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.
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Imaging The Sacrum and CoccyxWritten by Nicholas Joseph Jr. RT(R)(CT) B.S. M.S
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Objectives:
Upon completion, the reader should be able to:
- Discuss common reasons for imaging the sacrum and coccyx.
Introduction
The sacrum is imaged in general radiology as well as in subspecialties such as computed tomography (CT), magnetic resonance (MR), nuclear medicine (NM), and positron emission tomography (PET). Generally, the sacrum is evaluated as 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 sacroilitis. Other conditions such as gout, bone tumors, and Bechter’s syndrome are also common reasons for selective imaging of the sacrum or sacroiliac joints. Sacroiliac joint pain affects approximately 15 to 25% of patients with low back pain, the source of which is challenging to diagnose. While physical examination and radiological studies do contribute to understanding the etiology of SI joint pain, the most common method for diagnosing chronic low back pain originating from the SI joint is with small-volume local anesthetic joint blocks. In other words, when anesthetic SI joint block is successful, it is also diagnostic for the source of low back pain. Inflammatory conditions such as sacroilitis can also be confirmed with radiologic studies, which is essential to early detection of ankylosing spondylitis. Because some sacral lesions are difficult to visualize they are particularly imaged and localized using a cross-sectional modality such as CT. MR imaging is very useful for diagnosing congenital disorders such as meningocele, sacral agenesis, and other congenital lesions. So, the goal of this article is to review the anatomy, function, and mechanisms of common injury of the sacrum and SI joints, and discuss specific radiological imaging of the sacrum and SI joints.
Anatomy of the sacrum
The sacrum is part of the vertebral column articulating with the fifth lumbar vertebra at the lumbosacral junction. The sacrum and coccyx are the distal portions of the vertebral column and part of the pelvis. They are classified as irregular bones and atypical vertebrae. The sacrum is actually a composite bone formed by 5 vertebrae that fuse into a single bone by around age 20 years. This fusion presents distinct anterior and posterior sacral landscapes that will be described separately. On the whole, these fused sacral segments can still be identified on the adult sacrum. The main functions of the sacrum are to strengthen and stabilize the pelvis and transmit the weight of the body to the pelvic girdle through the sacroiliac joints. To perform these functions the sacrum forms four main articulations: 1) with the inferior articular processes of the fifth lumbar vertebra, 2-3) bilaterally with the ilium to form right and left sacroiliac joints, 4) inferiorly with the coccyx.
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 an important 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 further stabilize the pelvis.
The anterior surface of the sacrum or pelvic surface is concave, which is a shape that adds depth to the pelvic cavity. Four pairs of foramina are seen on the anterior surface, which are the remnants of the intervertebral foramen seen in other parts 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 largest is the first sacral vertebra, which receives the body’s weight transmitted along the vertebral column. It bears 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. The smaller fifth sacral vertebra articulates with the first coccygeal vertebra and may also fuse with it. Four transverse lines representing the original separations of the five sacral vertebrae can be seen on the anterior surface. Between two transverse lines is the body of each vertebra, which allows them to be accurately counted. The anterior sacral foramina (aka: pelvic sacral foramina), four in number are found on the lateral anterior walls of the sacrum. Anterior foramina are somewhat rounded opening in a lateral and forward direction. They are the remnants of intervertebral foramina through which anterior divisions of sacral nerves and blood vessels pass in route to structures in the pelvis and lower extremities. Sacral foramina are continuous with the sacral canal, which is continuous with the long longitudinal vertebral canal of the spine. The vertebral canal does not extend into the coccyx.
The posterior surface is convex presenting a prominent bony ridge in the midline 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 5 do not. Instead they form a U-shaped area called the sacral hiatus from which two small bony projections called the sacral horns (cornua) are prominent. Anesthetic agents are sometimes administered through the sacral hiatus to relieve pain (called an epidural anesthetic). Large wedge shaped surface called auricular surfaces (because it is shaped like the auricle of the ear), are seen on the lateral sides. They articulate with corresponding auricular surfaces on the ilium to form the two sacroiliac joints. The sacroiliac joint opens posterior and obliquely at an angle of about 30-degrees. The posterior surface presents four pairs of posterior sacral foramina that are irregular in shape. 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), the individual partially fused sacral vertebra are numbered from proximal to distal (1-5), and the transverse lines (SL) that indicate the body of individual vertebra are seen.
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, 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 to form 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 joint(s) (B). The SI joints stabilize the pelvis and are held in place by complex arrangements of ligaments.
When imaging the sacrum it is important to remember that the spinal cord is not the same length as the vertebral column. In infants, it terminates at L2 or L3, and in the adult at L1/L2 junction. The spinal cord is anatomically and functionally segmented. For example, the area corresponding to L4 is not at the level of the fourth lumbar vertebra because the cord ends at L1. The nerves for the lower lumbar segment, sacrum, and coccyx arise from their cord segment to descend within the vertebral canal. The length of the nerve roots increase, especially in the lumbar area to form a bundle of nerve roots in the subarachnoid space caudal to the termination of the spinal cord. The nerve roots that extend distal to L2 vertebra are collectively called the cauda equina (L. horse’s tail). The spinal cord ends in a tapered cone shape called the conus medullaris. Extending from the conus is a thin fibrous thread of meninges (mostly dura mater) called the filum terminale, which anchors the cord to the dorsum of the coccyx.
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These CT axial images are of the lumbar spine taken following a myelogram. The subarachnoid space and nerve root within the sac can be seen on both images. 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 information from the body to the brain, and send a motor response from the brain to various tissues. An example of sensory information to the brain would be those complex sensory inputs that tell the brain where you are going when driving a car. Motor responses would be those coordinated muscle movements to steer, accelerate or decelerate the vehicle. There are five pairs of sacral nerves and one coccygeal nerve that travel down the vertebral canal into the sacral canal to exit the sacral foramina. Therefore, the sacral canal and sacral foramina must be demonstrated on sectional images of the sacrum.
These coronal CT images demonstrate the sacral canal and sacral foramina (yellow asterisk) through which the sacral nerves exit. The sacral foramina are formed by the fusion of the sacral vertebra. Typically vertebrae do not fuse but form intervertebral foramen (yellow arrow) between adjacent vertebrae through which spinal nerves exit the vertebral canal.
The coccyx, also called the tailbone marks the most distal portion of the vertebral column. The coccyx is highly regressed in humans and does not resemble a vertebra. It 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 to imaging the sacrum and coccyx because they determine how much the central ray should be angled for a true AP projection. The frontal plain film radiograph of the sacrum requires a cephalic tube angle, but for the coccyx a caudal angle best demonstrates it. One of the more understood roles of the coccyx is the attachment of the meninges to its distal section. This attachment anchors the meningeal sac surrounding the spinal cord reducing cord movement within the vertebral canal.
This sagittal CT view of the sacrum (left) shows its major structures. The sacral promontory (A), the dominant concave curvature of the sacrum (B), forward curvature of the coccyx (C ), the superior articular processes of the first sacral segment (D-right CT image), and the sacral canal, which is continuous with the vertebral canal of the spine (yellow arrows).
Pelvic Stability
The bony pelvis lacks inherent structural stability, and so is stabilized by a system of tightly woven ligaments and muscles that provide its support. Strong ligaments arranged transversely resist forces that can externally rotate the pelvis, thereby opening it. Among these are the short posterior SI ligament, the anterior SI (sacroiliac) ligament, the iliolumbar ligament, and sacrospinous ligaments. Their function is to counter opposing forces such as AP compression. These ligaments fail when force exceeds their ability causing compression injury. Vertical stability of the pelvis is primarily due to the short and long posterior SI ligaments. Interosseous ligaments within the sacroiliac joints also provide some additional vertical stability.
Stabilizing ligaments help perform one of the main functions of the pelvis, to transmit weight from the trunk and lower lumbar vertebrae to the lower extremity. For example, while standing weight is transmitted from the trunk through the posterior pelvic arch to the sacrum and sacroiliac joints. Weight is transmitted through the acetabula of the hip bones to the femurs. The anterior arch functions like a strut maintaining the shape of the pelvic ring during active weight-bearing movements. Passive weight bearing such as during sitting is transmitted down a vector to the ischial tuberosities.
These three 3-D volume rendered CT images depict some basic patterns of those dense broad ligaments that support the bony pelvis. The illustrated CT image on the left demonstrate the arrangement of ligaments from the AP perspective; the middle CT image illustrate 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. Many ligaments support and provide short stabilizing struts for the pelvis. The greater sciatic notch forms an opening bridged by ligaments that guard the entrance into and out of the pelvis. The lesser sciatic notch provides an opening to the perineum. A dense array of ligaments holds the bony pelvis together; some are illustrated on the 3D volume rendered CT images above (blue lines). 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 from 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
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The take home point is that with pelvic fractures structural ligaments can be disrupted. Separation of the sacroiliac joints and severe fractures of the sacrum can cause instability of the pelvis. Of particular radiographic interest are the iliospinous ligaments that attach to the transverse processes of the fifth lumbar vertebra and to the ilium. This is illustrated on the posterior view above. 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 three coronal MR images show a portion of the tremendous 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 separates bone or joints causing pelvic opening. Pelvic compression can also separate bones and often results in rupture of major 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.
- The Tile classification system describes pelvic fractures that do not appreciably disrupt the pelvic ring (Tile type A).
- Type A, posterior arch intact stable fractures including avulsions of the iliac spine, iliac crest, ischial tuberosity, wing fracture, unilateral and bilateral pubic rami fracture, sacral fracture, or sacrococcygeal fracture. These are ring sparing injuries and fractures that do not affect the stability of the pelvic ring and do not involve the SI joints.
- Type B, incomplete posterior arch disruption occurring with pubic diastasis, anterior SI joint disruption, anterior sacral buckle fracture, AP and lateral compression injuries, internal and external rotational injuries, and accountability for stability of rotational and vertical injuries.
- Type C, complete posterior arch disruption. These are the most severe types of pelvic ring fractures caused by vertical shear, shear and AP/lateral compression. These occur with ipsilateral and contralateral internal or external rotation of the pelvis due to SI joint fracture/dislocation, and are rotationally and vertically unstable.
A rotational injury of the hemipelvis causes serious anatomical disruptions in 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 until permanent fixation reduction can be achieved. If bleeding is not stopped, vascular embolization by interventional radiology may be called upon to 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 possible cause of hemorrhaging. Hemorrhage is the most serious complication of pelvic fracture and is life-threatening in most cases.
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. 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. Always have the patient change into a metal free gown to eliminate the possibility of clothing artifact. Tight collimation is needed to reduce scatter radiation and improve subject contrast. Shielding of the male gonads is recommended; however, shielding for females is not possible since the ovaries lie within the pelvis. It is important to screen women of childbearing age for possible pregnancy prior to imaging the pelvis. The routine views of the sacrum are an AP sacrum, AP coccyx, and lateral sacrum/coccyx. When the sacroiliac joints are specifically evaluated the 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.
It is important that the pelvis is properly positioned so that the sacrum and sacroiliac joints are symmetrically displayed. Palpating the pelvis so that the anterior superior iliac spines 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 to reduce foreshortening of the sacrum. A properly positioned the AP sacrum view will demonstrate the entire 5th lumbar vertebra, symphysis pubis, and both sacroiliac joints.
When imaging the coccyx proper positioning and optimal exposure technique is essential to producing a diagnostic radiograph. 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. The bladder and colon should be emptied prior to imaging the sacrum or the 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 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. Align the entire long axis of the patient with the shoulders and posterior ribs superimposed, and place a pillow or radiolucent sponge between the knees to 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 opened and the median sacral crest is in profile.
The diagnostic criteria for the AP sacrum
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The following radiograph demonstrates structures that should be demonstrated on the AP sacrum (left) and AP coccyx (right) radiographs: 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).
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 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. Align the entire long axis of the patient with the shoulders and posterior ribs superimposed, and place a pillow or radiolucent sponge between the knees to 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 opened and the median sacral crest is in profile.
Diagnostic Criteria for the Lateral Sacrum
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This lateral view of the sacrum/coccyx demonstrates those structures that should be seen on a radiograph of the sacrum. 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 canal (C), sacral promontory (D), coccyx (blue arrow), and superimposed greater sciatic notches (yellow arrow).
Critique of Plain Films of the Sacrum/Coccyx
Radiograph #1
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Give your critique of this AP radiograph of the sacrum taken for chronic sacral pain and no history of recent trauma.
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This radiograph of the sacrum meets the diagnostic criterion for structures to be demonstrated. The entire 5th lumbar vertebra to the 1st coccygeal segment is seen without rotation of the pelvis. The midsagittal plane of the sacrum is aligned with the symphysis pubis. Laterally, both sacroiliac joints are seen and the visualized portion of the pelvis 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.
Radiograph #2
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Give your critique of this AP radiograph of the sacrum taken for acute sacral pain and history of fall landing on buttocks.
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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 sacrum without cephalic angulation of the central ray. This is evidenced by the sacral foramina not being profiled opened and there is foreshortening of the 1-3 sacral segments. This appears to be a male, which requires gonadal shielding when imaging the sacrum/coccyx. 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 through the sacrum/coccyx and application of the correct tube angle of 15-degrees cephalic.
Radiograph #3
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Give your critique of this AP radiograph of the sacrum given a history of chronic low back and hip pain and a negative radiograph series of both hips.
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The sacrum is elongated, and the symphysis pubis superimposes the 4th and 5th sacral segments. This is caused the tube being angled too much in the cephalic direction. Generally, an angle of 15 degrees is adequate for imaging the sacrum. Do notice that the visualized upper sacral foramina are opened and clearly demonstrated. Decrease the tube angle to 15-degrees cephalic and align the patient so that both sacroiliac joints are entirely demonstrated along with the entire 5th lumbar vertebra. Also use gonadal shielding since this is a male patient in keeping with ALARA. Exposure factors adequately visualize the sacrum with good bone detail.
Radiograph #4
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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?
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This radiograph does not meet diagnostic standards for the AP sacrum view. The main reasons are under penetration and improper tube angle. Notice that the sacrum lacks detail and overlying underpenetrated bowel completely obliterates subject detail. Keep in mind that the tube is not angled cephalic so the sacrum is foreshortened. Although the 1st sacral segment shows minimal foreshortening of the ala it is clear from observing the closed obturator foramina that either the tube is angled caudal or the patient has extreme lordotic lumbar curvature. This was the request of the ordering physician that the tube not be angled so that the post-surgical sacroiliac bridge is properly demonstrated. I would suggest aligning the tube with the sacrum to get a true AP view. The patient is poorly prepped as the urinary bladder is full and the rectum is not free of fecal material. When this radiograph is repeated properly prep the patient and increase the kVp 15 to 30% to penetrate the sacrum. Optimum exposure for the sacrum is 75 to 85 kVp for an average sized adult patient.
Radiograph #5
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Give your critique of this radiograph using the diagnostic criteria for the AP sacrum view.
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The diagnostic criteria for the AP sacrum states that L5 must be entirely demonstrated, the sacrum well-penetrated and having good bone detail, and the bladder and rectum emptied. None of these criteria are met by this radiograph. It is underexposed, lacks subject detail, and fecal material and a full bladder are reasons this radiograph should be repeated. Slight rotation of the sacrum is noted as its midsagittal plane is not aligned to the symphysis pubis. The tube angle seems to be appropriate as the sacrum is not foreshortened.
Radiograph #6
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Give your critique of this radiograph using the diagnostic criteria for the AP sacrum view. Consider a history of low back pain without trauma.
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Because this is not a trauma related study it would not be in keeping with ALARA to repeat this radiograph only because the entire 5th lumbar vertebra is not demonstrated. The entire sacrum is seen without foreshortening and the sacroiliac joints are 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.
Radiograph #7
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Give your critique of this radiograph using the diagnostic criteria for the AP coccyx view.
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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!
Radiograph #8
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Discuss why this radiograph does not meet the diagnostic standards for the AP sacrum view.
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This radiograph meets the diagnostic standards for including the proper anatomy. Both sacroiliac joints are demonstrated, L5 vertebra, the entire sacrum and symphysis pubis. This radiograph does not meet diagnostic criteria because the femurs are flexed (yellow arrows), the bladder and rectum full, along with poor penetration of the part. Even when properly prepped for imaging the AP sacrum view can be difficult to acquire due to fecal material 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 that may overshadowing the sacrum and symphysis.
Radiograph #9
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This patient slipped on some ice on the driveway while starting his car. His tailbone was determined to be the source of his acute pain. Does this radiograph meet the diagnostic criteria for the AP coccyx view?
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Two factors that make a good radiograph of the coccyx are proper tube angle and recorded detail. The coccyx is seen without superimposition on the symphysis indicating the tube angle is correct. However, this radiograph fails on collimation, which is a condition for ALARA. Close collimation improves subject detail when the part is small. Using the small focal spot also helps when imaging a small part in combination 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 in 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 the AP view alone is not used to evaluate the sacrum or coccyx. Therefore, repeating this radiograph because of lack of collimation is not in keeping with ALARA.
Radiograph #10
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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?
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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.
Radiograph #11
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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?
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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.
Radiograph #12
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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?
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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 (SI) joints are a part of the pelvis proper and are a 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. The diarthrodial joint is formed by the anterior third interface between the sacrum and ilium. The remainder and posterior portions of the joint involves intricate ligamentous connections. While the true synovial joint portion of the SI joint permits 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, and others that are connected to ligaments of the joint. These muscles provide support to the pelvis and can 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 SI joints. SI joint ligaments are stronger in males to resist normal pelvic forces. But in females, slight mobility of the pelvis is a necessary component of 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, or even hip, knee, or foot pain.
The SI joints are designed for pelvic stability as previously discussed. The load of the trunk is transmitted and 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 1/20th of the axial compression load 12. These last 2 motions may preferentially strain and injure the weaker anterior joint capsule13. Biomechanics studies of the SI joint show rotation about all three axes; however, it should be noted that these movements are very small and 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 be trauma, multiparity, and muscular dystrophy.
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. In the absence of trauma, the sacroiliac joints are 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 radiological studies of patient’s with SI joint pain have been disappointing. There are many issues in terms of diagnostic correlating findings with dysfunctional SI joint as the source of clinically assessed pain.
Most of the current data on SI joint pain correlated with imaging studies is retrospective and ongoing. Therefore, it is imperative that imaging studies of the SI joints is complete and properly made. In order to perform correct imaging it is important to note that the plane of the auricular portion of the joint can be directed from anterolateral to posteromedial. Accordingly, oblique plain film images should project the joint in this 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 radiological studies is reported to be between 13% and 46% and for CT imaging 57.5% sensitive and 69% specific in diagnosing SI joint pain source.
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.
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.
Computed Tomography (CT) of the Sacrum and SI Joints
Computed tomography scanning 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 assume the common practice that analgesic response to diagnostic local 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 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.
These axial CT images demonstrate the SI joint injection technique used to assess whether or not SI joint pain is from the SI joint or is referred from a 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. 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 SI joint with pain medication.
The two CT images above show the reformat plane for coronal images of the entire pelvis (left) and that of the sacrum (right). Notice that 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” views are reformatted 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 reformats so that the cuts are parallel to sacral promontory and coccyx. These cuts are made using the sagittal plane as a 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 purpose of the true coronals of 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 cohesively with these views.
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 the pelvic ring, sacrum, ischium, and ilium. Notice that the posterior pelvic ring, which is evaluated to determine stability of the pelvis, is well demonstrated. The diagnostic value of the coronal pelvis reformats require many thin cuts to demonstrate the sacral foramina, which are poorly demonstrated in these nine slices.
These three coronal CT images are reformats through the sacrum in a true coronal or “Book” views, so named because they open the sacral foramina and project the SI joints in relationship to the anatomical plane of the sacrum. Notice the complex relationship of the right auricular surfaces of the sacrum and ilium. Disruption of the auricular surfaces of the left sacroiliac joint and SI joint diastasis is easier to evaluate.
These three CT “Book” views of the sacrum demonstrate sacrum in true coronal planes. The sacral foramina are seen in profile and the sacroiliac joints symmetrically presented for evaluation. Notice that the sacrum is not foreshortened because the backward projection of the sacrum is compensated for by the slice angle. The sacral canal, which is a continuation of the vertebral canal, is seen on the far right image.
These three true coronal CT images demonstrate the importance of slices in this plane. Note the open longitudinal presentation of the vertebral canal and nerve roots of the caudal equine (left image). The true coronal plane of the sacrum better demonstrates vertical fractures of the sacrum, especially in the deep posterior areas than coronal pelvis slices alone (yellow arrows).
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 post posterior sacral elements 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.
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 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 it may be useful for comparison with the abnormal joint.
These two scanogram images of the pelvis show the proper alignment of reformatted slices for true coronal views of the sacroiliac joints. Both joints are reformatted in their true coronal planes. 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.
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 clear bone edges of the ilium and sacrum. It also demonstrates the complex interlocking aspects of the auricular shaped 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 indicate 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 joint is 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 that accurately displays the joint space. The relationship of the complex fracture of the ilium to the sacrum is well demonstrated
These sequential slices through the left SI joint allows for comparing the interlocking elements of the joint. The relationship of the sacrum to the complex fracture of the ilium is accurately demonstrated. 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. 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 acquired CT Images. Just as the coronal reformats of the pelvis are not sufficient for the SI joints, neither are the sagittal pelvis reformats alone sufficient. 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. Therefore, true sagittal reformats to the plane of the SI joint is required for proper assessment of the joint.
This is an example of an axial CT image slice that shows the SI joints. True sagittal images are indicated by the white interval lines. The true sagittal plane of the right SI joint is indicated by the yellow lines for specific imaging of the SI joint. Sagittal pelvis views demonstrate the SI joint as an open joint, which is a distortion of its true anatomical relationship. This occurs because sagittal images of the pelvis direct slices across open regions of the joint rather than parallel to the auricular surfaces that interlock.
These four CT images are sagittal to the pelvis and lumbar spine. Notice that the presentation of the right SI joint is as an open joint due to the slice plane being through open segments of the joint. This is a distortion of the true sagittal anatomy of the SI joint. Therefore, when fractures, spurring, or mass involves the joint space it is important to reformat images sagittal to the SI joint.
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 anatomical relationships. These views are important especially when the sacrum and SI joint is specifically imaged, or when fracture involves the joint proper.
These two selected CT slices show what is demonstrated with true sagittal images of the SI joint. The importance of demonstrating the interlocking mechanism of the auricular surfaces is seen.
CT 3D Imaging of the Sacrum
Multidetector computed tomography (MDCT) has provided high resolution thin slice axial acquired image data enabling 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 surface and transparent bone rendering. 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.
These 3D surface rendered CT images show the relationship of the sacrum to the pelvis. The anterior portion of the sacrum is cut out on the middle image to show the sacrum. Notice that the sacrum is reoriented from its natural downward tilt on the middle and right images to show its true anatomical relationship. Being able to manipulate images in the 3D formats is one of the reasons CT imaging is a powerful tool.
These 3D surface rendered images show the entire pelvis through 360 degrees of rotation. These are surface rendered images that do not demonstrate a see-through view of the pelvis as does a plain film radiograph. Therefore it is necessary to view surface images through 360-degrees of rotation. 3D surface anatomy images allows for viewing and evaluating structural relationships.
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