Phantom Testing: PET (Revised 11-30-2020)
Modified on: Mon, 30 Nov, 2020 at 10:46 AM
Phantom images must be obtained and submitted for review using the phantom that has been approved by the ACR Committee on Nuclear Medicine Accreditation and may be purchased from Data Spectrum (please see the order form for the ACR Phantom).
The PET phantom uses the base of the Jaszczak Deluxe Flangeless ECT phantom with the spheres removed (as described below) and a PET faceplate. The ACR-approved phantom is a cylinder with an internal radius of 10.8 cm. The faceplate has fillable thin-walled cylinders (8, 12, 16, and 25 mm in diameter), two additional 25-mm cylinders, one for air and one for “cold” water, and a Teflon cylinder (the PET phantom faceplate design is being used by the ACR with the permission of Peter D. Esser, Ph.D., Columbia University; the intellectual property rights belong to Dr. Esser and uses other than through the ACR require his permission). The lower portion of the cylinder contains six sets of acrylic rods arranged in a pie-shaped pattern with the following diameters: 4.8, 6.4, 7.9, 9.5, 11.1, and 12.7 mm. In addition, for the SPECT/PET version of the phantom, the upper section contains six solid spheres with the following diameters: 9.5, 12.7, 15.9, 19.1, 25.4, and 31.8 mm. The spheres must be removed for PET studies. RODS MUST REMAIN.
PET data must be collected and processed according to the instructions provided in the testing package. The acquisition and processing must be essentially the same as those used for clinical whole-body scans. Despite the use of a specific protocol, it is understood that there may still be some differences even if the data are collected on the same type and model PET unit.
The phantom was chosen for evaluating PET tomographic systems because it is relatively easy to fill and set up for a PET study and can be used to measure tomographic uniformity, spatial resolution, and the detectability of “hot” lesions. The variety of resolution and “hot” components enables the reviewers to see relatively subtle differences in system performance.
Acquire the best possible images on your system with the same protocol as a routine clinical whole-body scan. All settings must be documented. Reconstruct the entire phantom as you would for clinical studies.
Site scanning data forms (available on the ACR's Nuclear Medicine and PET accreditation website) should be completed for each PET system that is undergoing accreditation. In addition, for each PET system, the applicant is required to submit a list with the frequency of all PET-related quality control (QC) procedures, including yearly, quarterly, monthly, weekly and daily tests (see the PET Quality Control Summary Form).
The ACR requires semiannual testing and strongly recommends quarterly testing of each PET system with an appropriate ACR-approved PET phantom such as described here, in addition to other tests recommended by the vendor.
Preparation of Phantom
Please read all instructions before preparing the phantom. Confirm that all calibrations and QC procedures are current.
One scan will be acquired with the ACR-approved PET phantom. The phantom scan starts 1 hour after Dose A is measured. During the 1-hour preparation time the other doses are measured, vials are filled, background added to the phantom, and phantom positioned in the gantry.
1 ACR-approved PET phantom
1 1000 ml bag or bottle of distilled water or saline solution
2 tuberculin syringes (for measuring Doses A & B)
3 large syringes (60 ml)
Large-bore needles (18 gauge)
Dose: FDG or F-18 are acceptable (Most vendors provide F-18 for calibrations/tests of scanners at a significantly lower price than FDG)
Clock or timer
The Table of Dilutions to produce the 2.5 concentration ratio (2.5:1 ratio may represent a lesion in the liver) is presented below. From the left column on the table, select the administered FDG whole-body dose for your site. The activities that are used for the initial Dose A and for the phantom itself, Dose B, are calculated from the F-18 activity normally used by the facility for whole body scans. If the activity is between the values, you must use the one with the higher level. For example, if your facility uses a 5 mCi dose for the whole body scan, you must select the 6 mCi row from the Table.
The corresponding phantom Doses A and B are along the same row as the Patient dose. The second 25 mm vial must be filled with water and the third left empty (“air”).
The spreadsheet indicates the activity that must be used for filling the phantom (this will also be referred to as the “background”) and the “hot” cylinders (“lesions”). Care must be taken to ensure that the solutions in the background and 1,000 ml bag (or bottle) are thoroughly mixed.
Be sure to adjust the "zero" and "background" settings on your dose calibrator. Follow the directions below to measure (± 10%) the doses and activate the PET phantom. Please record all information on the Phantom Dilution Worksheet.
Phantom Dose Chart
Dose A (mCi)
Dose B (mCi)
Directions for Activating Phantom and Vials
Protocol Summary for the Two Required Doses:
Dose A will be added to 1000 ml bag (or bottle) to diluted activity for the 4 test vials
Dose B will be added to the phantom as background activity
Measurement of Doses A and B: Measure F-18 activity (Doses A, B) using the tuberculin syringes and enter the dose and time in the appropriate boxes on the Phantom Dilution Worksheet. The F-18 solution may need to be diluted to facilitate measurement because the concentration is often as high as 75 mCi/cc. For future reference, note the following (these will be needed for the worksheet): Dose A Measurement Time; Dose in mCi; Phantom Scan Start Time (1 hour after Dose A measurement time)
Fill the phantom with water. Be sure that the Teflon cylinder on the faceplate is secure before fastening the faceplate to the phantom.
Activation of Test Vials on Phantom Cover: Add Dose A to the 1000 ml bag/bottle of distilled water/saline. Flush syringe several times. Mix well. Then with the first 60 ml syringe withdraw 60 ml — this is test Dose #1 (set aside, see Step 5). Next, from this bag/bottle, using the second 60 ml syringe withdraw 40 ml and fill the 4 appropriate empty cylinders (8,12, 16 and 25 mm) in the phantom faceplate (see diagram above). Fill the cylinders at this time with the ‘hot’ solution. There should be two 25 mm cylinders remaining. Leave the 25 mm cylinder next to the primary extended filling cap empty (see the diagram above) and fill the neighboring 25 mm cylinder with ‘cold’ water.
Activation of the Phantom: Inject Dose B into phantom and flush syringe several times (phantom background activity). Thoroughly mix (a bubble of air will help ensure a well-mixed solution) Dose B into the main chamber of the PET phantom. After mixing, using the third 60 ml syringe, withdraw 60 ml from the phantom — this is test Dose #2 (set aside, see Step 5).
Test Dose Measurement with Time: Measure the activity of test Dose #1 and Dose #2 in the dose calibrator and record. Then inject the activity from Dose #2 back into the phantom background. (The plastic holder must be removed from the dose calibrator before the individual syringes are assayed; the change in geometry will not affect measurements because the parameters of interest are ratios). Fill any remaining air-space in the phantom with water and mix again.
Scan at the specified time. Dispose of syringes appropriately.
Data Acquisition and Processing
If the phantom does not have a flange on the top, place the phantom on its side at the end of the imaging table. Carefully align the phantom parallel to the axis of the table (left to right). Use a bubble level to position the phantom in the horizontal plane. A folded 3 x 5 card positioned under the end of the phantom can be used to make it level. If a metal plate is present, move the phantom to a position where the plate does not attenuate the photons.
If the phantom has a flange at the top, it should be placed on the table with its flange hanging over the edge. Carefully align the phantom so it is parallel to the axis of the table. If the imaging table has a metal plate at the end, the phantom should be moved to a position where the plate does not attenuate the photons. Material must be placed under the bottom of the phantom, to make it level. To avoid excessive attenuation, the material should only be under the bottom (rod side) near the end of the phantom, not the entire length of the phantom. If the table is continuous, material must also be used to level the phantom.
Use your facility’s whole-body protocol (zoom =1) with the same settings that are used for routine clinical studies.
Reconstruct the entire phantom with the same protocol used for whole body scans including pre- and post-reconstruction filters. Generate 1 cm thick transaxial slices for analysis (if unit is only being accredited for brain or cardiac, use the appropriate protocol for reconstruction). All acquisition and reconstruction parameters should be recorded on the form.
All images must be corrected for attenuation with the same protocol applied to patient data. Record the parameters on the form.
Mean Bkgd: 0.85 – 1.15
25 mm cylinder: >1.8 – < 2.8
16/25 ratio: >.7
ROI Analysis of 1 cm Transaxial Slice from PET Phantom
Regions-of-interest selection: use the minimum, maximum and mean SUV statistics from these regions for the analysis section. See the Phantom Dilution Worksheet for SUV parameter instructions.
A. Select the “best” 1 cm slice showing “hot” cylinders (A, B, C & D). Example:
B. Draw a background region (ROI) in the center of the slice as shown here (diameter = 6 to 7 cm). Small variations in size or location of this ROI are not important. Next draw an ROI just inside the largest hot cylinder found in the slice (the ROI is shown in white over the 25 mm cylinder).
C. Place copies of the smaller ROI (as shown in B) over the other visible objects in the phantom slice as shown here. The ROIs should be inside the Teflon, water, and air regions. Note that all four of the hot cylinders may not be observed; only the visible cylinders require ROIs. Make a hard copy of the final ROIs.
Images for Submission
Transaxial Slices for Phantom Scan: Prepare images that show all of the transaxial slices (1 cm thick) of the phantom. This should include the rod portion of the phantom.
Use linear mapping of the display with the lower threshold set at zero and the upper threshold set to the maximum count (or whatever is satisfactory to produce a good gray scale). When there is linear mapping in an image, an ROI with twice the number of counts will have twice the intensity. Label images appropriately.
ROI Images from Phantom Scan: Prepare an image that shows the “best” 1 cm slice of the hot cylinders from the phantom. Scan with the final ROIs image as described above.
Evaluation of the Submitted Images
Uniformity and noise are evaluated qualitatively by inspection of reconstructed tomographic sections. Optimal density ranges should be comparable to those used for clinical images. Spatial resolution is judged by identifying the smallest “cold” rods in the ACR-approved phantom and lesion detectability is determined from the “hot” cylinders using the region-of-interest protocol above. The same protocol is used for the “cold” cylinders that demonstrate the effectiveness of the attenuation and scatter corrections.
Revision History for this Article
Description of Revision(s)
Article created; FAQs incorporated; No criteria changes
|4-14-2020||All||Updated all links within the article|
|9-23-2020||Added link to NM/PET Phantom Image Atlas|
|11-30-2020||Added link to PET Equipment Evaluation Summary Form and QC Review|
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