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Citation:
900.12(e)(2)(i),(ii),(iii),(iv): (2) Weekly quality control tests. Facilities with screen-film systems shall perform an image quality evaluation test, using an FDA-approved phantom, at least weekly.
(i) The optical density of the film at the center of an image of a standard FDA-accepted phantom shall be at least 1.20 when exposed under a typical clinical condition.
(ii) The optical density of the film at the center of the phantom image shall not change by more than ±0.20 from the established operating level.
(iii) The phantom image shall achieve at least the minimum score established by the accreditation body and accepted by FDA in accordance with 900.3(d) or 900.4(a)(8).
(iv) The density difference between the background of the phantom and an added test object, used to assess image contrast, shall be measured and shall not vary by more than ±0.05 from the established operating level.
Approved Alternative Standards:
Discussion:
No, there is no specified upper limit. A trend has developed in recent years towards increasing the optical density of mammographic images, albeit at a somewhat higher dose, to take advantage of the resulting higher contrast. The amount of density increase will always be limited by the fact that the film contrast will peak (and so will the visibility of image details) before the density gets too high. Measurement of the optical density is specified to be in the center of the phantom because this location is most representative of the mean density and is easy to find consistently.
While a table of 52 values (over a year) contains all the necessary information, it does not provide an easy way to analyze the results and detect trends for the background optical density and contrast. For this reason, FDA recommends that the facility plot the phantom scores, the measured optical density, and contrast values.
It depends. For the majority of the QC tests, the type of screen-film combination used in the test is irrelevant to the test outcome. However, for the following QC tests, the regulations spell out specific requirements:
1. System Resolution - must be measured for each screen-film combination used at the facility with its corresponding unit(s).
2. Phantom Image and Dose – each of these must be conducted for each screen-film combination clinically used for the standard breast.
Note that the phantom image test applies to both the weekly QC and the annual test conducted by the medical physicist as part of the survey report. If only one combination is routinely used for the standard breast and the other combination is used for non-routine examinations of the standard breast, FDA recommends that the dose and phantom image QC tests also be conducted for the other combination, because the outcome of both tests is heavily influenced by the film-screen combination used.
Note that testing for the uniformity of screen speed must be conducted for all screens and cassettes respectively. Hence, by default, it includes all types of screens used, but this does not preclude performing this test with only one type of film. System artifacts must be performed for each cassette size.
A new baseline OD operating level may need to be established when switching to a new type of film or if the AEC density selector settings in the mammographic unit have been re-calibrated during servicing of the unit. Before changing operating levels, check that there isn’t an underlying problem that needs to be corrected. A new level may be established if the interpreting physician(s) has/have made an intentional decision to modify background optical densities of the clinical images. For example, many facilities are choosing to increase film density to take advantage of the film’s increased contrast at higher ODs. (See question 8)
A facility must follow the criteria established by its accreditation body in scoring weekly phantom images. Facilities accredited by the ACR may wish to review the latest ACR QC manual.
It is the intent of the regulation that the test object be placed on top of the phantom in a consistent, and if possible, permanent location in the image area. Consistency in positioning the added test object is necessary to achieve a meaningful operating level density difference between the background of the phantom and the added test object that must not vary by more than +/- 0.05 OD of the established operating level. However, the position of the test object is not specified in the regulations. If a facility believes it beneficial to place the "added" test object in a different position (e.g., adjacent to the phantom), it will have to assure consistent positioning of the "added" test object, as well as provide the additional x-ray attenuation needed to give a background OD equal to that of the phantom.
No. Since the regulations do not specify that the weekly test be performed on the same day every week (i.e., every 7 days), the facility can not be cited if it performs the weekly test every week but on different days in different weeks.
It is recommended that the phantom image evaluation test be performed using films from the box currently being used to produce clinical mammograms. If dedicated boxes of QC films are used for the phantom tests, the chance of detecting problems with the clinically used film is sacrificed.
FDA realizes that, due to differences in emulsion batches, a phantom image test with films from a new box may show variance in optical density and density difference greater than the allowed limits (when measured against the operating level established with films from the previous box). In such a case, facilities are advised to first check the whole imaging chain including the processor performance (facilities may wish to contact their medical physicist for help with this process). If no problems are detected, the facility may assume the change is due to different film emulsion batches. They may then adjust their typical clinical technique factors to meet the phantom optical density requirements.
A facility must follow the phantom image scoring methodology established by its accreditation body.
No. If the facility clinically uses the Full-Auto AEC mode for its standard breast patients, the weekly phantom images must be obtained using that mode. FDA requires the weekly phantom image be produced using the same clinical conditions that are used for its patients with the standard breast (compressed breast thickness of 4.2 cm, with breast tissue consisting of approximately 50% adipose (fat) tissue and 50% glandular tissue in composition). Prior to performing mammography on patients, the phantom image must achieve at least the minimum phantom score established by the accreditation body and must be within the action limits established for the three optical density requirements.
No. The only requirements on the weekly phantom image test are that the phantom image achieve at least the minimum phantom scores established by the accreditation body and must be within the action limits established for the three optical density requirements. FDA is aware that many facilities are monitoring kVp and/or mAs as part of their weekly phantom QC testing. This is not required. If a facility uses the Full-Auto mode and monitors kVp and/or mAs, it will probably observe that, over time, the Full-Auto mode leads to small variations in the kVp selected by the unit for the phantom exposures. Even small variations in kVp may lead to significant variations in the mAs values obtained. While small variations in kVp are to be expected when using the Full-Auto mode, large variations in kVp (greater than 1 kVp of the value usually obtained) may indicate a problem and should be further evaluated. Facilities using the Full-Auto mode that wish to monitor kVp and/or mAs may want to establish baseline mAs values corresponding to the specific kVp values usually encountered during phantom testing. In this way, they can account for the mAs variability that may be caused by small changes in kVp.
Mobile facilities should be aware of the following if they are monitoring mAs as part of their post-move-pre-exam testing. Performing the post-move-pre-exam test in the Full-Auto mode may be problematic (due to the variability of kVp and mAs as previously mentioned). In these cases, the facility may:
1. Use the AEC mode to perform the post-move-pre-exam test, even if they use the Full-Auto mode for their patients with the standard breast. Note: The weekly phantom QC test must be performed using the same clinical conditions that the facility uses for its patients with the standard breast.
OR
2. Use the Full-Auto mode and establish baseline mAs values corresponding to the specific kVp values usually encountered during phantom testing. If the mAs value is within 10% of the baseline value for the post exposure kVp value, the unit has passed that portion of the post-move-pre examination test.
If the OD at the center of the phantom image falls below the required minimum of 1.20 (and/or changes by more than +/- 0.20 from the established operating level), the facility should follow pathway A, B, or C; below, based on the situation at the facility:
If the film is of a different type (e.g., switch from Min-R 2000 to Min-R E) from the previous week’s passing test, the facility should establish new phantom QC operating levels.
If the film emulsion batch is unchanged from the previous week’s passing test:
Ensure that the phantom is exposed using typical clinical conditions and that the position of the phantom and, where appropriate, the position of the AEC detector have not changed from that used for prior images.
Reevaluate the daily processor performance and make sure the processor is properly optimized according to the film manufacturer’s specifications.
If the facility has been tracking mAs, check the function of the mammography unit by comparing the mammography unit’s current mAs output with values obtained for previous phantom images. If the mAs has changed by more than 15%, and the facility has been using the same kVp, the same mammography unit density setting, and the processor is operating within its action limits, then the medical physicist should be called to check the entire imaging chain, including the mammography unit. If the mAs has not changed by more than 15%, then proceed with step 4.
If the facility has not been tracking mAs, the facility should consult with its medical physicist for what to do next.
If no problems are found in steps 2 and 3, adjust the density control setting to obtain an optical density of at least 1.20 at the center of the phantom image (or obtain an optical density within +/- 0.20 of the established operating level).
Adjust the density control setting used clinically to be consistent with the changes made in step 4.
If the film is of the same type but of a different emulsion batch from the previous week’s passing test, the facility should follow the steps as described in B 1 through 5.
If the optical density again falls below 1.20 (and/or changes by more than +/- 0.20 from the established operating level) the next time the weekly phantom test is performed, the facility should follow the appropriate pathway (based on the film emulsion used) from the following three options:
If film of a different type (e.g., switch from Min-R 2000 to Min-R E) is used, the facility should establish new phantom QC operating levels.
If film of the same emulsion batch is used (assuming the same kVp and mammography unit density settings are used, and the processor is operating within its action limits), the facility should consult with its physicist and check the entire imaging chain before performing mammograms.
If film of the same type (but not of the same emulsion batch) is used, the facility should repeat steps B 1 through 5.
No, facilities are not required to perform phantom image evaluation for all image receptor sizes. Phantom image quality should not be significantly affected by receptor size. Because all currently approved phantoms simulate a standard breast, FDA recommends that the small image receptor size be used for phantom image evaluation.
No, the regulations do not require that the interpreting physician evaluate the weekly phantom QC test. However, the interpreting physicians may perform or evaluate the weekly phantom QC test, if the facility so chooses.
You are permitted to manually adjust the scale to the thickness of the standard breast before exposing the phantom using the BACE mode. Because you do not use the AEC mode to image your patients with the standard breast, you may not use a different AEC mode to perform the weekly phantom test.
For the purpose of making the density measurements, FDA interprets measurements taken at the "center" of the phantom to be equivalent to measurements taken on the center line as described later in this paragraph. As an alternative to repositioning the disk or replacement of the phantom cover, you may choose to keep the disk in place (as is currently recommended by ACR). You may then measure the background density at a different location along the center line of the phantom in the direction parallel to the chest wall, as long as this location does not obscure any of the phantom objects. In addition, you must measure the background density at the same location each time the test is conducted.
You should be aware that if you remove the disk, permanent defects may be produced in the cover plate. Such defects may produce "permanent artifacts" in the phantom image. As long as these "permanent artifacts" do not interfere with the scoring of the phantom (do not simulate masses, fibers or specks, and do not obscure the test objects), facilities may continue to use the phantom. The facility should not subtract these "permanent artifacts" from the phantom score. However, if these "permanent artifacts" interfere with the scoring of the phantom (simulate masses, fibers or specks, and/or obscure one or more of the test objects), the defective portion of the phantom must be replaced. The facility should also be aware that any artifacts on phantom images submitted for accreditation purposes may reduce the phantom image final score, which in some cases may result in a phantom image failure.
The answer depends on whether the x-ray units and processors are used interchangeably, whether the processors are matched (established operating levels for mid density and density difference for all processors are within 0.05 optical density), and whether each processor is operating within its own pre-established action limits.
If the processors are not matched and the facility is processing clinical films from its multiple x-ray units interchangeably through its processors, the facility must conduct the weekly phantom image test for each x-ray unit-processor combination. In this example, if a facility has 5 x-ray units and 2 processors, a total of 10 phantom images must be performed each week.
If the processors are matched and the facility is processing clinical films from its multiple x-ray units interchangeably through its processors, it is acceptable to produce a weekly phantom image from all x-ray units and process them through any processor, as long as each processor is tested with a phantom image at least once each week of use. (Note: in this scenario each processor must be operating within its own pre-established action limits). This will reduce the number of phantom images that must be performed. In this example, if a facility has 5 x-ray units and 2 processors, a total of 5 phantom images must be performed each week. Note: At least 1 phantom image must be processed through each processor.
If you are not using a cushion pad for the majority of your patients, you do not have to include the cushion pads when performing the phantom and dose QC tests. However, if you are using a cushion pad for the majority of your patients, you must include the cushion pads when performing the weekly phantom and annual phantom and dose QC tests in order to simulate as closely as possible your typical clinical conditions (21 CFR 900.12(e)(2)). If you routinely use the cushion pad on both the bucky and the compression paddle, you must use 2 layers of the cushion pad when performing the phantom and dose QC tests. When used clinically, the cushion pad is a single use device. Because of this, QC testing with the cushion pad in place is most appropriate when performing the phantom and dose tests. Therefore the facility does not have to include the cushion pad when performing other QC tests.
Related Topics:
Quality Control (QC) Tests – Other Than Annual