avulsions of the iliac spine, iliac crest, ischial tuberosity, wing fracture, unilateral and bilateral pubic rami fracture, sacral fracture, or sacrococcygeal fracture. The pelvic brim and SI joints are spared making these less severe injuries.
- Type B, incomplete posterior arch disruption occurring with pubic diastasis. Other injuries may occur such as anterior SI joint disruption, anterior sacral buckle fracture, AP and lateral compression injuries, internal or external rotation of the pelvis, and other types of rotational or vertical shear injuries that account for the stability of the pelvis.
- Type C, complete posterior arch disruption. These are the most severe types of pelvic ring fractures caused by vertical shear, anterior/posterior compression, or lateral compression. These occur with ipsilateral and contralateral internal or external rotation of the pelvis due to SI joint fracture/dislocation, or sacral fracture. These types are rotationally and vertically unstable and cause severe life-threatening hemorrhaging of pelvic blood vessels.
- Screen all females of child-bearing age for pregnancy.
- Empty colon and bladder prior to imaging.
- Change into a gown and remove all metal.
- Use collimation to reduce scatter and improve contrast.
- Use appropriate exposure factors and avoid repeats.
- Patient supine with the legs fully extended.
- Anterior superior iliac spines equidistant from the image receptor.
- CR angled 15 degrees cephalic, enters midsaggital plane midway between the ASIS and symphysis pubis.
- Demonstrates the entire 5th lumbar vertebra, symphysis pubis, and both sacroiliac joints.
- The sacrum centered to the film and collimated field.<./li>
- The midsagittal plane of sacrum aligned with the symphysis pubis.
- Equal distance of the lateral sacrum from the pelvic brim evidenced by symmetry of the ischial spines.
- The entire 5th lumbar vertebra, sacrum, 1st through 5th sacral segments, 1st coccygeal vertebra, both sacroiliac joints, and symphysis pubis should be demonstrated.
- Central ray is angled 15-20 degrees cephalic to open the posterior sacral foramen and reduce foreshortening of the sacrum. The pubis should not superimpose on the sacrum.
- Patient supine with the legs fully extended.
- CR angled 10 degrees caudad entering the midsaggital plane 2 inches superior to the symphysis pubis.
- The coccyx is demonstrated equidistant from the lateral walls of the pelvis opening.
- This view demonstrates the entire coccyx free of superimposition by the symphysis pubis and projected superior to the pubis rami.
- Structures demonstrated include: the entire 5th lumbar vertebra, entire sacrum/coccyx, and visualization of an opened sacral canal.
- Superimposed greater sciatic notches indicating the midsagittal plane is parallel with the image receptor.
- L5/S1 junction should be clearly demonstrated with the zygapophyseal joints superimposed and properly exposed.
- The coccyx should not be overexposed showing good recorded detail. It may be necessary to use a high grid ratio and contact lead along the collimated edge of the field to reduce scatter radiation reaching the image receptor.
A rotational injury of the hemipelvis causes serious anatomical disruptions within the pelvic cavity. An external rotation of the hemipelvis increases the volume (pelvic opening) of the pelvic cavity causing life-threatening hemorrhage. This type of injury must be reduced as soon as possible to control bleeding. Often these patients are immediately wrapped in a pneumatic antishock suit to reduce hemorrhaging in the interim of permanent bone reduction with fixation. An interventional radiologist may be called upon to perform vascular embolization in attempt control of bleeding. It is important that when imaging the pelvis, the radiographer is aware that they are demonstrating pelvic components in a way that helps determine the possible cause of hemorrhaging. Hemorrhage is the most serious complication of pelvic fracture and is life-threatening in most cases.
Imaging Low Back Pain
Plain films are the most common imaging procedure for initial imaging of low back pain. With the proper imaging technique plain films can be useful in diagnosing diseases of bone. It can also detect fractures, bone tumors, and extensive inflammatory bone disease. Plain film radiography is often used to identify disc degeneration with accompanying narrowing of the joint space, and other alterations of the adjacent vertebrae. Anteroposterior (AP) and lateral views provide an excellent survey of the lower back. A coned lateral “spot” film of L5/S1 better penetrates this region of the spine. Oblique views are associated with higher levels of radiation exposure to the reproductive organs of females but are useful when evaluating spondylolysis (a bony defect of the pars interarticularis). The objective of the oblique view is to demonstrate the classical “Scottie dog.” The diagnosis, spondylolysis, presents as the collar sign seen as a radiolucent collar about the neck of the “Scottie dog” indicating a separation of bone.
The articulation between two adjacent lumbar vertebrae produces the “Scotty dog” pattern on the oblique view. Both obliques views are taken to profile the articular processes, their facet joints, and the pars interarticularis. Parts of adjacent vertebrae form parts of the “Scotty dog.” The transverse process forms the nose; the superior articular process forms the ear; the pedicle forms the eye. Inferior articular processes form the forelegs; the pars interarticularis shapes the neck. The body (lamina and spinous process); hind leg ( the inferior articular process of opposite side); and tail ( the inferior articular process of opposite side). A well positioned oblique view will demonstrate all of these structures.
The “Scottie dog” is outlined on the radiograph on the left. Labeled parts are the intervertebral space (A), pars interarticularis (yellow line), the pedicle (B), inferior articular process (C) and superior articular process (D).
Plain Film Imaging of the Sacrum and Coccyx
One of the primary goals when selectively imaging the sacrum is reduction of patient radiation dose in keeping with ALARA (as low as reasonably achievable). To achieve low dose to the patient, especially females of childbearing age it is important to properly prepare the patient for the study. This begins with screening all females of child-bearing age for pregnancy. An exception may be acute trauma where the emergency room physician determines that radiographs will be taken regardless of pregnancy or not. Plain films of the sacrum and coccyx are best made with the patient’s bladder empty and the colon cleansed so that fluid and fecal material does not obstruct viewing. When ambulatory, have the patient change into a metal free gown to eliminate the possibility of clothing artifact. Tight collimation (four sided) is recommended to reduce scatter radiation and improve subject contrast. Shielding the male gonads is recommended; however, shielding for females is not possible since the ovaries lie within the pelvis. Reducing the milliamperage-second (mAs) using an appropriate change in kilovoltage (kVp) can significantly reduce exposure to the gonads.
The average exposure factor for imaging the adult sacrum is 75-90 kVp. In some cases increasing the kVp to 85-95 kVp with an appropriate mAs reduction the mAs by half will significantly lower patient radiation dose. Routine views of the sacrum/coccyx consist of three views: an AP sacrum, AP coccyx, and lateral sacrum and coccyx. When the sacroiliac joints are specifically evaluated both right and left posterior oblique views are included. Inlet and outlet views of the pelvis may also be requested to compliment routine views of the sacrum, posterior pelvis, or acetabulum following trauma.
Pelvic Imaging and ALARA
It is important that the pelvis is properly positioned so that the sacrum and sacroiliac joints are symmetrically displayed. Palpating and positioning the pelvis so that the right and left anterior superior iliac spines (ASIS) are equal distances from the image receptor reduces pelvic rotation. When accurately positioned the AP sacrum view will demonstrate the median sacral crest aligned with the symphysis pubis and the ischial spines equally demonstrated in relationship to the pelvic brim. With the patient supine and legs extended, the central ray is angled 15-degrees cephalic entering the pelvis at the midsagittal plane midway between the ASIS and symphysis pubis. A 15-degree cephalic tube angle opens the posterior sacral foramen and reduces foreshortening of the sacrum. A properly positioned the AP sacrum view will demonstrate the entire 5th lumbar vertebra, symphysis pubis, and both sacroiliac joints.
Patient Positioning for the AP Sacrum
Diagnostic Criteria for the AP Sacrum View
When imaging the coccyx proper positioning and optimal exposure technique is essential to producing a diagnostic radiograph. The bladder and colon should be emptied prior to imaging the sacrum or the coccyx. Because the coccyx is positioned forward (kyphotic curvature) relative to the sacrum it is not visualized anatomically with the AP sacrum. Often the pelvic brim will obstruct the coccyx unless the patient and central ray are properly aligned to the image receptor. When properly positioned the AP coccyx radiograph will demonstrate the coccyx aligned with the symphysis pubis and at equal distance from the lateral wall of the pelvic inlet. To achieve optimal positioning the legs are extended and the anterior superior iliac spines are equal distance from the image receptor. The central ray is angled 10-degrees caudal entering pelvis midsagittal plane 2 inches above the symphysis pubis. The exposure for adults is generally between 75 and 85 kVp; however, to achieve good recorded detail the technologist should also employ the following techniques: 1) instruct the patient to remain still during exposure, 2) suspend ventilation during exposure, 3) use the shortest OID, and 4) use the smallest focal spot size available. Four sides of collimation to the area of interest will improve contrast and is a practice in radiation protection in keeping with ALARA.
Imaging the AP Coccyx
These two radiographs demonstrate those structures that should be demonstrated on the AP sacrum (left) and AP coccyx (right). Labeled are: the 5th lumbar vertebra (A), wing of 1st sacral segment (B), wing of ilium (C), right sacroiliac joint (D), pelvic brim (E), left sacroiliac joint (F), symphysis pubis (G), and sacral foramen (white arrows). Notice the air and fecal material in the lower bowel partially obstructs viewing of the sacrum and coccyx.
The lateral view of the sacrum/coccyx is performed with the patient in the recumbent lateral position and the central ray directed vertical (perpendicular) to the sacrum. The lateral sacrum and coccyx are taken together to reduce gonadal dose to the patient. The patient is positioned in a true lateral recumbent with the spine straight along its long axis. Place sponges or pillows under the small of the waist and between the knees and ankles to keep the spine from rotating. The purpose of the lateral view is to demonstrate the 5th lumbar vertebra, entire sacrum, and coccyx free of superimposition by fecal material and gas, and without the urinary bladder and rectum obstructing viewing. Notwithstanding, proper positioning of the sacrum/coccyx in a true lateral is an essential diagnostic criterion. It is important that the entire long axis of the patient is aligned placing the shoulders in the longitudinal axis and superimposed. The posterior ribs superimposed and the right and left thoracic wall should also be superimposed. Also place a pillow or radiolucent sponge between the knees to align and superimpose the pelvis. When properly aligned the zygoapophyseal joints of L5/S1 are superimposed, the greater sciatic notches are superimposed, and the sacral canal is open demonstrating the median sacral crest in profile.
The lateral sacrum/coccyx radiograph will show the greater sciatic notches and femoral heads superimposed when there is minimal rotation of the pelvis. The CR enters the pelvis perpendicular to the tabletop 2 inches anterior to the posterior sacral surface at the level of the anterior superior iliac spine. Close four-sided collimation should be seen on the radiograph. Keep in mind that quite a bit of scatter is produced by the exposure technique. This will cause image fog and may be a cause for repeating the lateral radiograph. Therefore, be sure to place a lead contact blocker on the tabletop directly behind the patient to reduce the amount of scatter reaching the image receptor. The evaluation criteria for imaging the lateral sacrum/coccyx include showing the entire sacrum, L5/S1 apophyseal joints and lumbosacral junction, and the entire coccyx. An optimum exposure should clearly visualize a well-penetrated sacrum without burnout of the coccyx. To achieve optimum exposure automatic exposure control (AEC) is not recommended. Often the phototimer setting will cause early termination of the exposure resulting in an underexposed radiograph. A manual technique using fixed kVp and variable mAs from an exposure chart is preferred. Using fixed kVp exposure based on patient measurement is the best way to assure consistent radiographic contrast, optimal film density, and reduces repeat imaging that causes increased patient dose.
Diagnostic Criteria for the Lateral Sacrum/Coccyx
This radiograph demonstrates those structures that should be seen on a lateral view of the sacrum/coccyx. The zygoapophyseal joints of L4/L5 (A) are seen superimposed along with the entire L5 vertebra, superior articular processes of L5/S1 (B) and zygoapophyseal joints, sacral promontory (D), coccyx (blue arrow), and superimposed greater sciatic notches (yellow arrow). Notice the sacral canal (C) is open when the pelvis is in a true lateral position.
Critique of Plain Films of the Sacrum/CoccyxRadiograph #1
Give your critique of this AP radiograph of the sacrum taken for chronic sacral pain and no history of recent trauma.
This radiograph of the sacrum demonstrates those structures that should be seen on the AP view of the sacrum. The field of view includes from the entire 5th lumbar vertebra through the 1st coccygeal segment, and without rotation of the pelvis. The midsagittal plane of the sacrum is aligned with the symphysis pubis. Lateral margins include both sacroiliac joints and the pelvic portion is well collimated in keeping with ALARA. The urinary bladder and colon appear emptied although there is a gas shadow overlying the sacrum, but is not obstructive. This radiograph meets diagnostic standards for imaging the sacrum; however it does not show the sacral foramina opened, which questions whether a minimum 15-degree cephalic angulation was used. Because the AP view of the sacrum is only one of several views used to evaluate the sacrum, repeating this view would not be recommended unless other views fail their diagnostic criteria. The exposure technique adequately penetrates the sacrum and shows good bone detail.
Give your critique of this AP radiograph of the sacrum. The patient history involves sacral pain from a traumatic fall with landing on buttocks.
This radiograph demonstrates a well penetrated view of the sacrum; however, because the bladder is not emptied there is poor visualization of the lower sacrum and coccygeal junction. Additionally, the entire 5th lumbar vertebra is not seen; it should be remembered that an important radiological sign that could indicate pelvic instability is a fracture of the 5th lumbar vertebra transverse process. This appears to be a true AP view of the pelvis without cephalic angulation of the central ray required to profile the sacrum. This is evidenced by the sacral foramina not being opened and there is foreshortening of 1-3 sacral segments. This appears to be a male and there is no attempt to shield the gonads. It is not possible to shield female patients because the shield would obstruct the sacrum. Overall, this radiograph should be repeated to correctly demonstrate the 5th lumbar vertebra and the sacrum in profile using a 15-degree cephalic angle. If this view is requested along with an AP view of the pelvis because of trauma, one should consult with the ordering physician or radiologist to see if inlet/outlet views are needed rather than a coned AP sacrum view.
Give your critique of this AP radiograph of the sacrum given a history of chronic low back and hip pain and a negative radiographic series of both hips.
The sacrum is elongated, and the symphysis pubis superimposes part of the 4th and 5th sacral segments. This usually caused by the tube being angled excessively in the cephalic direction. Generally, an angle of 15 to 20 degrees is adequate for the AP view of the sacrum. Do notice that the visualized upper sacral foramina are opened and clearly demonstrated. Both SI joints are not entirely demonstrated. So, to correct this radiograph decrease the tube to 15-degree cephalic angle, align the patient so that both sacroiliac joints are entirely demonstrated, and include the entire 5th lumbar vertebra. Also use gonadal shielding since this is a male patient in keeping with ALARA. Exposure factors adequately visualize good bone detail.
This patient had recent surgery involving the right sacroiliac joint. The ordering physician wanted to evaluate the sacrum in a true AP projection without tube angulation. Does this radiograph meet the diagnostic criteria and physician’s request?
This radiograph does not meet diagnostic standards for the AP sacrum view. The main reason is that the sacrum is under penetrated. Subject detail is completely missing as there is very little contrast between bone and the internal fixation of the right SI joint. Under penetration of the full bladder and bowel gas completely obliterates the distal sacral detail. Keep in mind that the x-ray tube is not angled but there is significant foreshortening of the sacrum. However, the right sacral bridge is seen with minimal distortion. This radiograph meets the positioning request of the ordering physician, but the exposure is inadequate. All sacral segments are foreshortened with closure the sacral foramen due in part to the downward tilt of the pelvis and a vertically directed central ray. Notice the closure of the obturator foramina in the lower part of the radiograph. The tube is not angled at the request of the ordering physician so that the sacroiliac bridge can be demonstrated without elongation. To make this a properly exposed radiograph, increase the kVp at least 15% to penetrate the sacrum. Optimum exposure for the sacrum is 75 to 90 kVp for an average sized adult patient.
Give your critique of this radiograph using the diagnostic criteria for the AP sacrum view.
One of the diagnostic criterions for the AP sacrum view is that L5 be entirely demonstrated. Also, the sacrum must be well-penetrated and have good bone detail. To enhance image detail empty the bladder and rectum to optimize subject detail. It is important that the transverse process of L5 is seen when a history of trauma is given. About this radiograph, the sacrum is slightly underexposed and lacks subject detail. Fecal material and a full bladder distract from image detail. However, it may not be possible to improve this radiograph without emptying the bladder and rectum. When the sacrum is imaged for chronic pain a suppository should be given prior to the exam and the urinary bladder emptied. Slight rotation of the sacrum is noted by the midsagittal plane not being aligned with the symphysis pubis. None of these findings are reason to repeat this radiograph in keeping with ALARA. In consultation with the radiology it may be determined that there is too much fecal material to improve image quality. The tube angle is adequate as the sacrum is not foreshortened. If repeated, increase the kVp to penetrate the entire sacrum and coccyx.
Give your critique of this radiograph using the diagnostic criteria for the AP sacrum view. Consider a history of low back pain without trauma.
The entire sacrum is demonstrated, and because the history is chronic low back pain, not trauma, it would not be in keeping with ALARA to repeat this radiograph to demonstrate the entire 5th lumbar vertebra. There is no foreshortening of the sacrum and the sacroiliac joints are properly demonstrated. The exposure technique shows the sacrum with good bone detail. Slight rotation of the pelvis is noted, but it does not distort the sacrum sufficiently to warrant repeat imaging. Notice how bowel fecal material superimposes the lower part of the sacrum. Some pathologies could be hard to see such as a bone tumor, or lytic changes. Overall, this is a good radiograph that along with the lateral view meets the diagnostic standards.
Give your critique of this radiograph using the diagnostic criteria for the AP coccyx view.
This is a good radiograph of the coccyx that meets diagnostic criteria. In particular the coccyx is projected free of the symphysis pubis indicating the correct amount of caudal angulation (10-degrees) was used. A small amount of fecal material and bowel gas is seen overlying the coccyx and sacrum, but does not prevent diagnosis of alignment or fracture. The coccyx is seen in the midline equal distance from the lateral walls of the pelvic brim. Notice the ischial spines are seen in profile within the true pelvis. The exposure technique adequately displays the coccyx with good bone detail. The segments of the coccyx are well-defined. Excellent radiograph!
Discuss why this radiograph does not meet the diagnostic standards for the AP sacrum view.
While this radiograph includes the required sacral anatomy the flaw is that the patient is improperly positioned. Some good points are that both sacroiliac joints are demonstrated; L5 is demonstrated, and the entire sacrum and symphysis pubis are all seen. However, this radiograph does not meet diagnostic criteria because the femurs are flexed, which casts a soft tissue shadow over the sacrum (yellow arrows). Even when properly prepped for imaging (an emptied bladder and rectum) it can be difficult to get great subject detail due to retained fecal material and/or bowel gas. Failure to have the patient empty their bladder and rectum will almost guarantee that the image will be non-diagnostic if under penetrated or underexposed. Always extend the legs to reduce femoral soft tissue shadowing that may superimpose on the sacrum and symphysis. This radiograph should be repeated correcting positioning error.
On a cold winter morning this patient slipped on ice in his driveway and landed on his tailbone. Does this radiograph meet the diagnostic criteria for the AP coccyx view?
Two factors make this a good radiograph of the coccyx: proper tube angle and good recorded detail. The coccyx is seen without superimposition aligned with the symphysis. However, this radiograph fails to meet expected collimation goal in keeping with ALARA. Some technologists are uncomfortable with close collimation of the coccyx because over collimation may result in clipping the anatomy. This too will require repeating the radiograph. The importance of close collimation is twofold. First it reduces patient exposure to ionizing radiation, which reduces patient dose. Secondly, it improves subject detail, especially when the part is small like the coccyx. A small focal spot size should be used because it will improve subject contrast, especially when combined with close collimation. The area in the dotted blue box represents the area of acceptable exposure had good collimation been applied. Also notice that recorded detail is lacking on the large radiograph due to lack of collimation. When imaging a small part such as the coccyx it is important use close collimation, decrease OID, and use the smallest available focal spot. Fortunately both the AP and lateral views are needed to accurately diagnose coccyx injury. Therefore, repeating this radiograph because of lack of collimation is not in keeping with ALARA as long as the lateral view is acceptable.
A fall related injury resulted in low back pain that was clinically determined to be related to the sacrum/coccyx. Does this radiograph meet the diagnostic criteria for the lateral coccyx view?
To evaluate this radiograph using the diagnostic criteria we must consider positioning, structures demonstrated, and exposure technique. An accurately positioned lateral sacrum/coccyx radiograph demonstrates the third sacral segment centered within the collimated field and the fifth lumbar vertebra through the distal coccygeal segment is seen. Rotation should be minimal evidenced by superimposed greater sciatic notches and posterior pelvic wings. The median sacral crest should be seen in profile when the sacrum is not rotated. The exposure should demonstrate good contrast between bone and soft tissues. Cortical bone should show clear edges of the sacrum and sharp detail of the sacral canal. When we measure this radiograph by these standards it meets all diagnostic criteria except one. The fifth lumbar vertebra is not demonstrated. It is important that the entire 5th lumbar is demonstrated along with the apophyseal joints of L5/S1. The slight rotation of the pelvis is within acceptable limits. Good collimation is demonstrated in keeping with ALARA. Close collimation also adds to subject detail. Unfortunately this radiograph should be repeated to include L5.
This lateral radiograph of the sacrum/coccyx was rejected by the radiologist. Discuss why this radiograph does not meet the diagnostic criteria for the lateral sacrum/coccyx view?
Clearly this radiograph does not meet diagnostic standards for the lateral view of the sacrum/coccyx because the part is unacceptably rotated. The median sacral crest is not in profile, the greater sciatic notches are not superimposed, and the pelvic brim and femoral heads are also rotated and not superimposed. The femoral heads are at about the same level making it difficult to determine the direction of rotation. When one of the femoral heads is projected inferior it is farthest from the image receptor and is the side up. The femoral heads are about at the same level and the posterior iliac wings superimpose on the sacrum. The ischium superimposes on the coccyx indicating the pelvis is rotated posteriorly. Reposition the patient aligning the shoulders, posterior ribs, and posterior iliac wings perpendicular to the tabletop. Also place a sponge or pillow between the patient’s legs and knees to prevent anterior rotation.
This patient presented to a local emergency room with a complaint of lower buttocks pain after being kicked in the tailbone by a prankster. Is this a diagnostic radiograph of the lateral coccyx, why or why not?
The diagnostic criteria for the lateral coccyx view states that the entire 5th lumbar vertebra, entire sacrum/coccyx, and an opened sacral canal should be seen. Also the junction of L5/S1 should be clearly demonstrated with the zygapophyseal joints superimposed. Clearly these two criteria are not met in that the entire sacrum is not visualized. This is important because it is difficult to localize sacral vs. coccygeal pain so both are included on the lateral view. The coccyx as displayed is not overexposed and shows good recorded detail. The collimation is good for imaging the coccyx; however, it should be remembered not to collimate so closely as to omit the entire sacrum.
The Sacroiliac Joints
The sacroiliac joints (SIJ) are a part of the pelvis proper and are a pelvis component that undergoes significant stress from the weight of the body. The SI joint is the largest axial joint in the body. Its structure is variable but is described as a large auricular-shaped diarthrodial joint. Its diarthrodial joint component is formed at the anterior third interface between the sacrum and ilium. The remainder and posterior portions of the joint involves intricate ligamentous connections. It is a C shaped joint with two interlocking lever arms at the second sacral level. The contour of the joint contains numerous depressions and ridges which is crucial to its stabilizing function. While the true synovial joint portion permits only limited movement the ligamentous portion functions to restrict motion in all planes. The SI joint is further supported by muscles such as the gluteus maximus, piriformis iliacus, gluteus maximus, thoracolumbar fascia, and other structures that are connected to ligaments of the joint. These muscles provide support to the pelvis and may have a role in SI joint pain. Muscle movement can therefore affect joint mobility adding a potential for vertical shearing, which is seen clinically in nearly 30% of symptomatic SI joints. SI joint ligaments are structurally stronger in males resisting normal pelvic forces. But in females, slight mobility of the pelvis is a necessary anatomical modification for parturition so these ligaments are inherently weaker. Trauma, repetitive movements, poor posture, and arthritic changes to name a few can alter the biomechanics of the sacroiliac joints. Symptoms of SI joint dysfunction include low back pain, hip, knee, or even foot pain.
The SI joints are designed to help stabilize the pelvis. As the load of the trunk is transmitted down the spine it is dissipated to the lower extremities via the SI joints. Compared to the lumbar spine, the SI joints can withstand a medially directed force 6 times greater but only half the torsion and only one-twentieth the axial compression load. Medial and axial motions may preferentially strain and injure the weaker anterior joint capsule. Biomechanics studies of the SI joint show a potential to rotate about all three axes; however, it should be noted that these movements are very small and equally difficult to measure. Generally speaking the SI joint has mobility of less than 2 degrees; therefore, hypermobility is not a common cause of SI joint pain. Some exceptions may include trauma, multiparity, and muscular dystrophy. Likewise, any of the supporting muscles can be a confounding source of perceived joint pain. Because the SI joint itself is richly innervated by free nerve ending arising from L2 to S3, this wide innervation is partially responsible for different manifestations of referred pain to the joint.
This arthrogram of the left SI joint displays the synovial capsule of this synovial joint. Although it is a synovial joint the SI joint is not a diarthoroidal joint; it is only slightly movable mostly during the events of birthing. Only lower half of the joint is synovial. The upper S.I. joint is a strong syndesmotic articulation. Thus degenerative S.I. joint disease is relatively confined to the lower joint portion.
Imaging the Sacroiliac Joints
In addition to the routine plain films of the sacrum/coccyx, the sacroiliac (SI) joints are sometimes imaged using special views including tomography. In the absence of trauma, the sacroiliac joints are most often imaged because of low back pain suspect for sacroilitis (aka: Bechterew’s syndrome). Involvement of the SI joints is documented to be a first predominant finding in spondylarthorpathies even those that are seronegative. A hallmark of ankylosing spondylitis is involvement of the SI joints. Sacroilitis on radiographs is defined as erosion, sclerosis and irregular sacroiliac joint spaces. Keep in mind that bone erosion on a radiograph is a late sign, whereas early stages of sacroilitis may not be apparent on plain films. Other radiology modalities such as magnetic resonance imaging, computed tomography, or scintography may be needed. Early signs correlated to radiological studies of patients with SI joint pain have been quite disappointing. There are too many differentials preventing accurate correlation of clinically assessed pain with dysfunctional SI joint as the source.
Most of the current data on SI joint pain correlated with imaging studies is disappointing as well as retrospective. Because research is still ongoing, it is imperative that all imaging studies of the SI joints are complete and properly made. In order to perform correct imaging it is important for the radiographer to understand that the plane of the auricular portion of the joint is directed from anterolateral to posteromedial. Accordingly, oblique plain film images should project the joint along this tangential plane. The goal of the oblique views is to superimpose the anterior and posterior joint margins, which increases the sensitivity of the study for detecting joint abnormalities. Oblique views of the SI joints are taken at 25-30 degree angle to the anteroposterior plane with the side up being the subject of interest. The standard plain film views of the SI joint are 1) AP, 2) right posterior oblique-RPO and, 3) left posterior oblique-LPO. Joint widening with erosion or sclerotic changes at the bone margins is suggestive of inflammatory sacroilitis. The sensitivity of plain film radiological studies is reported to be between 13% and 46%, and for CT imaging 57.5% sensitive and 69% specific in diagnosing the source of low back pain being the SI joint. Plain films can be used to identify bone lesions; however, the low sensitivity of plain films is due in part because at least 30% bone destruction in the lesion must have occurred to be seen. Nuclear bone scan is much more sensitive and can identify lesions in which 2-3 percent bone destruction has occurred. However, nuclear imaging is not usually a first step in diagnosis of sacroiliac joint pain.
These two anteroposterior oblique views of the sacroiliac joints demonstrate the proper alignment of the anterior and posterior joint margins. The oblique views are taken with the patient positioned at 25-30 degree angle to the anteroposterior plane. An acceptable radiograph will demonstrate a closely collimated centered SI joint with the anterior and posterior joint margins superimposed. The LPO demonstrates the right SI joint as the side up, and the RPO demonstrates the left SI joint side up.
This right posterior oblique view demonstrates the left sacroiliac joint (side up). Notice the correct alignment of the anterior and posterior margins of the SI joint, which are superimposed. There is mild sclerotic presentation of the auricular surfaces of the joint without widening. This is a very diagnostic oblique view of the SI joint.
CT of the Sacrum and SI Joints
Computed tomography imaging is often used to evaluate the extent of reactive spurring, sclerosis or subluxation of the sacroiliac joint. It is also used to evaluate fractures, surgical repair, alignment, and complexity of the auricular surfaces of the sacrum and ilium. Many physicians rely on the common practice that an analgesic response to diagnostically localized anesthetic block is the best way to diagnose SI joint pain. So to reduce false-negative and false-positive results CT is used to localize needle placement into the suspected SI joint. Considering the many uses of CT imaging for SI joint diagnosis and treatment it is important that the CT technologist is aware of certain protocols for the SI joints. Routine axial, coronal, and sagittal CT images of the pelvis may not show the sacrum and SI joints in true anatomical projections. Therefore, specific reformats of the sacrum may include true coronal and sagittal views, and true coronal and sagittal views of each sacroiliac joint. Keep in mind that the pelvis is tilted downward in its anatomical position, and the sacrum is convex anteriorly with the coccyx projecting upward. Therefore, reformatting of the sacrum and coccyx is necessary to achieve true coronal CT images when these structures are specifically being evaluated.
These axial CT images demonstrate the SI joint injection technique used to assess whether or not low back pain is from the SI joint or is referred from another remote location. Procedure: Sterile technique using 5mL 1% buffered lidocane as local anesthetic. Using CT guidance in the prone position, a 25-guage needle is advanced into the SI joint (right CT image #3). A solution of 1mL kenalog (40 mg mL) and 2.5 mL Bupivacaine (0.5%) is injected for pain relief (yellow arrow). This procedure is used to diagnose the SI joint as the cause of low back pain and to treat SI joint pain. The patient’s pain level is assessed prior to and after injection of the suspected SI joint with pain medication. An absence of pain relief minutes after injection may indicate that the source of back pain is not from the suspected SI joint.
This image shows precision of CT guided needle insertion into the SI joint (yellow arrow). A local anesthetic can be administered directly into the joint space. CT guidance is an essential requirement for this therapeutic procedure since thin slice imaging confirms needle location. This is a relatively common imaging procedure for chronic back pain suspected of originating for the SI joint.
CT images of the pelvis to include all soft tissue structures are acquired in the axial plane. These images can be reconstructed to thin slices (.625 mm X .625 mm) and reformatted in coronal and sagittal planes. Coronal images of the whole pelvis are good for evaluating the multiple structures like the hip bones (ilium, ischium, and pubis), acetabulum, and hip joints. The large wings of the ilium and the pelvic ring are easily profiled and made in true coronal planes for easy viewing. But what is not demonstrated in the true coronal plane of the pelvis is the sacrum and coccyx. Only a small portion of the sacrum is demonstrated in each coronal slice because of the anatomical position of the sacrum and coccyx. But when coronal sacrum views are parallel to the sacrum, an “open book” presentation is seen. This demonstrates more anatomy per slice like the true anatomy of the sacrum. The “open book” viewing requires that reformats are made parallel to the long axis of the sacrum. Both reformat planes are valuable depending on the clinical history. Therefore, it is always a good practice to format the entire pelvis when the history is trauma, and also reformat the sacrum when injury involves the posterior pelvic ring. Reformatted coronal sacrum images should be obtained when there is trauma to the posterior pelvic ring, sacrum, or the SI joints.
The two CT images above show the reformat planes for coronal images of the entire pelvis (left) and the sacrum (right). The two imaging planes are very different though they are both coronal planes. The left image (A) will show the entire pelvis in the coronal plane and is preferred for general viewing of the entire pelvis. Remember the pelvis is tilted downward in its anatomical presentation. This will foreshorten the sacrum which is tilted backwards. The coronal plane of the pelvis is included in the protocol for imaging the sacrum because it shows the relationships of all pelvic structures. However, it does not show the sacrum in its true coronal plane, therefore, the “open book” reformats are needed. These are obtained by reformatting images coronal to the sacrum. The true coronal plane of the sacrum (B) demonstrates the “book” views of the sacrum. True coronals of the sacrum are made by aligning the coronal plane parallel to sacral promontory and coccyx. These cuts are made using the sagittal plane as the reference, whereas the coronal images of the pelvis uses the axial plane with the coronal reformats aligned to the iliac crests. Now let’s look at some images through the sacrum for both types of coronal reformats to illustrate the importance of true coronal images through the sacrum.
These three coronal reformatted CT images of the pelvis show the entire pelvis. Notice the comminuted fracture of the left ilium, which extends into the left sacroiliac joint. Coronal slices of the entire pelvis show the complex relationships of the entire pelvis in profile. Injuries to the ilium, ischium, acetabulum and pubic rami are better seen on coronal images of the whole pelvis.
These three slices through the pelvis further demonstrates the relationships of the ilium, ischium, pelvic brim, and femora as well as the sacrum. The left SI joint is further delineated showing the complex involvement of the auricular surfaces of the sacrum and ilium. Notice that because the sacrum is directed backward and is curved, it is not presented in a true coronal plane of its long axis.
These coronal views of the pelvis further demonstrate components of the posterior pelvic ring. The posterior pelvic ring must be evaluated to determine stability of the pelvis. While these CT images demonstrate the value of coronal pelvis views, anatomical orientation of the sacrum and SI joints is not truly anatomical. Furthermore, thinner slices through the posterior pelvis are needed to display all elements such as the sacral foramina and sacral canal. Because the left sacroiliac joint is disrupted, coronal reformats parallel to the anteroposterior plane would best demonstrate this area.
These coronal reformats are made in the true coronal plane of the sacrum. Images in this plane of the sacrum are often called “Book” views because they open the sacral foramina and SI joints. Notice how the complex relationship of the right auricular surface and ilium is displayed for comparison to the left SI joint. Disruption of the auricular surface of the left sacroiliac joint and associated joint diastasis is easier to evaluate.
Continuing this plane from anterior to posterior demonstrates the sacrum in its true coronal plane. The sacral foramina are seen in profile as we go posteriorly, and the sacroiliac joints are symmetrically displayed. The sacrum is not foreshortened in this plane because its backward projection is compensated for by the slice angle. The sacral canal, which is a continuation of the vertebral canal, is seen more posteriorly on the far right image. Coronal pelvis reformats do not demonstrate this area as well as the “Book” views does.
Distally and posteriorly the vertebral canal and sacrum are anatomically displayed. Note the openness of the longitudinal plane of the vertebral canal. This demonstrates the nerve roots of the caudal equine (left image) within the sacral canal. Also, because these are true coronal images of the sacrum we can better demonstrates vertical fractures (yellow arrows). These reformatted images are taken through the deep posterior architecture of the sacrum where pelvic stability is evaluated.
By comparing side by side the true coronal plane of the sacrum (left) and coronal plane of the pelvis (right) we can see that the ilium and sacroiliac joints are projected differently. Notice the fractures through the posterior sacral elements (yellow arrows). These are better seen on the true coronal sacrum slice than on the coronal pelvis slice. The “Book” views can be extremely valuable in evaluating the sacrum/coccyx when there is trauma.
Computed tomography scanning is also used to evaluate reactive spurring within the sacroiliac joint. Reactive spurring is thought to occur due to prolonged abnormal motion within the joint proper. CT adequately demonstrates sclerosis or subluxation of the SI joint and is used to evaluate post-surgical fixations of the joint. In order to properly evaluate the SI joint with CT it is important that reformatted images are provided in the plane that is true coronal to the joint not the pelvis. Reformatted images are made from very thin slices using bone kernel. Both SI joints are reformatted even when one is thought to be normal as these may be useful for comparison with the abnormal joint.
These two scanogram images of the pelvis show the proper alignment for reformatting slices in true coronal planes through the sacroiliac joints. Both joints are reformatted in their true coronal planes, which is different from the orientation of the SI joints in the coronal pelvis plane. Generally, thin slice axial images (1.2 or .6 mm) in bone algorithm are used when reformatting the SI joints. The coronal plane of the SI joint is not the same as the coronal plane of the pelvis or the sacrum since the SI joint are about 30 degrees offset from the anterior plane.
True coronal views through each sacroiliac joint gives an enface viewing of the joint. This is important because the spacing of the joint is accurately displayed showing sharp bone edges of the ilium and sacrum. It also demonstrates the complex interlocking aspects of the auricular surfaces. Consider the twelve selected slices through the right SI joint of a patient who suffered severe pelvic trauma with multiple fractures. Note the fracture of the right transverse process of L5 vertebrae, which may be indicative of pelvic instability as discussed earlier. The reformat field of view (FOV) should be small, usually less than 15, and should include the entire SI joint and related portions of the sacrum and ilium.
These true coronal serial reformatted CT images of the right SI joint demonstrate enface views of the joint. The sacral wing is displayed in profile as well as the SI joint. Both the right and left SI joints are reformatted individually since their true coronal planes are different.
These true coronal slices of the right SI joint demonstrate enface viewing of the joint. Notice how the interlocking aspects of the joint are clearly displayed (yellow arrows). There are two interlocking levers formed by the auricular surfaces of the sacrum and ilium at the level of S2 vertebra. Reformatting in the true coronal plane to the SI joint also demonstrates the joint space accurately allowing for evaluation of possible subluxation or bone spurring into the joint.
The posterior elements of the SI joint are clearly seen along with the interlocking components of the joint (yellow arrows). This area is important in determining pelvic stability when fractured. True coronals demonstrate the extent of injury or pathology involving the posterior structures. This is a normal SI joint that displays the normal joint anatomy.
These true coronal views of the left sacroiliac joint demonstrate the anterior joint elements, sacral and ilium auricular surfaces. Diastasis of the SI joint is apparent on these enface slices of the joint because we are oriented in the true coronal plane of the joint. The relationship of the complex fracture of the ilium to the sacrum is well demonstrated in the plane.
These sequential slices through the left SI joint further demonstrate the disruption of the interlocking elements of the joint. This is important when evaluating the relationship of the sacrum to the complex fracture of the ilium. These reformats can be made in bone and/or soft tissue algorithm for evaluation of bone and cartilage components of the joint. The sequential images below (9-12) demonstrate the posterior elements of the joint and interlocking auricular relationships. When present, vertical fractures involving the sacrum or ilium articulation are profiled.
Sagittal Views of the SI Joint
Sagittal CT views of the SI joint are also made to compliment axial and true coronal planes. Just as the coronal reformats of the pelvis is not complete for the SI joints, neither are sagittal pelvis reformats adequate for the SI joints. The reason is that shape of the auricular surfaces of the sacrum and ilium and their interlocking architecture are not sagittal to the pelvis. The SI joints open at an angle of 25-30 degrees and often as much as 40 degrees to the sagittal plane. This is why rotating the pelvis 25-30 degrees will open the SI joint for plain film imaging. In order to evaluate all pathologies of the SI joint it is necessary to see the interlocking levers of the auricular surfaces. In other words, to see the interlocking components enface, we must reformat sagittal to the SI joint. True sagittal views of the SI joint are made parallel to the sacroiliac joint, not parallel to the pelvis.
This scanogram image a show how sagittal reformates through the pelvis is different from sagittal through the SI joint. True sagittal images of the pelvis are indicated by the white interval lines on the image. The true sagittal plane of the right SI joint is indicated by the yellow broken line, which runs parallel to the SI joint. Sagittal pelvis reformatted images will demonstrate the SI joint as an open joint; however, they will represent slight distortion from the true anatomical relationship. This occurs because sagittal images of the pelvis are not parallel to the auricular surfaces of the SI joint and the interlocking components within the joint proper.
These four CT images show the SI joint sagittal to the pelvis and lumbar spine. This orientation does show openness of the joint as some slices will pass through the joint space. However, most of the anatomy is distorted because this plane is not parallel to the joint. Now let’s look at sagittal images that are parallel to the joint to emphasize the importance of true sagittal views.
These three CT images show what is demonstrated when the reformat plane runs parallel to the sacroiliac joint. Notice that the interlocking auricular surfaces of the sacrum and ilium are demonstrated in their true sagittal anatomical relationships. These views are important to evaluate preservation of the joint space and ankylosis extending across the joint space.
Look at how beautifully true sagittal images demonstrate the two complex interlocking aspects of the auricular surfaces. Bone spurring, sclerotic changes, fibrosis, traumatic bone fractures, and joint diastasis are best seen from this perspective. Although it is rare for SI joint interlocks to fail, shearing vertical forces can dislodge the joint. An example of this would be someone falling from height with one leg striking the ground before the other causing shearing.
CT 3D Imaging of the Sacrum
Up to now we have looked at how multi-planar reformats (MPR) in true sagittal and coronal planes provide better diagnostic information than pelvis images. Multidetector computed tomography (MDCT) provides high resolution thin slice axial acquired image data. Thin axial slices permit high quality precise 3D reconstruction. Raw image data allows for superb spatial and temporal resolution that yields precise image detail needed to evaluate anatomical relationships. The two most commonly used 3D displays of the sacrum are volume rendered (VR) surface and transparent bone reformats. Any number of images can be displayed through 360 degrees of rotation along any selected plane. For example, images can be rotated from anterior to posterior along a medial/lateral revolution, or tumbled along the superior/inferior axis. 3D images can be oriented in an infinite number of axes for static imaging as well. What is nice about 3D imaging is that images are closer to the anatomical appearance of the structure than are planar images.
Surface rendering (SR), also known as shaded surface display, looks at the apparent surface of structures within the specified volume of data. Surface rendering was an early development in CT post-processing and has evolved to high quality finished images. It has limitations in that it does not adequately demonstrate surfaces that do not have inherent well-differentiated appearance. Surface rendering has been largely replaced by volume rendering techniques. Volume rendering not only shows excellent surface anatomy, but also shows a significant amount of image detail below the surface. Images generated using volume rendering make excellent multimedia presentations. There are a lot of positive gains with volume rendering, for examples, image display with varying levels of transparency, ability to vary color for air, fat, soft tissue and bone, show interior structural detail, and produce cut away dissection quality images.
These 3D surface rendered CT images show the relationship of the sacrum to the pelvis. Portions of the pubic bones are cut out (a feature of volume rendering) to show the anterior surface of the sacrum (middle image). The sacrum is reoriented from its natural downward tilt (middle and right images) to show it in a true anterior facing. Being able to manipulate images in the 3D formats is one of the reasons 3D CT imaging is such a powerful tool.
These 3D volume rendered image samples show the entire pelvis through 360 degrees of rotation. The scanogram image in the upper left corner represents the orientation of each image. These are surface volume rendered images that do not demonstrate a see-through view of the pelvis as does a plain film radiograph. But you can see that it does show various color assignment that adds detail to the overall images.
These images of the pelvis show 3D volume rendered images using a transparent bone algorithm. This algorithm provides see-through like images similar to plain film radiographs. A major difference between this algorithm and plain films is that images can be rotated through 360 degrees in many planes. This algorithm is also useful when viewing metal implants used for bone fixation. Transparent bone algorithms are useful for viewing surface relationships that are more apparent when 3D images are rotated. Rotation of images is beneficial when viewing structures that would otherwise be superimposed on plain film.
Volume rendering is an excellent tool for evaluating surface anomalies such as fractures, congenital malformations, surgical alterations, and the like. It also offers transparent bone algorithm that is similar to a plain film x-ray while allowing the viewer to rotate images through 360 degrees. Images can also be tumbled so that the north to south orientations can be viewed through 360 degrees of rotation. This can be extremely important when viewing the pelvis because the pelvic inlet and outlet can be viewed en face. The next two images below demonstrate how varying the transparency of the 3D image contrast between metal and bone is enhanced. Varying transparency coupled to image rotation provides significantly more information than standard x-rays with oblique views.
These images demonstrate how varying the degree of transparency we can see the exact arrangement of the metal fixation screws to the bony structure. This is a very useful tool especially when metal may extend into or pass through a joint. The image on the left shows low contrast with see-through of the metal fixation. The right image has higher contrast, which may be useful in some cases.
One important feature of volume rendering is that the technologist can perform cut-away images to display selective anatomy. On the images below some of the anterior bony anatomy is removed so that the sacral anatomy can be viewed without obstructing superimposition. As will be seen is some of the cut-away images structures like the femurs and soft tissue structures like the rectum and urinary bladder can also be cut away.
These two 3D CT images show some of the important manipulations that are possible with CT. The image on the left displays color whereas the right image does not. Both images have the anterior pelvis cut away to display the anterior sacral surface and SI joints.
These three CT images show cut out of the sacrum. The orientation of the sacrum is rotated to show it in a true AP (left), superior (middle), and lateral (right) viewings. Volume rendered 3D CT allows for image rotation in unlimited planes as well as cut out images of the specific part of interest such as the sacrum.
Pathology of the Sacrum/Coccyx
There are several common pathologies of the sacrum medical imaging professionals encounter. Some are simple benign lesions; others are malignant lesions that occur as primary or secondary metastasis. Noncancerous lesions include ankylosing spondylitis, oseoma, osteochondroma, and giant cell tumors to name a few. Primary malignant species include osteosarcoma, Ewing’s sarcoma, and multiple myeloma. An initial diagnosis of bone abnormality is usually made radiographically from plain-films, CT, MRI, or scintillgraphy. The goal is to identify these lesions and determine if they are benign or malignant. CT is very useful for determining the pattern and extent of bone involvement, and for identifying precise biopsy location.
Plain films can be used to identify lesions, but the down side is that at least 30% bone destruction in the lesion must have occurred. Nuclear bone scan imaging is the most sensitive as it can identify bone changes as little as 2-3 %. However, scintillgraphy is generally not a first step in bone imaging. With exceptions, most benign bone tumors occur in the first 2 or 3 decades of live and malignant tumors are seen in the elderly. Most benign lesions are expansile having sharp sclerotic margins. Malignant lesions are generally lytic often penetrating and destroying bone margins. It is also important in some cases to image remote location from the site of interest to check for distant metastasis. Inflammatory conditions are also of great interest as these may respond to nonsurgical treatments.
Inflammatory Bone Diseases
Ankylosing spondylitis (Bechterew’s syndrome) is an inflammatory disease that affects bone and soft tissue structures. It usually manifest in young adulthood and progresses with age. Early symptoms may include stiffness and pain in the lower back and may involve the hips. Almost any joint can be affected, but the most common areas are the lower vertebrae, hips, cartilage of the ribs and sternum, and areas where ligaments attach to bone, such as the heel. The cause of the disease is still unknown; however, it is thought to have some genetic association. Those with the gene HLA-B27 have a significantly higher risk for developing the disease, but having the gene is not diagnostic for the disease. Often the key to the diagnosis is that the patient presents with a complication associated with the disease. These include compression fracture, difficulty breathing, uveitis, and occasionally heart problems.
Risk factors for ankylosing spondylitis include young age, morning stiffness, and diminished symptoms with exercise, alternating buttock pain, and awakening back pain. Radiological confirmation of sacroilitis is one of the early diagnostic signs of ankylosing spondylitis. The dilemma is that conventional radiography of the SI joints is too insensitive for the diagnosis. Nuclear bone scan is highly sensitive; however, it is also non-specific for the disease. Therefore computed tomography is used to evaluate the SI joint for sacroilitis if conventional radiography is normal or equivocal.
Spondylolisthesis (spon + dee + lo + lis + thee + sis) is a condition whereby one vertebra slips forward or backward in relationship to an adjacent vertebra. A forward slippage is termed anterolisthesis, while backward slippage is referred to as retrolisthesis. Spondylolisthesis can cause deformity of the spine and is some cases may narrow the spinal canal thus compressing exiting spinal nerves. A common site for spondylolisthesis is at the L5/S1 junction (approximately 90% occur here). Spondylolisthesis is best seen on a lateral radiograph. There are five types of spondylolisthesis: dysplastic, isthmic, degenerative, traumatic, and pathological. Prior to spondylolisthesis a condition called spondylosis may exist. Spondylosis is a condition in which there is a break in the pars interarticularis (region between the superior and inferior articular processes). This is usually caused by a developmental congenital anomaly and less often due to trauma. Its radiographic appearance is seen on the oblique views of the lower spine as a “collar” or broken neck on the “Scotty dog.”
Dysplastic spondylolisthesis is a congenital disorder in which a facet is defective allowing the vertebra to slip forward. Isthmic spondylolisthesis is most often caused by trauma, especially repetitive hyperextension injuries. There is seen a defect in the pars interarticularis (portion of the vertebra between the superior and inferior articular process). The presence of a defect without accompanying slippage is termed spondylolysis. This condition is seen in athletes like gymnasts and football players. Degenerative spondylolisthesis is a term for arthritic changes seen in the joints between vertebrae due to degeneration. This is most often seen in elderly persons and those with arthritic syndromes. Traumatic spondylolisthesis is caused by trauma resulting in fracture of the pedicle, lamina, or apophyseal joint causing a slippage of the vertebra. As the name implies, a pathologic spondylolisthesis is caused by a bone defect such as tumor or bone erosion due to cancer and the like.
Degenerative joint Disease
Degenerative joint disease (DJD) a.k.a. osteoarthritis, is characterized by loss of joint space, sclerosis, and spurring osteophyte formation. This condition can affect the lower half of the sacroiliac joint because this portion of the joint is a synovial joint. Joint pain that radiates to the back or hip is common since the S.I. joint is a weight-bearing joint. While DJD is the most common form of arthritis that affects the S.I. joint other forms do occur including gout, rheumatoid arthritis, psoriasis, and ankylosing spondylitis. Ankylosing spondylitis, for example, tends to affect males between the ages of 10 and 30 years. Its classical presentation shows bilateral narrowing and fuzzy appearance of the S.I. joints. As the disease progresses the S.I joints eventually become obliterated. As the disease progresses the bones of the spine become calcified as do the intervertebral discs and spinal ligaments. The spine eventually becomes a ridged block of bone giving it the characteristic nickname “bamboo spine.”
Congenital Sacral Lesions
A transitional vertebra is frequently seen as a developmental normal variant of the spine. It is often an incidental finding on radiographs, but is found in approximately 20% of humans. It most often occurs in the lumbosacral or sacrococcygeal regions. When L5 is incorporated into the sacrum it is said to be “sacralized.” When S1 is incorporated into the lumbar spine it is said to be “lumbarized.” These distinctions are mute points since transitional vertebrae have partial features of the above and below segments. The clinical significance of a transitional vertebra is the possibility of confusing the counting of vertebra number during medical or surgical treatment. Only rarely is the transverse process enlarged to cause clinical symptoms of encroachment of adjacent structure(s). Plain film radiographs, CT, fluoroscopy, or MRI is used to correctly label transitional vertebrae prior to any surgical intervention.
Sacral agenesis (caudal regression syndrome) is an abnormal developmental disorder. Sacral agenesis is unfortunately a very severe disorder of rare occurrence, occurring in approximately 0.005 to 0.01% of the population. The incidence is higher in children of diabetic mothers. Children of diabetic mothers also have an increased frequency of other spine and spinal cord abnormalities. There are varying degrees of sacral agenesis that can occur with and without spinal cord and meninges abnormalities.
Meningocele is a protrusion of the membrane (meninges) that surround the spinal cord through a bony defect in the vertebral column. An osseous defect can occur on either the ventral or dorsal side of the vertebral column. The bony defect is due to failure of the vertebra to fuse during embryonic development. The classic example of this is spina bifida. This term specifically refers to a bony defect through which the meninges and/or spinal cord may protrude, e.g. spina bifida cystica. The meninges and cord also may not protrude through the defect and remains hidden by a covering of tissue and skin (spina bifida occulta). Thanks to the work of the March of Dimes organization, worldwide awareness that ingestion of folic acid by the mother during pregnancy greatly reduces the risk of meningocele and other neural tube defects. Today, prenatal vitamins and minerals nutrition is a major part of pregnancy care. Posterior meningocele is more common than anterior meningocele and are classified according to the contents of the protrusion as myeloceles, myelomeningoceles, or lipomyelomeningoceles.
Myelocele /my•elo•cele/ (mi´ĕ-lo-sçl) is a term for the protrusion of the spinal cord through the bony vertebral arch. This usually occurs through a posterior defect in the vertebra. Myelomeningocele /my•elo•me•nin•go•cele/ (-mĕ-ning´go-sçl) describes the protrusion of both the spinal cord and the meninges through a bony defect in the spine. Lipomyelomeningocele is a term from a myelomeningocele that has an overlying lipoma. In some cases all of these conditions may occur as an open wound putting the meninges and/or spinal cord in danger of infection. These developmental lesions of the spine and sacrum are optimally imaged with magnetic resonance imaging, although plain film or computed tomography may also be used to assess the associated osseous defects.
Sacral cyst commonly occurs and is usually an incidental finding during cross-sectional imaging or myelograpy. There are many names for these cyst including sacral meningeal cyst, perineural cyst, Tarlov cyst, sacral arachnoid cyst, and occult intrasacral meningocele. However, they basically have two broad classifications, those that communicate freely with the subarachnoid space and those that do not. Overall, they are benign and asymptomatic; however, symptomatic sacral cysts can be aspirated or modified with fibrin glue therapy.
Benign Bone Lesions
Imaging the sacrum can be an important step in the diagnosis of low back pain. Often the failure to properly image the sacrum may lead to delayed diagnosis of sacral tumors. The sacral canal can shelter and hide slow growing asymptomatic tumors until they become large and compress adjacent soft tissue structures. Low back pain is often ignored and when it becomes problematic for the patient plain films are often the only screening images. Plain films of the lower spine and sacrum is known to be insensitive for detection of most sacral lesions. This is mainly due to poor visualization of the sacrum because of its anatomical orientation, bowel gas and fecal material superimposing the sacrum, or poor subject contrast. Although CT, MRI, and scintigrapy better image the sacrum, a definitive diagnosis is often elusive. Advanced imaging can provide differential diagnosis, but definitive diagnosis may require image-guided biopsy. Sacral tumors such as giant cell, aneurysmal bone cyst, chordoma, infections, osteoid osteoma, osteoblastoma, and other must be precisely diagnosed to avoid an undiagnosed risk of malignancy.
he sacrum is the most common site of giant cell tumor involving the spine. Giant cell tumor is the second most common primary sacral tumor. Chordoma is the most common sacral tumor. Giant cell tumor is considered a benign but aggressive bone tumor; however, approximately 5-10% of giant cell tumors are malignant. They most often occur in women in the 2nd through 4th decades of life. These neoplasms are best imaged using CT or MRI. Giant cell tumor is a lytic lesion presenting as a soft tissue mass with a thin sclerotic rim. CT may show hemorrhage or necrosis of the lesion. Generally, the tumor may have growth across the sacroiliac joint and into the ilium. MRI signals show intensity of both T1 and T2-weighted images. Angiography may be used to demonstrate the tumor’s vascularity and/or to embolize the tumor prior to surgical resection.
Another type of bone cyst is the aneurysmal bone cyst. This lesion is often filled with blood and is expansible. It is more common in females than males and occurs greater than 80% of the time before the age of 20. It is a hypervascular lesion often crossing the ingervertebral disc space to involve the adjacent vertebrae. It is often difficult to differentiate aneurysmal bone cyst from other lesions such as giant cell tumor, chordoma, infection, or aggressive malignancy.
Osteoid osteoma and osteoblastoma are types of rare lesions that do occur in the sacrum. Only about 2% of osteomas occur in the sacrum, usually in young males. These lesions are similar histologically, but can be differentiated by size. Osteoid osteomas are less than 2 cm in dimension, while osteoblastomas are larger and occur more often. Osteoid osteomas arise within cortical bone to erode deep underlying bone tissue. As a result it is a lytic lesion called a nidus surrounded by a zone of dense sclerotic bone. These lesions can be identified on CT as having a characteristic high-attenuation nidus surrounded by a zone of low attenuation sclerosis. There may also be a zone of reactive sclerosis. Osteoblastoma subtypes are pathologically similar to osteoid osteomas, except they have a different clinical neurological presentation. Patient with osteoid osteoma present with classical night back pain that is relieves by aspirin (salicylates).
Some sacral neoplasms occur within the sacral canal proper. Examples of these include meningiomas, schwannomas, and neurofibromas that develop from lower lumbosacral sensory nerve roots. They are all quite rare in the sacrum and difficult to see on routine plain film, but can usually be seen with advance imaging techniques such as CT, MRI, and radioisotope scanning. As these tumors grow they may involve the sacral canal and neural foramina. Imaging alone does not definitively differentiate these lesions, but can identify potential malignant transformation. Tomography does identify precise biopsy localization, which does differentiate the type of lesion.
Ependymomas are malignant tumors that can arise in the sacrum from ependymal cells of the filum terminale. These tumors also can also come from ependymal cells of the central canal of the spinal cord. This is another type of tumor that can manifest as a destructive sacral lesion. There are several other types of tumors that develop within the sacral canal such as gerinomas, neuroectodermal tumors, glioblastomas and carinoid types.
Malignant Sacral Lesions
Unfortunately, some spine and sacral lesions are malignant. The spine plays a major role in hematopoiesis and serves as a reservoir for red marrow in the adult. As a result it is also a common site for hematological malignancies like myeloma, lymphoma, plamacytoma, and Ewings sarcoma. Because the spine is a blood forming tissue it also provides passageway for metastatic diseases to spread. Metastasis is the most common cause of sacral neoplasm with primaries from the prostate, breast, lung, uterus, rectum, bladder, and kidneys reported. Both lytic and sclerotic metastatic lesions have been found in the sacrum; however, lytic lesions are the most common presentation of malignancy of the sacrum. When multiple lesions are found, it often suggests that metastatic disease has occurred.
Cells types from the immune system can also cause sacral malignant lesions. Lymphocytes are among the most important cells of the immune system. T-lymphocytes are so called because they are derived from the thymus gland, whereas B-lymphocytes come from the bone marrow. These cells along with others of the immune system ingest foreign materials and produce immunoglobulins like IgG, IgM, IgA, IgD, and IgE. While these antibodies have a positive effect on the body there can be some adverse reactions such as those seen with organ transplant. Another adverse effect occurs when there is a malignant transformation of lymphocytes known as lymphoma. All of these lymphocyte transformations are malignant and based of their radiological presentations and clinical behaviors are further classified. Hodkin’s disease (lymphoma) is the most common type of lymphoma occurring at all ages, but especially in the young to middle-aged adults. In spite of its radiological presentation, Hodkin’s is diagnosed primarily from biopsied tissue that shows Reed-Sternberg cells.
The most common primary malignancy of the sacrum is chordoma. Chordomas arise from what was the embryonic notochord. The notochord is not present in the adult, but contributes to some spinal elements including the intervertebral disc. Among the primary tumors of the sacrum, chordoma is the most common accounting for about 2-4% of bone neoplasms. They occur later in life, usually in the 4th to 7th decades. The chordoma is a midline structure because of its notochordal origin, approximately half of which occur in the sacrococcygeal reagion.
Chordomas are relatively large lytic lesions that are recognizable on plain films and on CT because of their characteristic calcium deposits. These tumors usually have soft tissue involvement within the sacral canal and involving adjacent intervertebral disc space and SI joints. They are considered a low grade metastatic tumor that infrequently spreads, but are very aggressive locally. Because they develop within the sacral structure they are difficult to resect surgically. MRI and direct angiography are often required to display the vascular characteristics of these tumors. Chordoma shows low to intermediate signal intensity on T1 MR images and marked signal increase on T2-weighted images.
The sacral lesion seen in the CT images above are thought to be chondroma. However, at the time of this publication the biopsy results were not complete to confirm the diagnosis. The sacral lesion is extensive and shows marked bone destruction and inflammatory changes.
Another type of tumor that occurs in the sacrum is the teratoma. Teratoma is a type of tumor that produces many types of tissues. Traditionally teratomas are said to have tissue representatives from all three embryonic germ layers. Although teratomas are more commonly seen in the testes they do occur in the sacrum too. The sacrococcygeal region is the most common location of teratomas discovered in infancy. The prevalence of teratomas in infancy is 1 in 35,000–40,000 births. These tumors do occur in adults, but rarely. Most are benign in infants and malignant in adults. Also, most teratomas in infants are visible externally, whereas in adults they are usually not so obvious.
The sacrum and coccyx, being at the distal end of the spine, is often ignored as an important site for pathologic conditions. The distal spine is optimally imaged with cross-sectional techniques to include soft tissue and bone algorithms. The normal development of the sacrum or failure thereof is manifested by imaging. Both congenital and neoplastic sacral lesions are importantly imaged with plain films, CT, MR, and radioisotope imaging. Anatomical implications of sacral lesions are related to the tissues that form the sacrum and its surrounding soft tissues (i.e., bone, cartilage at the sacroiliac joint, meninges, and neural elements). Some structures have a predisposition for affecting the sacrum including sacral agenesis; primary tumors, and particularly teratoma, chordoma, giant cell tumor, and chondrosarcoma. Sacroilitis, perineural cysts, metastases, stress fractures and traumatic injuries also commonly affect the sacrum. Familiarity with those conditions that may affect the sacrum enhances the ability of radiographers to properly image the sacrum. Correct imaging protocols allow radiologists to arrive at a specific diagnosis or give an appropriate differential diagnosis. Thus our role in imaging the sacrum contributes to clinical management of sacral conditions.
Coming July 2016
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