Questions Key Words/Related Topics
Citation:
900.12(e)(5)(i)(A)(B)(C): Annual quality control tests. Facilities with screen-film systems shall perform the following quality control tests at least annually:
(i) Automatic exposure control performance.
(A) The AEC shall be capable of maintaining film optical density within ±0.30 of the mean optical density when thickness of a homogeneous material is varied over a range of 2 to 6 cm and the kVp is varied appropriately for such thicknesses over the kVp range used clinically in the facility. If this requirement cannot be met, a technique chart shall be developed showing appropriate techniques (kVp and density control settings) for different breast thicknesses and compositions that must be used so that optical densities within ±0.30 of the average under phototimed conditions can be produced.
(B) After October 28, 2002, the AEC shall be capable of maintaining film optical density (OD) within ±0.15 of the mean optical density when thickness of a homogeneous material is varied over a range of 2 to 6 cm and the kVp is varied appropriately for such thicknesses over the kVp range used clinically in the facility.
(C) The optical density of the film in the center of the phantom image shall not be less than 1.20.
900.12(b)(10)(i),(ii)(A)(B),(iii): Automatic Exposure Control.
(i) Each screen-film system shall provide an AEC mode that is operable in all combinations of equipment configuration provided, e.g., grid, nongrid; magnification, nonmagnification; and various target-filter combinations.
(ii) The positioning or selection of the detector shall permit flexibility in the placement of the detector under the target tissue.
(A) The size and available positions of the detector shall be clearly indicated at the X-ray input surface of the breast compression paddle.
(B) The selected position of the detector shall be clearly indicated.
(iii) The system shall provide means for the operator to vary the selected optical density from the normal (zero) setting.
Discussion:
In its Diagnostic X-ray Performance Standard, FDA defines an automatic exposure control (AEC) as a device that automatically controls one or more technique factors in order to obtain a desired quantity of radiation at a pre-selected location. Such a device would automatically terminate the exposure when the selected quantity of radiation had been delivered. The AEC may control the selection of target material, focal spot, filter material, time, mA, mAs, kVp or a combination of any or all of these factors.
AEC mode refers to the type of AEC being used. Typically available AEC modes can range from fixed kVp and mA (where the kVp and mA are selected by the operator and the time is varied by the AEC), to fixed kVp (where the kVp is selected by the operator and the mAs is varied by the AEC), to various AEC modes in which all factors are varied by the AEC. Some of the more automated AEC modes are known by brand names such as BACE, OPDOSE, AUTO FILTER and AOP.
Mean Optical Density (MOD) means the average of the optical densities measured on the images produced with a given equipment configuration during the AEC performance test using 2, 4, and 6 centimeter thicknesses of a homogeneous material.
For AEC testing purposes, the only equipment configurations that need to be tested are the contact configuration, the magnification configuration (if used clinically), and the various image receptor sizes. Due to advances in AEC design, the example of a target-filter combination as an equipment configuration given in the regulations is no longer applicable. Therefore, we will not enforce testing a target-filter combination as a separate equipment configuration.
Yes. Facilities should continue to develop and use technique charts for their clinical exams, especially for AEC modes where kVp and other technique factors must be selected by the technologist. The only place where the words “technique chart” appear in the regulations is in the annual AEC performance test requirement. This regulation places a restriction on the use of a specific factor of the technique chart, the density control setting, when the medical physicist is performing the AEC test. The medical physicist may not adjust the density control setting while performing the AEC test in the 2 to 6 cm range. In other words, the medical physicist may not use the density control setting to compensate for inadequate performance of the AEC. When performing this test, the medical physicist may use a technique chart to adjust other factors such as kVp, filter, anode track or AEC mode to the extent such factors are used clinically. If the AEC performance test fails, the medical physicist may create a temporary technique chart that includes the appropriate density settings (in addition to the other technique factors). This temporary technique chart may then be used by the facility for up to 30 days, or until the problem has been corrected and the equipment passes the AEC performance test, whichever comes first.
When the AEC is functioning properly, the radiologic technologist shouldn’t need to adjust the density control setting while imaging patients who are in the 2 to 6 cm range. If the radiologic technologist needs to continually adjust the density control to achieve films of adequate density, the AEC may need adjustment and the medical physicist should be consulted.
The regulations do not restrict the use of technique charts by radiologic technologists. While a properly functioning AEC should reduce the need to use the density setting component of a technique chart, radiologic technologists may use these charts to change the density control settings whenever they believe it appropriate during the performance of clinical mammographic examinations. In addition, the regulations do not preclude the use of the manual mode and under that scenario, the use of technique charts is essential.
Due to the proliferation of mammography units with multiple AEC modes, testing of AEC performance has become more complex in recent years. When units had only one AEC detector, a single AEC mode, and a single target-filter combination, testing was relatively straightforward. That is no longer the case for most units. The following guidance is designed to help medical physicists adequately test a unit’s AEC performance without over-testing the unit.
During the annual physics survey, the physicist can limit testing of AEC performance to the contact configuration. To fulfill MQSA requirements, all AEC detectors (that can be individually selected by the operator) and all AEC modes used clinically over the 2 to 6 cm range in the contact configuration must be tested. While there are several ways to do the test, medical physicists who use the following guidance will have fulfilled this requirement. Note: Facilities that do not clinically use their AEC in the 2 to 6 cm range (only use manual techniques) must still test the AEC to ensure that at least one AEC mode for each available AEC detector meets the regulatory requirements.
In order to minimize sources of variability, the physicist should use a single cassette (or same cassette type), film from the same emulsion batch, and the same processing conditions throughout Steps 1 and 2 below (see question # 6 below).
Step 1: Determine the Mean Optical Density (MOD)
A) For an AEC detector used in the contact configuration, perform three exposures using 2, 4, and 6 cm thicknesses of a homogeneous material. The exposures are to be performed using an AEC mode clinically used at each of the thicknesses. For example, if a facility typically uses fixed kVp mode at 2 cm, fixed mA mode at 4 cm and OPDOSE mode at 6 cm, then the medical physicist should use these same modes at those thicknesses when conducting the AEC performance test. Note: Even if a facility clinically uses more than one AEC mode at a particular thickness, no more than one of the AEC modes should be tested at each thickness to establish the MOD. For example, if a facility clinically uses both the fixed kVp and the AOP CONTRAST modes at 2 cm, the medical physicist should use the more commonly used of these modes to determine the MOD.
B) Measure the optical density of the images obtained at 2, 4 and 6 cm (total of three images) and average them. This is your MOD.
Step 2: Determine if the AEC detector used in Step 1 is within the regulatory action limit of +/- 0.15 OD of the MOD
A) Check to see that all three of the optical densities obtained in Step 1B are within the action limit when compared to the MOD
B) If ALL three ODs are within the action limit AND no other AEC modes are clinically used in the 2 to 6 cm range, then this AEC detector has passed. The medical physicist then needs to repeat Steps 1 and 2 for each additional AEC detector clinically used in the 2 to 6 cm range (See question #5 for additional guidance on testing multiple AEC detectors).
C) If ALL three ODs are within the action limit AND the facility clinically uses an additional AEC mode(s) in the 2 to 6 cm range (other than the ones used to originally establish the MOD), the facility must test the additional AEC modes. The medical physicist needs to test EACH additional AEC mode(s) at any ONE clinically used thickness in the 2 to 6 cm range. If the OD(s) is within the action limit when compared to the MOD, then this AEC detector has passed. The medical physicist then needs to repeat Steps 1 and 2 for each additional AEC detector clinically used in the 2 to 6 cm range (See question #5 for additional guidance on testing multiple AEC detectors).
The medical physicist does not have to test the other clinically used equipment configurations during the annual physics survey, but will have to test these configurations whenever a mammography equipment evaluation involving the AEC is performed.
During a mammography equipment evaluation, the AEC must be operable in all equipment configurations (contact, magnification, and various image receptor sizes) used clinically by the facility. The term "operable," means the AEC must meet the performance requirements of 900.12(e)(5)(i) within the 2 to 6 cm range. Compliance with this requirement may be demonstrated by any of the following three methods:
1. Confirming AEC performance in the contact configuration. In the contact configuration, the AEC must maintain the film optical density (OD) over the 2 to 6 cm range within the action limit of +/- 0.15 OD of the MOD (See question 3 for additional guidance).
AND
Confirming AEC performance in all other clinically used configurations. This can be done by demonstrating that the AEC meets the density and reproducibility limits established by the manufacturer for those other configurations.
Note: Method #1 can be used only in those cases where the manufacturer has established AEC performance standards for the non-contact configurations provided.
2. Confirming AEC performance in the contact configuration. In the contact configuration, the AEC must maintain the film optical density over the 2 to 6 cm range within the action limit of +/- 0.15 OD of the MOD.
AND
Confirming AEC performance in all other clinically used configurations. This can be done by comparing the contact configuration MOD with measurements obtained using the 4 cm thick phantom in the other configurations used clinically at the facility. When results across different configurations are compared, the facility may use the action limit of +/- 0.30 OD.
3. Confirming AEC performance by demonstrating that the AEC maintains the MOD within +/- 0.15 OD in all configurations used clinically by the facility. The action limit applies only within each specific configuration tested and does not apply to data collected across the different configurations.
Note: During mammography equipment evaluations, facilities that measure AEC performance only in the contact configuration will be subject to citation. 21 C.F.R. 900.12(b)(10)
The general principle is that all AEC detectors must be tested. What is considered adequate testing will depend on the arrangement of the AEC detectors in the mammography unit.
1. Where a mammography unit has different AEC detectors in the different size cassette holders (buckys), each detector must be tested separately as described above in questions #3 or #4.
2. Where a mammography unit has more than one AEC detector in a single cassette holder (bucky), the physicist must test all the individually selectable AEC detectors and may test the detectors using either of the following methods:
3. Where a mammography unit has multiple AEC detectors that are not individually selectable by the operator, the AEC can be tested as if it was a single detector. An example of such a system is one with three fixed detectors in which the system automatically chooses which detector will be active during the exposure. Similarly, a large field detector that automatically selects its active area needs to be tested only as a single detector. However, a system with three fixed detectors, each of which can be selected individually by the operator, needs to have all three detectors tested as described in section #2 above. Please note that a detector that can be moved to different positions by the operator is still considered a single detector and needs to be tested at only one of those positions.
No. Because the AEC performance test involves many parts of the imaging chain, the medical physicist needs to make sure that the AEC is the part responsible for the failure. For example, problems with the processor, film emulsion or the use of different cassettes during the performance of this test may lead to a failure that is not the fault of the AEC. Facilities are reminded, however, that whatever the cause of the failure it needs to be corrected within the appropriate time frame.
Note: When conducting the AEC performance test, the physicist should try to minimize the variation introduced by components such as film, cassette and processor. Different film emulsion batches, cassettes or processors should not be used while conducting the AEC performance test. An exception would be where prior testing has shown that the different film emulsion batches, cassettes or processors would not introduce excessive variation in the test results. Introducing such variability into the AEC performance test may lead to an inappropriate “failure” of this test.
Different image receptor sizes are considered different configurations and have to be tested separately during the mammography equipment evaluation. With respect to AEC performance testing during the annual physics survey, the medical physicist can limit testing in the contact configuration to one image receptor size (usually the small size). However, FDA does recommend that in addition to this required testing, the medical physicist also measure the optical density (MOD) obtained using the large image receptor and a 4 cm thick homogeneous material and compare it to the mean optical density obtained for the small image receptor. When results across different size image receptors (different equipment configurations) are compared, the physicist should use the action limit of +/- 0.30 OD.
During the annual physics survey, the unit is not required to meet the AEC performance action limit outside the 2 to 6 cm range and the medical physicist is not required to test the AEC using thicknesses outside this range. However, we recommend that in addition to the required testing in the 2 to 6 cm range, the unit also be tested at all clinically used thicknesses outside this range and that the action limits specified in the regulations be applied to the extended test. If the unit cannot meet these action limits outside the 2 to 6 cm range, FDA recommends that a technique chart be developed showing appropriate technique factors (kVp, AEC mode, target/filter, and density control setting) for the different breast thicknesses and compositions so that optical densities (OD) within +/- 0.15 OD of the MOD under AEC testing conditions can be produced.
During the mammography equipment evaluation (as defined in 900.12(e)(10)), the medical physicist must evaluate the AEC in all clinically used configurations (See Question 4). Section 900.12(e)(10) requires that the AEC meet the requirements of 900.12(b) and (e). Under 900.12(b)(10), the AEC is required to be "operable" under "configurations provided." The term "operable," means the AEC must meet the performance requirements of 900.12(e)(5)(i) within the 2 to 6 cm range. FDA also recommends that in addition to the required testing in the 2 to 6 cm range, the unit also be tested in all configurations at all clinically used thicknesses outside this range and that the action limits specified in the regulations be applied to the extended test. If the unit cannot meet these action limits outside the 2 to 6 cm range, FDA recommends that a technique chart be developed showing appropriate technique factors (kVp, AEC mode, target/filter, and density control setting) for the different breast thicknesses and compositions so that optical densities (OD) within +/- 0.15 OD of the MOD under AEC testing conditions can be produced.
No. The intent of the regulation is to ensure that the AEC mode is operable in all equipment configurations used clinically by the facility. The term "operable," means the AEC must meet the performance requirements of 900.12(e)(5)(i) within the 2 to 6 cm range. One way is to have the AEC tested in all the configurations provided by the system. An alternative method is to ensure that the facility does not clinically use the AEC in those configurations not previously tested by the medical physicist. This can be accomplished by placing a label on the unit’s control panel listing the configurations that cannot be used because they were not tested. These non-operational configurations must also be identified in the facility’s quality assurance records.
Yes. It is not necessary that the AEC detector be mobile over the entire area of the breast.
Yes. The size and positions indicated at the input surface should be indicative of the size and positions of the detector in the plane of the detector. Compliance could be achieved by representations permanently marked on the paddles or by a projected image that approximates the size and position of the detector.
Yes. The relative position of the selector would be an adequate display of the detector position, and this display need be visible from only one location.
Yes.
The regulations do not specify the range of variability that must be provided; only that some variability is available.
The intent of this regulation is to help the radiologic technologist optimally position the AEC detector. Under some AEC detector designs it may be difficult to show all possible positions. Some detectors cover the entire area of the image receptor and once the exposure begins, they automatically select the region of maximum density as the active area. For these systems, indication of the entire potential active area, along with appropriate instructions (usually found in the user manual), would satisfy this requirement. Since the area is automatically selected, the display of the size of the detector is not required. Other designs may have an essentially infinite number of locations under all or part of the image receptor. An indication of the complete range and detector size, coupled with adequate instructions, would be sufficient. Still others may indicate the range of multiple positions on the paddle. Again, this would satisfy the requirement. There may be other methods employed that also satisfy the requirement. The key is that the operators know what areas they may select and the size of the detector.
An indication of the range of coverage and the detector size, along with appropriate instructions (usually found in the user manual), would satisfy this requirement.
No. Paddles designed to be smaller than the full size of the image receptor do not have to display AEC detector position and size. Paddles used only for invasive procedures do not have to display AEC detector position and size because they are not covered by the regulations.
The answer to the first question is yes. According to 900.12(e)(5)(i)(A), when the AEC performance is found to be outside the action limit during physicist testing, the medical physicist may create a temporary technique chart that includes the appropriate density settings (in addition to the other technique factors) to be used with the malfunctioning AEC. The facility can use this temporary chart for up to 30 days, or until the problem has been corrected and the equipment passes the AEC performance test, whichever comes first. If the AEC is completely non-functioning, the medical physicist may create a manual mode technique chart that includes all the appropriate manual technique factors. Use of the manual mode would be acceptable under the complete failure situation raised by the question. The facility can use manual techniques for up to 30 days while the non-functioning AEC is being repaired and can continue to use the unit on patients during this period.
The answer to the second question depends on the repair needed to fix the problem. If the repair is classified as “major” (see Table: Medical Physicist Involvement in Equipment Adjustments, Changes, or Repairs), then the medical physicist must be onsite to perform the post repair testing. If the repair is not classified as “major” then the post repair testing may be done under the medical physicist’s oversight. In either event, the appropriate testing must be performed and passed within the specified time frames.
Automatic Exposure Control (AEC)
Related Topics:
Air Kerma and AEC Reproducibility Annual Quality Control Test
Mammography Equipment Evaluations