Quality Assurance and the Helical (Spiral) Scanner
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This article reviews quality assurance program requirements including quality control tests and tools used to follow the guidelines set forth by various governing agencies.
Author: Nicholas Joseph Jr. RT (R)(CT) BS, MS., Taffi Rose, BS, RT (R),
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In today’s world of rapidly changing medical technology, it is important to maintain quality diagnostic information for medical professionals. However, it is equally important to minimize patient and personnel radiation exposure risks. Helical (spiral) computed tomography (CT), was one of the most significant developments in the field of radiography. This article will review the evolution of CT and the corresponding quality assurance program requirements; including quality control tests, and the test tools used to follow the guidelines set forth by various governing agencies. These guidelines must be maintained by medical facilities across the country in order to continually provide patients the quality medical information they expect, and minimum radiation dose they deserve. CT has made rapid developments within the past 10 years, and continues to be an integral part of the field of radiologic special imaging. Quality assurance testing assures periodic performance testing of the CT scanner and comparison to standards.
History of CT and Evolution of Spiral Scanners
The term tomography stems from the Greek word "tomos" meaning "section".1 Scientists and mathematicians have described, "body section radiography" in many different ways since the 1920's. It wasn't until the 1960's after much research, that the world's first CT scanner emerged.1,2 The inventor was Godfrey Newbold Hounsfield, born in England 1919.1 He and Alan Cormack, a medical physicist, together developed and placed the first brain scanner into operation in 1971 for a company called EMI Ltd. 1,2 In 1979, they were awarded the Nobel Prize in medicine and physiology.1 Initially data acquisition in CT scanning was very slow. The first experimental brain scan in 1967 took 9 days.1 By 1971 they had reduced the scan time to 20 minutes.1 In 1989, the helical (spiral) concept was considered one of the most significant developments in CT.1,2 This development meant continuous rotation of the x-ray tube without reversal between images.2 The new continuous motion was given the name "slip-ring" technology, and it reduced brain scan times to as low as 0.8 seconds.2 As technology continues to develop with multi-slice systems, times are getting even shorter (0.4 sec.).2
Data from spiral or helical scanners is often referred to as "volumes" of tissue rather than individual cross-sectional slices.2 These images are "overlapped" and do not have a gap in between them. This allows complete coverage with no "missed" areas of tissue. This modern equipment has allowed more rapid image production for trauma and pediatric patients; however, they can sometimes deliver a higher dose of ionizing radiation.2 Since Hounsfield's first operational scanner in 1971 to today's modern "multi-slice" spiral scanning devices, a lot of progress has been made to improve image quality and most importantly, to decrease patient ionizing radiation dosage. With this evolution of technology has come the need for more comprehensive quality assurance programs, new phantoms specific to spiral CT, and higher standard safety guidelines. Not only are specific tests conducted to maintain equipment operation at an acceptable level, but these programs are also designed to recognize and create a corrective action for quality assurance issues.3
The development of faster scan times has also created some drawbacks such as: the need for x-ray tubes with higher heat ratings; more powerful generators to sustain added heat volumes; and, increased image noise consistent with the rapid reconstruction of images.2 This "image noise" can cause an artifact known as the venetian blind artifact.4 This occurs with multi-slice scanners and appears as bright and dark bands superimposed on three dimensional images.4 Another drawback from helical scanners is the notable difference in low contrast resolution. This problem has created the need for additional test tools and more suitable phantoms for spiral and multi-slice scanners.5 Even after a close review; the benefits of spiral scanners definitely still outweigh the drawbacks.
Governing Quality Assurance Agencies for the CT Helical (Spiral) Scanner
There are many agencies that guide our medical physicists or QA program coordinators in creating the best quality control program for their facilities both at the national and local level. The AAPM (Association of Physicists in Medicine), The ICRU (International Commission on Radiation Units and Measurements), and the ACR (American College of Radiology), just to name a few, are involved in setting x-ray equipment standards. The use of CT for medical purposes in the U.S. is primarily controlled at the state or local government level.6 However, the guidelines set forth in the equipment manufacturer's manuals for quality assurance are the primary resource for compliance concerns in diagnostic x-ray systems. The Food and Drug Administration (FDA), as well as the applicable state health department, should first approve all equipment prior to being put into operation for patient use.6 Each equipment manufacturer should include a list of standard tests to be performed and the proper phantoms for their equipment.
Under the Radiation Control for Health and Safety Act, all CT equipment is subject to an equipment performance standard.6 This consists of: minimum radiation safety requirements and manufacturer requirements regarding safety standards and performance.6 Manufacturers must ensure that their equipment meets the standards set forth by this act.6 Any additional tests are chosen at the facility's discretion and in accordance with national and their state regulatory agencies.6
There are typically three categories for regulation and guidance: laws- which are passed through legislation and provide an authority on the subject matter; regulations - which provide specific requirements that are authorized or required by law; and, guidance -which are agency decisions or policies that describe how to comply with a regulation.7 These laws, regulations, and guidelines ultimately allow the nationï¿½s medical professionals to maintain high quality images with the lowest possible patient dose.
Regulations and laws governing CT scanners insure both quality assurance (QA) and quality control (QC) programs are implemented. Quality assurance is the component that measures CT scanner performance to assure operation is at an acceptable level. Quality control takes action to correct inadequacies before they are problematic. These measures assure high imaging performance standards, and reduce the risk of patient harm due to change in equipment performance characteristics.
There are three basic tenets of an acceptable quality control program. QA must be performed on a regular basis, there must be prompt interpretation of test results, and the third tenet is accurate bookkeeping. Some test are required daily, others monthly; annually or at equipment acceptance. The CT technologist usually performs daily tests, which means for these tests they must recognize when results are out of range. Test results must be recorded in a logbook, data log, or computerized record for as long as the scanner is in use. Daily, weekly and monthly results can be compared to acceptance data. This can be very useful especially if there appears to be a malfunction of the equipment. Often the CT technologist is too busy to perform daily tests; however, you should always find time to perform daily tests since a properly performing CT scanner eliminates the equipment as the cause of an improper interpretation of CT images.
Phantoms and Required Test Tools for Spiral Scanners
The ICRU defines tissue substitutes and phantoms as the following: tissue substitute - any material that simulates body tissue interaction with ionizing radiation8; and, phantom - any structure that contains tissue substitutes, at least one, which can simulate radiation interactions in the human body.8 There are two categories of phantoms: calibration phantoms and imaging phantoms.8 Calibration phantoms are for testing detectors and correcting quantitative information obtained from digital images.8 Imaging phantoms assess image quality and are usually further classified as head, body, standard, or reference phantoms.8
This photograph of a CT phantom used to perform quality assurance testing. Phantoms are made of Plexiglas, which has a density of 120 Hounsfield units. The black holder is also used to perform fine alignments of the phantom in the gantry and to the localizing laser lights. There are several types of phantoms, for example, one type may be used for head dose calculations, and another for body dose calculations. Different manufacturers will recommend testing their equipment with specific phantoms. All phantoms must meet performance standards set by the Food and Drug Administration (FDA).
Quality assurance for the spiral CT scanner consists of these basic required elements of testing: contrast scale and mean (standard deviation), CT number for water, high-contrast resolution, low contrast resolution, laser light alignment and accuracy, image noise, uniformity and artifacts; slice thickness and localization, and patient dose.9,10 There is a wide array of tests that may be performed as well as test tools that can be used. The facility's quality assurance manager or medical physicist generally decides this. The selection of these tests should be based upon the type of equipment and the frequency in which the equipment is to be utilized. This will usually limit some of the more complex tests to an annual survey.
Here is a brief description of the basic QA tests done for a CT helical scanner. Again, these may vary from facility to facility and from state to state. There are daily, monthly, semi-annual, and annual test categories, as well as acceptance testing.2,9,10 Tests are routinely performed by the medical physicist or staff QA manager upon receiving new equipment. Special tests may also be required if equipment is relocated, or the x-ray tube is replaced.2
A good QA program will provide regular testing, prompt interpretation of test data, and faithful record keeping.3 A current logbook or computer file should be readily available for viewing by any regulatory agency in the event of an unscheduled inspection. In addition to the following tests, daily visual checks should be done on table/gantry movement, cables, cords, operating console, controls, and print system if installed.2 These visual tests should be documented as well, for future comparison.
Acceptance testing includes the following baseline type tests:11
Quality assurance and performance phantoms assess system performance and allows for the establishment of an ongoing quality assurance program. Phantoms are designed so that maximum performance information is gathered with minimum effort. These tests are simple to perform and provide unquestionable accuracy for quality control. (Please see your equipment manufacturer specifications manual or refer to the appropriate requirements and guidelines for your state to learn what QA tests will best fit your program). This module will explore a phantom type used to perform quality assurance testing on General Electric HiSpeed CT/I scanners. While the specific methods of testing described in this module may differ slightly from one typer of CT scanner to another, these test are required on all scanners and their results are uniformly regulated.
This CT phantom (left photograph) is used to make various images used to gather quality assurance measurements. Image "A" is used for high contrast resolution, contrast scale, slice thickness, and laser accuracy. Image "B" is used to measure low contrast detectability. Image "C" is used to measure noise and uniformity of the scanner. We will look at each of these and other images and their respective tests in detail in this module.
Basic Quality Assurance Tests
Test 1 - CT number for water (average & standard deviation)1,3
Computed tomography involves complex processing of digital data using mathematical principles called reconstruction algorithms. Scan data is based on penetration and attenuation measurements of photons as they traverse matter. This raw data must then be converted to digital data and displayed as a CT image. Each pixel in the image is assigned a number collectively referred to as a CT number. CT numbers are referred to as Hounsfield Unit (HU) named after the inventor of computed tomography. The Hounsfield scale (H scale) is a calculation from the linear attenuation coefficients of tissues that make up an image slice. CT numbers are relative to the attenuation of water, which is assigned a value of 0 HU. Using water as the reference the maximum brightness of a pixel is -1000 HU and will appear white on the CT image. The opposite end of the H scale is maximum darkness, which is +1000 HU and the image will be black. Between these extremes are various shades of gray that make a diagnostic CT image. Therefore, when we check the CT number for water we are effectively checking the reconstruction algorithm that computes CT numbers across the image.
This chart shows the various CT numbers (Hounsfield units) calculated for various tissues and substances based on the density of water. Notice that at the extremes is bone (+1000 HU) and air (-1000 HU). Water has a CT number of zero, which is used to test for the function of the algorithm that calculates CT numbers.
To perform the CT number test a water phantom test tool is used. The phantom is a water-filled cylinder with a 20cm diameter for head technique calibration, or 32 cm diameter for body simulated calibration. Scan with usual technique for the body part represented by the phantom type. A 20 cm diameter phantom and head technique is most frequently used. Select a ROI (region of interest) of about 2-3 cm or containing about 200-300 pixels and measure the average CT number. Air measures -1000 HU; water should be close to 0 (zero). Water should not exceed +/-3 HU at the center of the image, and no more than +/- 5 HU from center to periphery. If the CT number in the center of the image is not within 3 HU the scanner fails this test. Recalibrate and retest if not within limits. Keep in mind that although the CT number for water is measured daily there are two media used in calibration, these are water and air. At least once a month a ROI outside the phantom image in a region representing air is taken. This reading should measure -1000 HU, +/- 5 HU. This reading is to check contrast since water is zero and air measures –1000 HU. When the algorithm that calculates CT numbers is accurate, water is within +/- 3 HU of zero, and air is within +/- 5 HU of –1000 HU.
The CT number for water- (average & standard deviation) test is done to ensure equipment manufacturer specifications for CT number, field uniformity, and noise. The test for CT number of water is done daily. Possible causes for the CT number of water to be out of range is miscalibration of the algorithm generating CT numbers. This is a type of problem that needs immediate attention of the biomedical engineer or radiation safety officer. When the CT number for water and air fail the recommended range it must be immediately corrected to insure accuracy of the displayed CT image. If the CT number fluxuates significantly, but remains within the acceptable range, this too should be brought to the attention of the radiation safety officer.
The water phantom test images shown here demonstrate a clear uniform field free of artifacts. The image on the right shows a ROI placed in the center of the image to measure the CT number for water. In this example the ROI measured 0.07 HU with a standard deviation of 3.33 HU. This measurement is within acceptable performance guidelines of the manufacturer. This is a simple test that confirms the proper functioning of algorithms that calculate CT numbers and provides a quick check of the field for artifacts.
Test 2 - Noise and Field Uniformity1
When we think of image noise in traditional radiographic imaging using screen-film imaging we are referring to the overall graininess of the image. CT is a type of digital imaging processing in which image noise can be caused by a variety of factors. Noise in CT is mainly related to the following: (1) number of detected photons; (2) matrix size (pixel size); (3) slice thickness; (4) algorithm; (5) electronic noise (detector electronics); (6) scattered radiation; and (7) object size.1 Noise limits low contrast resolution and may hide anatomy similar to surrounding tissue. Most pathology imaged in CT is seen in soft tissues such as the lungs, kidney, liver, and brain. To test for image noise a simple cylinder or container of about 20 cm. in diameter is used. The phantom used to calculate the CT number for water and air is used for the noise uniformity test. The phantom is scanned at different slice thicknesses, and gradual increases in mAs. Measure an ROI of about 200-300 pixels and find the standard deviation of the CT number at the center of each image. The image field is sampled along the periphery as well as in the center of the image. There should be uniformity in the CT numbers throughout the image. The noise level can be stated as a percentage of image contrast in CT numbers. The stand deviation for noise should be +/- 3. Since CT numbers range from +/-1000 HU, noise is less than 0.3%. The maximum standard deviation between the center ROI and any peripheral ROI is less than +/- 5 HU.
The result should show the noise in the image being directly proportional to the standard deviation of the CT number of water. The number should decrease as the slice width and mAs are increased. Increased noise can be a result of poor beam/detector alignment, reduced detector sensitivity, or reduced output from the tube.1,2 Be sure that the noise levels produced by the equipment do not increase with age. A higher noise level will result in a lower dose to the patient and lower noise level results in a higher dose. There must be a balance between the two to maintain quality images and low doses. Out of range CT numbers for standard deviation could be caused by decreased dose or increased electronic noise. Standard deviation describes the difference between the lowest ROI value and the maximum ROI value. Testing is performed daily with the CT number for water and also upon acceptance of new equipment.
These two CT images taken of a water phantom show an ROI in the center and periphery of the field to measure field uniformity. The same locations for measuring should be used each day. The image on the right shows a grid that has been inserted on the image to help with consistent daily placement of the ROI's. The image on the left shows the ROI's without the grid. The center ROI measured 0.25 HU and the four ROI's in the periphery measured within the acceptable < +/- 5 HU of center measurement. Comparing peripheral field HU's is necessary since attenuation of x-rays by different body tissues when displayed in different parts of the image field must be accurate.
Region of interest selected from the scanners monitor is also capable of measuring the standard deviation within the ROI. Keep in mind that the standard deviation is dependent on several factors including kVp, mA, scan time, slice thickness, phantom size, and ROI position. This is why the technical factors used to measure HU for phantom tests must be standardized so that they are constant each day. Likewise, when measuring field uniformity it is important not to place peripheral ROI's too close to the edges of the image since at this location standard deviation is lower than towards the center. Ideally, the standard deviation should be small; however, more importantly it should be compared to the latest service record standard. It should not vary by much from day to day and an increasing pattern should be brought to the attention of the service department. An increase in the standard deviation indicates the image is becoming "noisier." Possible causes include decrease in tube output, increased detector, amplifier(s), analog-to-digital converter, or other issues.
This CT image shows two ROI’s taken during the CT number for water phantom test to measure image noise. The standard deviation within each ROI was measured by the computer’s quality assurance software. The CT number for water is shown in the three measurements along with their standard deviations. For ROI 3, 4, and 5 the standard deviations are 02.71, 02.88, and 02.85 respectively. A low SD indicates low image noise, which is desirable for diagnostic image interpretation. In this CT image, the mean CT number is given along with the standard deviations for selected peripheral ROI’s. A center and peripheral ROI is demonstrated.
Test 3 - High-contrast (spatial) resolution1,2,3,5,11,12
Spatial resolution is important in detecting the edges of structures, margins of tumors, small foreign bodies, and small bony structures. This test measures spatial resolution by measuring the high-resolution pattern in a phantom image. This test measures how the scanner distinguishes between two high contrast objects placed close together, and how small an object that can be visualized. Resolution phantoms come in a variety of test patterns. Generally, the test tool will have a bar pattern or series of holes cut in the Plexiglas within the phantom. Generally, each bar patter contains a set of 5 holes or bars and spaces of constant equal dimension. Each block decreases in size from one pattern to another. Measurements are taken of the depths of different drilled holes usually into an acrylic or hard resin-like substance. The holes may be filled with air giving 100% contrast or water giving 12 to 20% contrast. All holes or bars should be seen on the scan image; however, we are only interested in the smallest row in which all five bars and spaces can be clearly seen. The resolution block contains the following bar sizes 1.6mm, 1.3mm, 1.0mm, 0.6mm, and 0.5mm. The smallest the row clearly seen indicates better performance of the scanner and image quality. Expected result is that complete set of bars or holes and accompanying spaces will be in the range of 0.75 to 1.0 mm. All modern scanners should have a resolution of 0.5% contrast for 5mm. The minimum size of the holes visualized should not increase over the life of the equipment.
High-contrast resolution tests should be performed upon acceptance of equipment and monthly. Baseline is established at acceptance or referenced to manufacturer's specifications. Test failure is related to an enlarging x-ray tube focal spot, poor registration, detector failures, mechanical misalignments, mechanical wear and so forth. In any case the biomedical engineer should be notified if resolution degrades from baseline measurement.
The photograph on the left is of a phantom designed for multiple QA testing. The yellow arrow points to the bar pattern in the phantom that is used to measure scanner high-resolution. The resulting image is seen on the right. Notice the bar pattern is filled with air giving 100% contrast. The smallest row of bars is recorded and compared to baseline value in this test. Expected result is that a complete set of bars or holes in some rows in the range of 0.75 to 1.0 mm or 0.5% contrast for 5 mm for modern CT scanners.
This CT image is used to determine high contrast spatial resolution. Spatial resolution is a measure of how well two high contrast objects placed close together are distinguished. In this phantom image the two high resolution objects are the Plexiglas (120 HU) and water (HU 0) seen in the spaces between the bars. Each pattern consists of five bars and spaces called line pairs. The size of each pattern is 1.6mm being the largest and 0.5mm being the smallest. The smallest line pairs discernable are seen at 0.8mm.
To validate this test the 1.6mm bar pattern is measured using a box ROI (yellow arrow). The box ROI should be sized until it fits into the pattern. Measure the standard deviation of the pixels in this ROI to get a quantitative assessment of changes in system resolution. The standard deviation measurement should be 40 +/- 4. This should be compared to the baseline measurement at equipment acceptance for accuracy. Then, take a measurement of the other patterns, or alternately of the smallest pattern with discernable line pairs and record.
As we have discussed, spatial resolution is a measure of detail resolution. Sometimes it is practical to evaluate the spread of information within the CT system. To do this we look at what is known as the modulation transfer function (MTF). The MTF is the most common method of describing spatial resolution in CT, digital radiography, and film-screen radiography systems. The MTF analysis allows us to compare system performance on a day-to-day basis, or to compare a system's performance against another CT system. Modulation transfer function is expressed in line pairs per centimeter (lp/cm). When counting a line pair, one line and its adjacent space are called a line pair. To measure MTF directly a line pair phantom is imaged and the number of line pairs is counted. If 5 line pairs are counted, the spatial resolution is reported as 5 lp/cm. If 10, 15, or 20 line pairs are seen, the spatial resolution is reported as 10 lp/cm, 15 lp/cm or 20 lp/cm respectively. The number of line pairs seen in a given length is known as the spatial frequency.
Because CT scanners are not created equally, how well a given scanner displays an object is also a function of the size of that object and the spatial resolution of the scanner. An object's size in a given length is also known as its' spatial frequency. What this means is that how frequently an object fits into a given space is it spatial frequency. Generally speaking, the smaller an object is the higher spatial frequency and the more difficult it is to be displayed accurately. Likewise, a large object will have low spatial frequency and will be more accurately displayed. If an object is displayed accurately as it is, then the MFT is given a value of 1.0. The modulation transfer function scale is from zero to 1. A MTF value of 0 would mean the image is blank and contains no information about the object scanned. Scanned objects will have values between 0 and 1; however, the closer to 1 an object is the better the MTF of the scanner.
This picture demonstrates how object size is related to their spatial frequency. Small objects have low spatial frequency since more of them fit into a prescribed length. Large objects have high spatial frequency since they fit fewer times into a given length.
In practical terms, the information needed during quality assurance testing or when comparing CT scanners for purchase appear in graph form called MTF graphs. Modular transfer function is plotted along the y-axis and object spatial frequency along the x-axis. When comparing the function of a scanner over time, or when comparing the performance of different scanners, we look at what is called limiting resolution. Limiting resolution is the spatial frequency at MTF of 0.1 for any scanner. MTF of 0.1 is referenced because it is the lowest MTF that will result in a visible CT image. A scanner with a higher spatial frequency will be able to image small objects.
This graph of MTF and spatial frequency for three scanners is shown. At limiting resolution (0.1 MTF) scanner "A" have a spatial frequency of 11.5 lp/cm. Scanner "B" at a MTF of 0.1 gives a spatial frequency of 17.0, and scanner "C" the spatial frequency is 20.0. Interpretation of the graph implies that scanner "C" is better able to display small objects than scanners "B or C".
Test 4 - Low Contrast Resolution/Detectability1,2,3,5,11,12
Purpose of this test is to determine the ability of the scanner to discriminate low contrast objects. Keep in mind that most relevant detail in the human body is made of soft tissue, and is low contrast. Arguably, this is perhaps the most important quality control performance test. This test measures the scanners ability to detect objects that vary only slightly from its background. This is especially important when trying to detect low-density tumors that lie in soft tissues. The most common areas where this is important are the brain, kidney, and liver. Often intravenous or oral contrast media is used to increase density difference that can be detected by x-ray. In other words, contrast agents increase the sensitivity of low contrast resolution. The visibility of low contrast objects is constrained mainly by amplitude and frequency characteristics of the image noise11.
Subject contrast is a product of both high and low contrast within and surrounding a structure. Subject contrast in a CT image is in simple terms the difference in average CT numbers between two adjacent regions of the image. So we must be assured that low-contrast resolution is accurate and there is accuracy in the display of high-contrast image resolution. Low contrast sensitivity is where CT excels over conventional radiography. CT can resolve small differences in tissue densities because there must be at least a 10% density difference to be detected with conventional x-ray imaging. This is why we use a 15% increase or decrease in the kVp to change the scale of contrast in conventional radiography. CT on the other hand is able to resolve density differences as little as 0.1%. Notwithstanding, the size of a low contrast object, its inherent density (calcium vs. fat), image noise, and viewing window setting will in part determine its detectability.
To perform this test various types of low-contrast inserts are available for the CT phantom: therefore, scanning for this test is manufacturer specific. Our low contrast detectability test phantom image is defined by the smallest hole size visible for a given contrast level and dose. The phantom contains a doped polystyrene membrane suspended in water. The membrane is pierced with holes ranging from 10.0mm, 7.5mm, 5.0mm, 3.0mm and 1.0mm. The basic of this test is that the number of object visualized on the phantom image is determined, and the mean value of each visualized hole and surrounding material is recorded. The smallest holes that should be visualized is 5 mm in diameter or smaller for 5% contrast objects.
This low contrast detectability phantom image displays various sized holes used to determine low contrast (left). The various sizes are labeled on the right image; however, it is difficult to see the smallest holes. This test measures the scanners ability to detect an objects density when it is close to background density.
Low-resolution contrast is determined as the difference in HU of objects and background. High noise in the image will cause a decrease in low-contrast resolution. To get an accurate measure of low contrast we need to know the CT number for the polystyrene membrane. This is accomplished by taking the CT number for water over an area that does not include the membrane. A second measurement is taken over an area that includes the membrane superimposed on water (called water plus the membrane). The CT number taken for water is subtracted from the water plus membrane to get the CT number for the membrane. To perform this test, measure using a box ROI above and below the membrane in the water section (labeled A and B in the phantom image below). Take a box ROI in the polystyrene membrane above the holes (labeled B) and below the holes (labeled C). Subtract "A" from "B" and subtract "D" from "C". When the measurements are completed and recorded adjust the Window Width to 20 and the Window Level to the CT number recorded for water. This will allow for an accurate reading of the number of holes visible.
This CT phantom image shows boxed ROI's placed over the polystyrene membrane and over water when calculating low contrast detectability. The membrane shown by yellow arrow contains spaced holes of different densities. There are various types of low contrast testing phantom inserts that are equipment specific. This on is used for the GE Lightspeed scanners.
Low contrast test tools are still being perfected as there remain questions as to the most optimal means of testing. The low contrast sensitivity test shows ideal results with the new polyurethane resin material phantoms. The low-contrast sensitivity in the image plane was easily measured with no dependence on temperature or beam quality. Additional phantoms are also available for testing low resolution in three dimensions. Contrast measurements should be within equipment manufacturer specifications. Tests for contrast are done monthly and upon acceptance.
Test 5 - Slice thickness (sensitivity profile) 1,2,3,13,14
The purpose for the slice thickness test is to determine if collimators, which shape the x-ray beam, correctly open to the appropriate size set at the console. Traditionally, 45 degree tilted ramps (aluminum or plastic), a spiral, or step wedge are used. There is a hole drilled in them that allows the beam to pass through it projecting an image of the width and length of the hole needed to match the width of the x-ray beam exactly. For example: slice widths of 7mm or greater should match nominal slice width within 2mm or less. But for narrower slice widths, there is a larger margin for error, possibly even doubled. This is generally just a matter of adjusting the calibration. This traditional test was once fairly time-consuming however; today's spiral and multi-slice scanners, new phantoms have been developed that can be used as an insert with a CT performance phantom. It has virtually no set-up time and is much more accurate. The new phantoms also reduce partial volume averaging and do not compromise z-axis (patient coverage). They can even perform accurately when not parallel to the imaging plane.
After the phantom and test tool have been imaged, the stainless steel bearings within the phantom create graphs on the test monitor providing information as to whether or not the equipment is within limits. The pixel values obtained from each image are plotted and the data processed and normalized. This test is done to determine if the beam is actually creating an exact match with the specified slice programmed. So letï¿½s look at how this test is performed on the GE Lightspeed scanner.
For this test the phantom insert contains a block pattern of air filled holes designed to demonstrate slice thickness. Each visible hole or line in the phantom is at 1mm thickness that is aligned perpendicular to the scan plane. It is important when determining slice thickness that the display image is viewed at the recommended window level and width for counting visible lines. The width is always set at 250 HU, and for a slice thickness of 10mm the level is set at 10 HU. For 5.0mm slice thickness viewing is at 250/0 HU; 3.0mm slice thickness viewing is at 250/-50 HU; 1.5mm slice thickness is viewed at 250/-100 HU. Standard limits are +/- 1mm. This test is done semi-annually and upon acceptance.
The quality assurance phantom on the right is used to perform several tests including the slice thickness test. On the left is the resulting image used to determine slice thickness. The block pattern showing line thickness appears along the edges (yellow oval) of the pattern.
Because several factors affect viewing of the line pairs for width sensitivity profile, it is recommended on some scanners that the window/level setting be standardized. This chart shows the standard viewing windows for the GE Lightspeed scanners for each slice thickness being tested for. A slice thickness of 10mm is viewed at W/L 250/50, whereas for 3mm the recommended viewing is 250/-50 W/L. When this test is properly performed and the collimators working correctly, the number of visible lines should equal the chosen slice thickness.
This image taken on the CT phantom shows 10 one-millimeter lines on the corners of the image. The slice thickness is 10mm according to this image, which is what was set at the console for the slice thickness test. The window and level setting for this image was set at 250/50 according to the manufacturerï¿½s recommendation. The results of this test confirm that the collimators that shape the x-ray beam are open to the appropriate size. One can also vary the slice thickness to test for linearity of the system. Collimators that shape slice thickness should be accurate to +/- 1mm of the setting at 10mm.
Test 6 - Localization device accuracy 1,2,3
There are two centering lights used in computed tomography imaging, an internal and an external laser positioning markers. The importance of the centering lights is to place the object at isocenter, and to accurately represent slice location, especially during entry needle placement for procedures such as a biopsy or fine needle aspiration. Different scanners will have a different procedure and/or test tool to evaluate localization light accuracy. The traditional method uses a test device that has a "target." The beam is adjusted to be on the center of the "X". Generally, the test tool consists of a plastic phantom with 2 holes drilled at 45-degree angles crossing one another but not touching. Centering the phantom markings to where the two holes cross to the laser beam is how the scan is performed. The resulting image should show the targeted image holes and the actual image of the holes to be exactly aligned. If "L" (the distance from the center of the CT slice to the target location) is 3mm., then adjustments should be made. Because this is a clumsy method to identify accuracy of the localization light newer more accurate methods are used.
This photograph of a head phantom demonstrates the "X" pattern seen with the localization lights. Notice that there is an internal (green arrow) and an external (purple arrow) light source that can be used to place the part in the isocenter of the gantry. Coronal and sagittal (blue arrow) light field alignment must also be tested. The photograph on the right shows the phantom setup with the light localizer on to demonstrate testing procedure. Testing the accuracy of the vertical and axial plane light localizers is performed monthly and whenever new equipment is installed.
Localization light testing for the GE Lightspeed 16-slice scanner, and most multidetector row CT scanners is simple and can be tested for along with the slice thickness test. The insert used for the slice thickness test has two deeper center holes on the reference that are distinctly visible on the image. The position of the center holes corresponds precisely to the line scribed on the circumference of the phantom, which is aligned with the light field. When this alignment is accurate to the axial light and vertical fields, the resulting image should demonstrate a symmetrical hole around the center hole in the slice thickness pattern. When using a line insert the longer line in the pattern for slice thickness is the center alignment.
These CT phantom images demonstrate light localization images from one type of phantom insert apparatus. Notice the small holes in the image on the left that is aligned with the laser light on the phantom. The holes are bored to the vertical center of the phantom and aligned with the circumferential line of the phantom. The white broken line on the right image connecting the alignment holes is vertical alignment center. When a grid is placed on the image the alignment with these holes indicate correct light alignment. A grid placed on the phantom image allows us to evaluate sagittal and coronal alignments of the light to the phantom. Both internal and external lights are examined in this fashion for accuracy.
The CT image on the left with grid lines shows the alignment of the center holes to be accurate with the alignment of the laser lights in the axial and vertical planes. The image on the left demonstrates both the slice thickness and the light localization tests. The red arrows point to the center hole lines that represent alignment of the test tool with the circumferential lines of the phantom. Sagittal and coronal line accuracies must also be checked using a grid to assure they are accurate. Laser light localization is tested upon acceptance and monthly.
This CT phantom image on the left shows good alignment of the axial laser lights to the center holes of the phantom. Relative points on the image used to measure sagittal and vertical alignment of the light field are marked with red and yellow lines. These markings on the grid indicate relative points of light-phantom alignment are within sagittal and vertical plane limits. For QA purposes the measurement is taken from the visualized hole on one side to a selected point on a grid; the same relative measurement is performed on the opposite side too. Coronal alignment is verified by visualization of the holes in the phantom image.
Test 7 - Table/bed indexing accuracy 1,2,3
This test is done to ensure the distance the bed is moving between scans is accurate to what the equipment reads. This test is performed at same time as localization test. One way to do the test is with a piece of x-ray film taped to the table. The piece of film is generally covered or in a holder to prevent exposure. A series of 10 to 12 scans are done 10mm apart. A scan can be done at any thickness to check indexing. Using a ruler or tape measure, determine the distance between bands. The distances should be equal between selected increments. If there is more than a +/-2 mm. difference between any slice, equipment needs to be serviced. This test is done monthly and upon acceptance.
Patient dose 1,2,3,7,9,12,15,16
When we talk about image quality, especially contrast resolution, noise, spatial resolution, image artifacts, it is important to realize quality assurance assures high quality images and acceptable levels of patient radiation dose. Image quality is not always performed at maximum resolution because it requires an increase in patient exposure and patient dose. When it comes to patient dose and dosimetry it is important that the radiographer understands that dosimetry and performance standards must comply with Federal Regulation 21CFR 1020.33(C). The Food and Drug Administration (FDA) have established Specific CT dose indices in this report. The purpose of the report is to relate radiation dose and image quality of the CT scanner. Often CT technologists find themselves in discussions concerning patient dose for a given scan type. In order to properly relate dose information to the patient or physician it is important to understand how dose is calculated and the terminology of CT dose, which is slightly different from general radiography dose.
This example of a daily phantom test report gives the calculated CTDI and DLP. By monitoring this parameter daily the radiation safety officer is able to determine that the CT computer is accurately calculating dose. In order for these measurements to be valid it is important the radiation physicist performs the quality assurance testing for dose. It should be pointed out that QA dose calibrations and the are perform quality by the radiation physicists. However, technologists should understand how this test is performed and its relationship to the dose calculated by the scanner.
The whole purpose of dose calculation is to assess patient biological risk since dose is related to risk, and highly dependent on the tissue exposed. In the past, for example in general radiography, studies such as chest x-rays, fluoroscopy, and so forth were performed for many years without calculating patient dose. This is because the effective dose, which is the proper way to express dose, is very difficult to determine for a plain film x-ray. Now with computed imaging, especially CT scan, it is easier to calculate effective dose. What we mean by effective dose is that dose summed from the weighted dose and radiosensitivities of specific organs or tissues exposed. The doses are reported in documents published by the National Council on Radiation Protection (NCRP) and ICRP 60 (International Committee on radiation Protection, Publication 60). It is not possible to characterize the specific dose any one individual may receive.
Because it is difficult to determine specific dose of a patient the CT computer calculates what is called the CT Dose Index (CTDI). The CTDI is determined from calibrated doses at specific points in a phantom. A head phantom is used to calculate head dose, and a body phantom for body dose. The dose is taken in the phantom at center and peripheral locations. Therefore, the CTDI dose is the dose absorbed in the phantom material polymethyl methacrylate (PMMA) at a point volume of +/- 7 contiguous slices adjacent to the point. A calibrated CT scanner is able to calculate patient dose in the same manner; however, this is a limited estimate of dose because only 14 slices are used in the calculation (+/- 7 slices). The calculated dose for CTDI is defined by U.S. Federal Regulations as:
The formula for CTDI is specified in U.S. Federal Regulation 2121CFR 1020.33 (C). n=number of image slices per scan, T + slice width per image, D(z) = Z-axis dose profile (absorbed in PMMA). What is important for technologists is that CTDI is a calculated dose based on phantom calibrations to calculate patient dose. It is a limited dose calculation because it represents the dose from a single CT slice.
Another term called the CTDI100 provides a better relative index when thin slices are taken on the patient and when helical scanning is used. The CTDI100 is a single reported dose that consists of 2/3 CTDI100 peripheral doses and 1/3 CTDI100 central dose. The dose spans 100mm thickness whereas the CTDI only covers 14 slices (+/- 7 slices from adjacent selected point). The combined peripheral and central doses are reported as CTDIw, a relative dose for the entire scan. The CTDIw, which is specified in the international standard IEC 601-2-44 drafts is calculated from the CTDI and reported on the patient dose report. You may be asking why we need both a CTDI and CTDIw calculation. The answer is that data collected during helical scan length is greater than the reconstructed image region, and CT irradiation is from a series of narrow x-ray beams. This is because multiple helical scanning requires overlap exposure areas. This is more pronounced when the scan is short, so this overlap is calculated into the CTDIw.
The mathematical formulas for CTDI100 and CTDIw are shown above. What is important to know is that a body or head phantom is used to calibrate the scanner to calculate these doses. The scanner calculates the CTDIw for a patient scan. The technologist does not need to perform these calculations, but should understand their relationship to calculated patient dose.
The dose length product (DLP) is the final calculation performed by the scanner’s computer. This is a raw calculation that must be converted using a tissue-weighted factor to arrive at the effective dose. The effective dose is the best estimate of patient risk because we can compare this dose to other x-ray procedures. The dose length product is the volume CTDI (CTDIvol) multiplied by the scan length (slice thickness × number of slices) in centimeters. The volume CTDI is defined as CTDIw divided by the beam pitch factor, which is the most commonly cited index for modern MDCT equipment.15 DLP is expressed in mGyCm (milliGray Centimeters). DLP displayed at the end of a scan is computed for each group in the scan and for the sum of the groups. While DLP is a good measure for managing patient dose a minor downside is that it is nonspecific for what is actually scanned. It does not matter if the patient is a 10-pound infant or a 200-pound adult, or if the scan is of the brain vs. chest or abdomen. So DLP data is used retrospectively to estimate effective dose equivalence (ED). To more accurately calculate effective dose a weighted factor for organ dose is applied.
A classical scenario encountered in CT imaging is that the patient or patient’s physician inquires about dose prior to ordering a CT scan. For example, the patient is 5 months pregnant, presents with new onset chest pain and swollen right leg. The physician wishes to perform a pulmonary CT angiogram to rule out pulmonary embolus. However, the question is what is the estimated patient dose and risk for the scan? Average broad estimates of effective dose (E) for common CT procedures should be available at the scanner. These estimates may be derived from values of DLP for an examination using appropriately normalized (weighted tissue) coefficients:
The mathematical formulas for CTDI100 and CTDIw are shown above. What is important to know is that a body or head phantom is used to calibrate the scanner to calculate these doses. The scanner calculates the CTDIw for a patient scan. The technologist does not need to perform these calculations, but should understand their relationship to calculated patient dose.
For a CT procedure in the chest, one would then multiply the DLP for that procedure (in mGy-cm) by the factor 0.017 mSv mGy-1 cm-1 to get an effective dose in mSv.
Now let’s get back to our reading as calculated by the CT scanner for the daily phantom test. In the example below we see that the scanner calculates the DLP, but does not calculate the effective dose equivalency. ED is not calculated routinely, but is a retrospective calculation should you need to explain risk to the patient or their physician. As a CT technologist you should know how to read the DLP dose report produced at the completion of a scan. The dose from the scout images is not reported since there is currently no standard to follow. Generally the scout images are not made for diagnostic reading purposes, just for alignment and are low dose. Scouts are a small part of the total patient dose, which is considered to have no practical dose significance.
Note the volume CT dose index (CTDIvol) (arrow) and dose length product (DLP) (asterisk). Scan parameters: mAs 150, kVp 120, rotation time 0.6 sec. Effective dose in mSv = DLP × conversion factor: 193.14 mGy-cm × 0.02185 mSv/mGy-cm-1 (conversion factor) = 4.23 mSv. (Conversion factor ICRP Report 103).
The dose from the scout images is not reported since there is currently no standard to follow. This is also true for routine computed radiography exams too. Generally, the scout images are made with substantially low dose for image alignment. Because of the low exposure technique and low dose these images are not used for diagnostic reading. The DLP calculated for this head CT is based on accuracy of the calculation-performed daily as in the previous example.
Now that we have examined dose data and how the CT scanner calculates dose, we have to ask, “what do we do with effective dose data?” The answer is that this data permits us to compare patient EDs from CT with those of other x-ray based imaging examinations, as well as with benchmark values, including natural background radiation and regulatory dose limits. Since effective dose is measured in mSv it is possible to compare dose from different modalities. Table 1 below lists of representative diagnostic procedures and associated doses.
Average effective dose in millisieverts (mSv) as compiled by Fred A. Mettler, Jr., et al., "Effective Doses in Radiology and Diagnostic Nuclear Medicine: A Catalog," Radiology Vol. 248, No. 1, pp. 254-263, July 2008. This chart compares the average “effective dose” of selected imaging studies compared to the average effective dose from PA chest x-ray of 0.02 mSv. The assumed average "effective dose" from natural background radiation in the United States is 3 mSv per year.
A CT examination with an effective dose of 10 millisieverts (abbreviated 10 mSv; 1 mSv = 1 mGy in the case of x rays.) may be associated with an increase in the possibility of fatal cancer of approximately 1 chance in 2000.7. This increase in the possibility of a fatal cancer from radiation can be compared to the natural incidence of fatal cancer in the U.S. population, about 1 chance in 5. In other words, for any one person the risk of radiation-induced cancer is much smaller than the natural risk of cancer. Nevertheless, this small increase in radiation-associated cancer risk for an individual can become a public health concern if large numbers of the population undergo increased numbers of CT screening procedures of uncertain benefit18.
A CT examination with an effective dose of 10 millisieverts (abbreviated 10 mSv; 1 mSv = 1 mGy in the case of x rays.) may be associated with an increase in the possibility of fatal cancer of approximately 1 chance in 20007. This increase in the possibility of a fatal cancer from radiation can be compared to the natural incidence of fatal cancer in the U.S. population, about 1 chance in 5. In other words, for any one person the risk of radiation-induced cancer is much smaller than the natural risk of cancer. Nevertheless, this small increase in radiation-associated cancer risk for an individual can become a public health concern if large numbers of the population undergo increased numbers of CT screening procedures of uncertain benefit18.
When the radiation safety officer or radiation physicist performs QA dose measurements either a head and body technique and head or body phantom is used. A pencil ionization chamber attached to an electrometer is placed within drilled 1 cm holes at various locations of an acrylic phantom and measured. This is the easiest and most accurate way to measure dose. Dosage is measured in mR (millirad) and is specified as the CTDI or CT dose index. The ionization chamber is placed anterior, posterior, in the center, and around the perimeter of the phantom to determine various simulated patient doses. Holes that are not utilized during testing should be filled with an acrylic plug. The typical positions tested are anterior center and at surface max which is usually at the anterior 12:00 position. Results should not vary more than 10% from one assessment to the next. This test is usually done annually and upon acceptance.
This is a picture of a pencil ionization chamber that is inserted into a head or body phantom for patient dose calibration testing. A head phantom of 16 cm diameter is used for head dose calculation, and a 32 cm diameter body phantom is used for calibrating dose to the torso. Phantom material must be made of polymethyl methacrylate (PMMA) with holes greater than 14 cm thick.
The pencil ionization chamber is inserted into the center hole (A) and peripheral holes that are 1 cm from the surface (A through E) in the 12 o’clock, 3 o’clock, 6 o’clock, and 9 o’clock positions to determine various simulated patient doses. Positions B through E measure peripheral doses and center dose is calculated at position A. Holes that are not utilized during testing should be filled with an acrylic plug.
For a list of complete tests or for those not mentioned, please see the equipment manufacturersï¿½ specification manual for your specific model or unit. The Quality Control survey from each facility's medical physicist will also outline all required tests at the state and federal levels along with data outlining past test results for comparison.
As CT has evolved through the years, various phantoms and test tools have been used to test CT scanners and equipment performance. These tests reveal a great deal of information to medical physicists, patients, physicians, and quality assurance personnel. Conventional CT phantoms for single slice scanners are not adequate for newer helical multi-slice technology. The Food and Drug Administration has established standards for phantoms and specific tests for CT scanners. Manufacturer and model specific phantoms that meet these standards are widely available. New phantoms are constantly being developed to evaluate problem areas in maintaining quality images. Proper use of quality assurance phantoms for daily QA testing, recording daily test data, as well as understanding tests described in this module remain a duty of the CT technologist.
CT is considered a high-dose procedure and accounts for 35% of total patient dose.2 The dose from CT is approximately that of an average fluoroscopic study.2 Currently, there are no dose limits for patients receiving a CT scan. However, the need for the exam must outweigh the risks to the patient. Refer to maximum permissible dose (MPD), and the ALARA principle (as low as reasonably achievable) in the standards and goals of the (NCRP) National Council on Radiation Protection and Measurements.2 The general non-patient population is limited to 5 mSv/yr (500 mrem). Occupational dose limit is 50mSv/yr (5000 mrem).2,15 Each state and facility may have different limits set forth by their own governing agencies.
The measured dose from a spiral scanner can vary from manufacturer and model, to the age of the equipment. It is important to base all dose results on the specific equipment for your facility. You may, of course, choose to compare your results with other facilities and their quality assurance programs. A good quality assurance program containing accurate documentation, along with the newest testing devices, are necessary in maintaining the highest standard of quality and the minimum quantities of radiation exposure possible to our patients. Because of the excellent diagnostic information that CT provides, it will continue to be an essential tool for medical diagnosis.
For a list of complete tests or for those not mentioned, please see the equipment manufacturer specification manual for your specific model or unit. The Quality Control survey from each facility's medical physicist will also outline all required tests at the state and federal levels along with data outlining past test results for comparison.