The intent of the MRI Accreditation Program is to use the information obtained from the review of both clinical and phantom images to assess overall image quality. Your facility will need to perform two specified phantom scans using ACR protocols as well as a second set of phantom scans using your site’s routine clinical head protocol as outlined in these instructions. It has come to our attention that some manufacturers of MRI systems have sent “sample” or “recommended” phantom site scanning protocols to their users. Please be aware that the requirement for MRI accreditation is that in the second set of scans, facilities use the same protocol for the phantom that the facility uses for head imaging. Failure to comply with this requirement could result in failure to achieve accreditation.
Phantom Set-Up and Alignment for Scanning
The MRI Accreditation Large Phantom should be scanned in the head coil with the cylindrical phantom aligned as a head would be in the coil. Transaxial slices should result in circular cross-sections of the phantom. The phantom should be positioned so that the word “Nose” is where the nose would be for a standard head study and the word “Chin” is where the chin would be located in a standard head study. The center of the phantom (the dark notch on the side of the phantom) should be placed in the center of the head coil and aligned with the positioning indicator light so that it will be in the isocenter of the scanner. Once grossly positioned, it is then necessary to “fine tune” the position of the phantom along all three axes. For this, you will need to use the non-metallic bubble level enclosed. Place the level along the top of the phantom running in and out of the scanner (along the z-axis) to ensure that the phantom is horizontal. Place a gauze pad under either end of the phantom to level the phantom horizontally. Next, place the level on top of the plastic bar at the chin surface, rotating the phantom so that the plastic bar is horizontal. With the phantom then clamped or wedged inside the head coil, check to see that the sagittal laser alignment light is parallel to the line running along the “nose” surface of the phantom (to see the laser light reflection, it may be necessary to place a piece of white paper on top of the phantom). After each position adjustment, recheck that the top of the phantom and the chin bar are still horizontal. After the phantom has been moved into the center of the magnet, verify its positioning by performing sagittal and, if desired, coronal plane localizer scans, until correct.
Note: Some systems require that a weight be entered in order to scan the phantom; the ACR recommends that your site enter a weight of 200 pounds. For some 3T scanners, using a weight of 200 pounds will cause the ACR T1 series scan time to double from 2:16 to over 4 minutes. The scan time doubles in order to meet head SAR model restrictions. You could reduce the entered weight to approximately 50 lbs so that the scan time is reduced to 2:16. The image quality should not be affected.
Once correctly aligned, the phantom should be kept in the same position during the entire series of scans.
You may use a nonmagnetic bubble level (not provided) for positioning the phantom.
It is not acceptable to cool the phantom before scanning, to improve SNR.
Scanning the Phantom
If your facility uses an eight-channel head coil, it is necessary to perform all phantom scans using the surface coil intensity correction option. A sagittal locator sequence should be acquired with the acquisition parameters listed on the Site Scanning Data Form. Use exactly these pulse sequence parameters, if possible, placing a check mark under each prescribed parameter to indicate that it has been used. If alternative parameters must be used because of machine or software limitations, enter the alternative scan parameters actually used below the ACR-prescribed scan parameters. Fill in alternative parameters only for those parameters that differ from the ACR-prescribed parameters. Deviations from the specified imaging parameters will often require a different overall study time. List the actual scan time required on the data form.
The sagittal locator scan should result in an image similar to Figure 1. If the pairs of 45° crossed wedges are not visible in the scan, the phantom must be repositioned and rescanned. A horizontal line used for slice prescription (see Figure 2) should be parallel to the low contrast disks located at the top of Figure 1 or Figure 2. If not, the phantom must be repositioned.
Figure 1: Sagittal localizer view of MRI Phantom with several inclusions of the phantom labeled
Figure 2: Sagittal locator image with slice locations for transaxial scans indicated
The next two scan acquisitions are transaxial pulse sequences acquired with identical spatial parameters: 5 mm slice thickness, 5 mm gap, 25 cm FOV, 256 x 256 matrix. At least 11 slices should be obtained, aligned using graphic prescription from the sagittal locator as shown in Figure 2 (this is the preferred method for slice positioning). The center of slice #1 should be aligned with the vertex of the crossed wedges (visible on the lower left in Figures 1 and 2) and through the center of the dark chemical shift and resolution insert (visible on the lower right). Slice #1 should result in a transaxial image that looks like Figure 3. The centers of slices #8–11 should align with the four low-contrast discs shown toward the top in Figures 1 and 2. Record this sagittal locator image, using a 12 on 1 format, showing the locations of the prescribed transaxial slices. If your scanner cannot obtain enough slices in a single scan, then perform multiple scans with the specified TR/TE and with the maximum number of slices allowed by the system. Repeat the scans, each with the specified scan time, until all 11 transaxial images have been obtained in the proper locations.
Figure 3: Slice #1 of the SE 500/20 transaxial scan
Figure 4: The sagittal localizer and all 11 slices of the SE 500/20 transaxial scan filmed in 12 on 1 format
If your scanner is not capable of obtaining 5 mm slices with 5 mm gaps, then use the closest slice thickness to 5 mm for the specified TR and TE. Set the slice gap so that slice thickness plus slice gap equals 10 mm and be careful that the images are positioned as specified in Figure 2. If necessary, perform multiple scans, each with the specified scan parameters, to get these 11 images in as close to the proper locations as possible. Record each of these 11 transaxial images on the same sheet of film as the sagittal localizer image. The full sheet should look like the 11 images in Figure 4 in terms of window settings and image positions on the film. Please note: Your axial slices must be positioned as shown in Figure 2 in order for your images to be acceptable for evaluation. If your MRI system is unable to prescribe a 5-mm slice gap then you should either perform an interleaved multislice acquisition or perform 11 single slice acquisitions. Please make sure that each slice is positioned as shown in Figure 4.
If you are unable to perform the axial slice positioning as indicated, then stop and contact the ACR before proceeding any further.
The conventional spin-echo (SE) 500/20 scan should be acquired with one acquisition per phase encoding step (one signal average acquisition or NEX) and the bandwidth used routinely for brain studies. This should take a total scan time of approximately two minutes. Enter the exact scan time required, along with the bandwidth (in kHz) on the Site Scanning Data Form. Remember to place check marks below the scan parameters that are used exactly as they appear on the table of Pulse Sequence Acquisition Parameters or enter the alternative parameters in each blank.
Acquire the SE 2000/20, 80 double-echo scan with one acquisition per phase encoding step at the same 11 slice locations as used for the previous scan. If a double echo at TEs of 20 and 80 ms cannot be obtained, then use the closest multiecho TEs to 20 and 80 ms (e.g., 40 and 80 ms). This scan should take approximately 8.5 minutes. Record the sagittal localizer and each of the 11 SE 2000/80 images only on one sheet of film. The PD weighted images do not need to be filmed. Enter the bandwidth for the scan in the space provided on the Site Scanning Data Form; enter the exact scan time required in the blank below scan time on the Data Form if it differs from the scan time specified on the form. Place check marks or enter the revised scan parameter in each block of the Data Form.
The ACR protocols do not specify any scan options, such as autoshim or image filtering, and you are not required to use any. If you wish, you may use the scan options you normally use for clinical head imaging, provided those options do not interfere with attaining the scan parameters and slice prescriptions specified for the ACR protocols. Record all scan options used in the space provided on the Data Form. For the ACR protocols, on scanners that have a range of image filter settings available, we recommend against strong filter settings because they are often detrimental to high-contrast resolution.
Next, scan the phantom using your site’s T1- and T2- weighted scan protocols. It is important to acquire images with 5 m slice thickness, if possible, or as close to 5 mm slice thickness as possible, and to acquire slices with center-to-center spacing of 10 mm for both T1- and T2- weighted images. Please try to adapt your normal scan protocols to obtain the 5 mm slice thickness and the specified 11 slice locations for both T1- and T2-weighted images. Enter the precise scan parameters used for T1- and T2-weighted scans (adapted to 5-mm slice thickness and the 11 prescribed slice locations) in the Site Scanning Data Form.
When adapting your site’s sequences, only change slice thickness and slice spacing. Do not change other scan parameters. Many sites normally use a reduced field of view in the right-left dimension on their axial head images. This leads to wrap-around (aliasing) artifact when scanning the ACR phantom. Do not change the field of view of your sequences to avoid this artifact. This artifact does not interfere with our assessment of the images, and you will not be penalized for it.
Evaluating Phantom Image Quality
After scanning the phantom, you and/or your physicist will use the Large Phantom Test Guidance document to evaluate your images using the same procedures that ACR physicist reviewers will use. If the images do not pass, the physicist will inform the supervising physician and service engineer, as corrective action may be warranted. If your site service engineer makes system adjustments and/or the supervising physician makes scan protocol changes, rescan the phantom. All images will be uploaded electronically for accreditation review. In order to ensure that your phantom data passes all of the measurements the phantom reviewer will be making, it is recommended that you check your phantom measurements after uploading but prior to submitting the online testing material.
Revision History for this Article
Description of Revision(s)
Article created; FAQs incorporated; No criteria changes