The Accreditation Phantom
Disc Placement: In the 1999 ACR Mammography Quality Control Manual, for the weekly screen-film phantom QC and density difference measurement, the ACR recommends placing the disc between and slightly below the first and second largest fibers. However, any location is acceptable as long as it is in a consistent location in the image area so it will not obscure details in the phantom and where it cannot cast a shadow on any portion of the AEC detector. With current equipment, significant variability in film optical density can result from placing the disc along the central anode‑cathode axis, where a varying fraction of the AEC detector area might be covered by the disc’s shadow, depending on the position of the detector. To ensure consistency, glue such as “SuperGlue” may be used to attach the disc permanently to the phantom as long as it does not produce artifacts.
Phantom Control Chart: On pages 171 and 173 of the mammography quality control manual, the mAs graph of the Phantom Control Chart indicates a permissible variance of ±15%. The y-axis of the graph indicates a variance of ±15 points, but no units are given. The intent was to have the mAs remain as close to ±15% as possible but also to make this data as easy as possible for the QC tech to plot. Assuming that most mAs values obtained with the phantom will be around 100, the ±15% will be equal to ±15 mAs (85 mAs to 115 mAs). If the phantom’s baseline mAs is 90, then ±15% becomes 76.5 mAs to 103.5 mAs. This is more difficult for the tech to plot on graph paper with 10 divisions per grid. Using ±15 mAs (75 mAs to 105 mAs) in this case will be easier for the tech to plot and still provide reasonable control limits. With very low or high baseline mAs values, the upper and lower levels should be modified to be as close to ±15% as possible but still make it easy for the staff to plot.
Finally, the ±15% is not a MQSA or ACR requirement but is an ACR recommendation to help facilities determine possible causes for density changes on the phantom image.
Radiologic Technologist’s Tests
Processor Quality Control Operating Levels: When establishing processor QC operating levels, as recommended in the 1999 ACR Mammography QC Manual, the MD step should be the step that has an average density closest to but not less than 1.20. The HD step should be the step that has an average density closest to 2.20, and the LD step should be the step that has an average density closest to but not less than 0.45. B+F is derived from the average of the densities from step 1 or any clear or the unexposed area of the 5 strips used when establishing the processor QC.
When doing the daily processor QC, MQSA requires that the B+F density shall be within +0.03 of the established operating level, the MD shall be within ±0.15 of the established operating level, and the DD shall be within ±0.15 of the established operating level.
The information provided in our final report, under Section IV, reflects the above information. As indicated in this particular section, the ranges shown are recommendations for processor QC operating levels. Therefore, because the recommended range for B+F is ≤ 0.25, with a control limit of +0.03, the B+F could be 0.28; although that is on the high end of the control limit and should be investigated.
If you are re-establishing your quality control operating levels, you may still perform mammography. We recommend that you continue to monitor and plot the data from your processor for those five days using your old chart to check for stability. At the end of the 5 days, determine the new operating levels and plot the values over the past five days on the new chart. Be sure to note in the remarks section why the operating levels were re-established.
Temperature: The 1999 manual does not have a chart for daily plotting of temperature. The temperature only needs to be measured (and recorded) when establishing operating levels, when problems are suspected (if points show a trend or go out of limits), or at other critical times, (e.g., during “crossover”). Some film and chemistry manufacturers have included a temperature chart in their control charts. The facility is free to track temperature on a daily basis if staff feels it provides a benefit to operations.
Sensitometer/Densitometer Maintenance and Repairs: The ACR manual recommends that you follow the manufacturer instructions for preventative maintenance and calibration of these devices. See page 158.
If your sensitometer is out for repairs, you may still use the processor to develop mammography films. The FDA has approved an “alternative standard” that may be used for a period up to 2 weeks in this situation. Under the alternative standard, processor performance is considered satisfactory if:
The optical density of the film at the center of an image of a standard FDA-accepted phantom is at least 1.20 when exposed under typical clinical conditions
The optical density of the film at the center of the phantom image changes no more than ±0.20 from the established operating level
The density difference between the background of the phantom and an added test object, used to assess image contrast, is measured and does not vary by more than ±0.05 from the established operating level
To evaluate base + fog, an additional measurement of density must be made, either of a shielded portion of the phantom image film or of an unexposed film. In accordance with 21 CFR 900.12(e)(1)(i), the base plus fog density must be within + 0.03 of the established operating level
This alternative test must be conducted “each day clinical films are processed, but before processing of clinical films.” All results must be recorded and charted. If processor performance fails to meet any part of the alternative test, the problem must be corrected before processing is resumed.
If your densitometer is out for repairs, you may not perform mammography. Without a densitometer, you cannot measure the optical densities or degree of blackening of film. The daily QC is required to be conducted prior to processing patient films. No alternative is currently available to verify processor performance without a densitometer. If your facility plans to use a loaner densitometer, reintroduce one into service after repair/calibration, or obtain a new densitometer, you should contact your medical physicist for assistance on how to proceed.
The Crossover Procedure: The purpose of the crossover procedure is to “recalibrate” your control chart to reflect the slightly different sensitivity of the film emulsion of the new QC film compared with the old (it’s analogous to weighing yourself on one bathroom scale that gives your true weight, and another one that reads high by 2 pounds… you would adjust the second scale down 2 pounds to read the correct value). When your box of QC film is almost empty (at least 5 sheets left), the crossover procedure should be performed using film from both the old and new boxes. The control chart is then adjusted to compensate for the differences. This procedure should be performed on the same day. See page 161 of the 1999 manual for detailed instructions.
A crossover needs to be performed whenever a new box of film is opened. Even though the new and old batch of film have the same emulsion number, film aging and storage conditions can affect the sensitometric characteristics of the film.
1999 ACR Mammography QC Manual Errors and Clarifications: The ACR is aware of the following in the 1999 manual:
The processor QC chart is missing a line (9 instead of 10). Corrected forms are available and can be printed from our website.
On page 211, the performance criteria for fixer retention of the second paragraph reads “if the stain indicates that there is more than 0.05 μg/m2 residual hypo in the film, the test should be repeated.” In the paragraph above, it states “0.05 g/m2.” It should say “0.05 g/m2” in both paragraphs.
On page 205 of the Radiologic Technologist’s Section, there is an example of a completed Mammography Report – Reject Analysis form. The numbers and percentages in the “Repeats” and “Rejects” columns are reversed (the number of repeats is larger than the number of rejects). The repeats should be the sum of items 1-11 (23) for a percentage of 1.9%; the rejects should be the sum of all the rows (1-14) for a percentage of 3.2%.
The 1999 ACR Mammography Quality Control Manual and the FDA regulations state that “the maximum compression force for the initial power drive shall be between 111 newtons (25 pounds) and 2000 newtons (45 pounds).” The regulations do not require that this amount of compression be used for each mammogram but rather that your mammography unit be capable of providing this maximum amount of compression force.
Medical Physicist’s Section
Collimation and “White Gap”: On page 236 of the manual, the ACR recommends that the field not be within ‑4% SID on the anterior side of the film. The purpose of this criterion is to minimize the amount of unattenuated light that reaches the reader’s eyes that can cause a decrease in perceived contrast. However, this is a “recommendation” and not a “requirement” for accreditation. As an FDA-approved accrediting body, we can only require for accreditation what is required under MQSA.
“Deviation Between X-ray Field and Edges of the Image Receptor” Error: In this section of the Collimation Assessment Test, the ± sign under ACR/MQSA Action limit should be a plus (+) sign only (It is our hope that readers will figure this out since the rest of the test says “If X-ray field exceeds image receptor…” It’s hard to “exceed” something with a “-” sign). This has been corrected in the forms available on the Web site.
Artifact Tests on Systems with Daylight Loaders: The 1999 ACR Mammography Quality Control Manual specifies that 2 exposed films must be processed orthogonally to each other in order to differentiate processor-caused artifacts from other causes. Generally, you cannot do this with a daylight processing system, as daylight processing systems will only accept cassettes to unload in one direction. Furthermore, attempting to “trick” the system by loading an 18 x 24 cm film crosswise in a 24 x 30 cm cassette may result in error messages or jam the system. Remember, the purpose of this maneuver is to help you localize the source of the artifact. Fortunately, many processor-caused artifacts have characteristic appearances (e.g., roller marks, pi lines, runback, etc.). If you cannot readily identify the artifact based on its appearance, try sequentially removing (or changing) various components of the x-ray system (such as the filter, compression paddle, bucky, or cassette) to see if the artifact goes away. If there are several mammography units at the facility, you can also check if the same artifacts appear on all the films. If they do, the processing system is most likely the cause. Finally, check with the daylight loader/processor manufacturer for any recommendations for this test that are specific to their unit.
Screen Uniformity and Standard Deviation of Control Cassettes: The 1999 ACR Mammography Quality Control Manual (page 308) states that if the standard deviation of control cassette densities is less than 0.05 and the density range exceeds 0.3, corrective action is needed. Both the 1994 and the 1999 manuals are correct. This test is designed to evaluate the uniformity of the screens and cassettes. However, the procedure is only valid if the processing and x-ray generator remain constant. If the standard deviation of the control cassette densities exceeds 0.05, it means that there is another problem not associated with the cassettes. This variability should be evaluated and reduced before the cassettes are evaluated (see page 247 under “Data Analysis and Interpretation”). Corrective action on the cassettes should only be taken if the standard deviation of the control cassette does not exceed 0.05 and the optical density exceeds 0.03. In other words, don’t blame the cassettes if it is an x-ray unit or processor problem.
Siemens Units and Density Settings: For the density control test, Siemens has recommended that its units be tested at the stations ‑12, ‑8, ‑4, 0, +4, +8, +12 in order to get appropriate increments in mAs and optical density. The ACR manual specifies that the test be performed at each integer density setting. The intent of the ACR recommendation was to ensure that there are fine enough steps in density control to make small adjustments to film optical density. The Siemens recommendation for testing is acceptable. You should also note that the density control test is not an MQSA requirement. It is ACR guidance and applies to most of the equipment that was in use at the time the QC manual was published. The ACR does not require that facilities meet our guidance/recommendations to pass accreditation. However, they must meet MQSA requirements/regulations.
Viewbox Luminance: According to the 1999 manual, the luminance of viewboxes for mammography must be 3000 cd/m2 (nit). This measurement or performance criterion is not required under the FDA’s Final Rules nor is it required for ACR accreditation. This test is recommended to ensure viewboxes have adequate luminance to view the high-density mammograms that are commonly recommended. See the Medical Physicist’s Mammography QC Test Summary and page 290 in the test section of the 1999 manual. Also note that, based on more recent data, the 3000 cd/m2 recommended in the 1999 manual has been reduced from the 3500 cd/m2 recommended in the 1994 manual.
Also note the error on page 289: Although the text in step 6 on page 287 states “take the following illuminance measurements with the viewbox lights of the viewbox being evaluated turned OFF,” figure 13 on page 289 specifies that illuminance “measurements should be made with the viewbox lights ON.” Figure 13 is in error. The caption should read, “measurements should be made with the viewbox lights OFF.”
Finally, note that on page 288, the 1999 manual states that “the illumination levels should be 50 lux, or preferably less.” Much more is known now about the importance of low ambient lighting than at the time the manual was written. Even though the authors could not specify a lower number at the time, they did indicate that they would prefer the illuminance to be less than 50 lux.
Revision History for this Article
Description of Revision(s)
Article created; FAQs incorporated; No criteria changes
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