Phantom Testing: CT (Revised 11-9-2022)

Revision History

The Gammex 464 phantom can be scanned either by a technologist or medical physicist. If performed by a technologist, it is strongly recommended that a physicist check all images and data forms before submission. A medical physicist must perform the dosimetry scans. The dosimetry scans can be done on a different day; however, they must be within the time frame of all other images required for submission. The medical physicist must verify that the protocols on the phantom data form accurately reflect the clinical use of the unit and may need to assist the technologist in determining “average” protocols for each examination by viewing previously-scanned patients.

1. Fill Out the Phantom Data Form

This will entail listing information about the CT scanner and the acquisition parameters typically used for several types of clinical scans. You must use average manual techniques for each protocol when scanning the phantom. Do not use any type of automatic dose reduction technique other than iterative reconstruction when scanning the phantom. If iterative reconstruction is used clinically, then it can be used on the phantom scanning; tube current modulation should be turned off and iterative reconstruction should be noted as a “Dose Reduction Method” in the phantom scanning data form. If you use an automatic dose reduction technique for clinical scanning, you must look at images from patients, and determine the average technique (mid-brain for head, mid-liver for body), and enter that on the form; use this value for scanning the Gammex 464 phantom and for dosimetry measurements. You will report your use of an automatic dose reduction technique on the phantom data form, but not use it for scanning the phantom. 

These parameters will be used for imaging the phantom and should be consistent with the acquisition parameters used for the clinical exams. For each individual protocol, you will be able to choose to enter mA, mAs, or effective mAs as an acquisition parameter. The examination protocols required for the phantom portion of your accreditation submission depend on the examinations and patient types routinely performed on your unit and chosen in your accreditation application. The phantom data form available to you in your online testing package will reflect the phantom scans necessary for your submission. You will be required to select Single Energy/Single Tube, Single Energy/Two Tubes (Dual Source) or Dual Energy for each individual protocol. Scanning the phantom with any deviation from the protocols recorded on the phantom data form, with the exceptions noted below, may result in failure to achieve accreditation.

• Adult Abdomen: All units must scan the phantom with an average adult abdomen protocol. This is because some measurements are only analyzed using this protocol. Enter the average technique used on a mid-liver image on an average sized patient.

• Adult Head: If your facility chose the head/neck module on your application, you must enter the average parameters used for the cerebrum portion of your facility’s adult brain protocol (for accreditation purposes, adult examinations are patients over the age of 18).

• Pediatric Abdomen: If your facility chose the pediatric patient type and chest and/or abdomen modules, you must enter the average mid-liver protocol for a pediatric abdomen examination on a 40-50 lb. child.

• Pediatric Head: If your facility chose the pediatric patient type and the head/neck module, you must enter the average parameters for the cerebrum portion of your facility’s pediatric brain protocol. This protocol must be what you would use on a 1 year old child. 

2. Calibrate the Scanner

Prior to scanning the ACR CT accreditation phantom, perform tube warm-up and any necessary daily calibration scans (air scans, water scans) as recommended by the manufacturer. The ACR recommends that the site’s water phantom be scanned and tested for accuracy of the CT number of water, absence of artifacts, and uniformity across the field of view prior to proceeding. Stop and contact your field service engineer or medical physicist if any problems are found.

3. Perform Phantom and Scanner Alignment

Pull back the table padding and position the ACR CT accreditation phantom so that it is “HEAD” first into the gantry. (Be sure to choose a patient orientation of “supine head first” on the scanner.) Carefully position the phantom so that the CT scanner’s alignment lights are accurately positioned over the scribe line corresponding to the center of Module 1 (FOOT END of the phantom). Use the set of alignment lights, internal or external, that is used clinically. Align the phantom in the sagittal, coronal, and axial planes. Zero (or landmark) the table at this point (or be sure to note the table location, as all scans will be acquired in reference to this location). While maintaining careful alignment, secure the phantom so it will not move (Figure 1). 

Use a single axial scan at the landmark location (0 or S0). Use an image thickness ≤ 2 mm to verify adequate alignment Use a display field of view (reconstructed image diameter) as close to, but not smaller than, 21 cm.

Figure 1: Examine the image to determine whether all four BBs are visible in the image (Figure 2). Use WW = 1000 and WL = 0. All four BBs should be visible. 

The longer central wires should be symmetrically located in the center of the image. This indicates good positioning of the phantom.

Figure 2: All four BBs are visible, and the central wires are symmetrically located (± 1 wire) in the center of the ramps

4. Scan the Gammex 464 Phantom with your Facility's Protocols

Using your protocols listed on the phantom data form, scan the Gammex 464 phantom from slice positions 0 through 120. Use a display field of view (DFOV) as close to 21 cm, but not less than, as possible. You may adjust the scan field of view (SFOV) to a size appropriate to the size of the phantom. No other adjustments to your routine protocols should be necessary except as noted above. If you are routinely using other than 120-130 kV for the adult abdomen protocol and your CT numbers do not meet the criteria listed below, please submit an additional scan of module 1 using 120-130 kV. If you are submitted a DE protocol, submit the dual energy reconstruction used for interpretation.

5. Evaluate Your Phantom Images

Using the images obtained from scanning the phantom with your facility’s protocols listed on the phantom data form, evaluate your images for pass/fail criteria. Use the best images from each scan series for each phantom module to evaluate the images before submitting them to ACR for review. The method and criteria for the measurements are: CT Number Calibration; Low Contrast Criteria (Contrast to Noise Ratio, or CNR); Uniformity; and Artifacts. Each of these is described in detail below.

CT Number Calibration

View the best Module 1 image scanned with your facility’s adult abdomen protocol. Place a circular region of interest (ROI, approximately 200 mm²) within each cylinder (Figure 3) and record the mean CT number for each material for your records. It is important to center the ROIs within each cylinder. The water cylinder is subtly seen as a large gray ring. Be sure to place the water ROI as shown in Figure 3. 

Figure 3: Regions of interest (ROIs) for each material and for the water-equivalent background material.

The CT number calibration criteria are shown in Table 1. Values outside each of the listed criteria will result in a minor deficiency.

Measure only the water number for all of the other protocols you are submitting for the phantom portion of your accreditation submission. Values outside of the water HU criteria will result in a minor deficiency.

Low Contrast Criteria (Contrast to Noise Ratio - CNR)

View the best image located in Module 2 for all submitted protocols using a WW = 100 and a WL = 100. Note that there are four cylinders for each of following diameters: 2, 3, 4, 5, and 6 mm (Figure 4). Place a ROI (≈ 100 mm2) over the large (25-mm diameter) cylinder and between the large cylinder and the 6 mm cylinders. 

Figure 4: Module 2 low contrast resolution image at WW = 100 and WL = 100 with correct ROI placement 

Record the mean signal in the ROI inside the 25mm rod (A), the mean signal in the ROI outside the 25 mm rod (B), and the Standard Deviation (SD) from the ROI outside the 25 mm rod for your records. Use this formula to calculate the Contrast to Noise Ratio (CNR):

CNR = |A-B|/SD

Use the absolute value of the difference – that is, do not take into consideration whether the CNR is a positive or negative number. The CNR must be greater than 1.0 for the adult head, and adult abdomen protocols. The CNR must be greater than 0.4 for the pediatric abdomen protocol and greater than 0.7 for the pediatric head protocol.

Uniformity

View the Adult Abdomen protocol image in Module 3 (uniformity image) with a WW = 100 and a WL = 0. Place an ROI of approximately 400 mm² at the center of the image (A) and the four edge positions shown in Figure 5. For the edge ROIs, place the edge of the ROI approximately one ROI diameter away from the edge. Record the mean CT numbers for all five ROIs for your records. Additionally, record the standard deviation of the center ROI. All CT numbers for all five ROIs must be within ± 5 HU of the center ROI mean value. Differences between 5 and 7 HU values from the center ROI mean value will result in a minor deficiency. Differences of more than 7 HU from the center ROI mean value will result in a major deficiency.

Figure 5: Placement of uniformity center and edge ROIs

Artifacts

With all graphics turned off, view the same image carefully with the room lighting reduced. Examine the image for artifacts such as rings or streaks and record the presence and appearance of any artifacts. If artifacts are present, your medical physicist or service engineer may be needed to investigate and correct them. Artifacts may be evaluated as minor or major deficiencies depending on the severity and the discretion of the reviewer.

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