For a QC program to be practical and able to be followed by all facilities, some pragmatic decisions about the usefulness of individual tests and scope and extent of site survey testing must be made. It was found in the DMIST trial, that the testing process was quite time consuming and that, while some of the information was relevant to the initial characterization of digital systems, it was of limited use for QC purposes. In addition, if one test was the same as a number of other, that test should be used first, and only if the system fails the initial test should it be necessary for the physicist or service person to perform further, more selective diagnostic tests. In this work, the tests used in DMIST are considered in three categories: A) THE PERFORMANCE OF THE IMAGE ACQUISITION SYSTEM, B) the dose and image quality, and C) the image display system. The objective of each test was reviewed briefly and the pass/fail criteria are presented. Based on experience from DMIST, the utility of the test is discussed and modifications to the method of carrying out the test, including its elimination from the program and/or changes to the pass/fail criteria, are recommended. The intent is to develop a harmonized set of tests, and also allow for cross-vendor validation of system compatibility.
- THE IMAGE ACQUISITION SYSTEM
Objective: To ensure that all locks, detents, angulation indicators, and mechanical support devices for the x-ray tube and breast support assembly are operating properly and that the DICOM header information is correct. The overall safety of the equipment is verified, and problems that might interfere with general operation are detected. A nonexclusive list of items to be checked regularly by the technologist covers most areas of outwardly observable physical faults. The physicist does a more thorough evaluation.
Pass/fail criteria: A number of the items on the list are subjective with suggested performance targets. Evaluation requires diligence and discernment on the part of the technologist. Where tests have objective measures, pass/fail criteria are similar to those for SFM systems through MQSA.
DMIST results: There were very few significant problems found at physics inspections, but it should be noted that these facilities were highly motivated, and had regular reinforcement of QC policies. The most common failure identified by physicists was the absence of posted technique charts. This was justified in most cases because the computer displayed a recommended technique, and there were no manual techniques used at those facilities. The most common problems seen by the technologists were those related to viewing conditions and monitors.
Effectiveness: Regular checks by the technologist ensure that monitors are appropriately cleaned and that viewing conditions are appropriate. Some mechanical problems were found and service visits were scheduled to prevent downtime.
Recommendation: DMIST recommended that this test set be retained, and that it be performed weekly by the technologist. The physicist should perform a thorough inspection on each annual visit. Additional items which must be documented include: checking DICOM image header compliance for proper labeling, date, time, and time zone, all of which might get changed when software is upgraded. It is especially important that indicated breast thickness is accurate, as this affects technique selection and resultant breast dose.
CR Imaging plate fogging:
Objective: To confirm that computed radiography CR plates are not fogged by radiation in their storage location.
Pass/fail criteria: There should be no evidence of fogging on the image. The shadow of a coin taped to the front of a cassette and left in place through a full day of imaging should not be visible even at the narrowest display window setting.
DMIST results: No evidence of imaging plate fogging was seen.
Effectiveness: There is a low probability of failure.
Recommendation: This test is not recommended to be performed for routine QC.
Collimation and alignment:
Proper collimation of the x-ray field is necessary to ensure there are no unexposed portions of the image receptor and that patients are not needlessly exposed to stray radiation. Proper alignment of the edge of the compression paddle with the chest-wall edge of the image-receptor holder assembly is necessary for proper positioning and compression of the breast. Note that for CR, the image receptor is used with a number of different x-ray units and it is the alignment of the unit that is being evaluated in the following tests.
- X-ray field, field indicator and image field congruency.
- Compression paddle and image receptor—Excluded tissue at chest wall.
Objective: To evaluate whether the field as indicated by the machine, positioning light or other indicator matches the true x-ray field and whether the x-ray field is congruent to the image receptor.
Pass/fail criteria: The sum of the x-ray field-indicator misalignments in the left-right and anterior-posterior directions should not exceed 2% of the source-to-image receptor distance (SID). The x-ray field should cover the entire displayed area, and no edge of the x-ray field should extend beyond the image receptor by more than 2% of the SID. Additionally, the x-ray field must not extend beyond the shielded area provided by the breast support except at the chest wall side.
DMIST results: There were some inconsistent results in the different units. The high failure rates for the Fischer system arise from discrepancies between the points when x-ray exposure begins and ends during the scan, adjustable by the service engineer and the field markings printed on the tabletop at the factory. There is no radiation hazard associated with this failure, provided that the x-ray detector or surrounding shielding absorbs the full area of the primary x-ray beam.
Effectiveness: Accurate indication of the active image area is necessary for correct patient positioning. From experience with film/screen systems, this test is useful for detecting gross errors in collimation adjustment or damage to the collimator device.
Recommendation: We recommend that the collimation tests be performed annually, and whenever major components that could affect alignment of collimation, x-ray tube, collimator parts, detector assembly and/or scanning drive are repaired or replaced. In the future, this test will need to be done with a fluorescent screen, self-developing film, or an electronic “edge of field” imager because most facilities will not have access to a film processor.
Objectives: To ensure that the compression paddle is in the appropriate position and to determine the amount of tissue that is not imaged by the mammographic unit when a patient is positioned as closely to the unit as possible. A phantom was imaged to assess the amount of tissue excluded from the image at the patient’s chest wall.
Pass/fail criteria: The edge of the paddle is not to be visible in the image. Not more than 7 mm should be excluded.
Effectiveness: On some units, the paddle extension is adjustable, and improper alignment could result in poor positioning of the breast. If the edge of the compression paddle extends too far beyond the image receptor edge, the patient’s chest is pushed away from the image receptor and some breast tissue will be excluded from the image. If the edge of the compression paddle does not extend far enough, the breast tissue will not be properly pulled away from the chest wall, resulting in poor compression at the chest wall, and the vertical edge of the compression paddle could obscure clinical information. Mechanical support structures or clearance for the chest-wall edge of the detector may result in un-imaged tissue.
Recommendation: It is recommended that the collimation test be performed annually and following service to the x-ray tube or collimator or whenever the alignment of the breast support to the detector is adjusted. The 7 mm limit for missing tissue was found to be satisfactory in that all systems could be adjusted to achieve compliance.
Objective: This test evaluates the kilovoltage provided by the generator.
Pass/fail criteria: The measured kV must be within 5% of the nominal kV and the coefficient of variation between four successive exposures at the same kV setting must be less than 0.05.
DMIST results: Only one measurement exceeded the 5% limit.
Effectiveness: Given the stability of modern x-ray generators, kV accuracy and reproducibility need not be tested as part of routine QC. Furthermore, noninvasive test instruments estimate kilovoltage based on beam quality and are less precise than voltage meters that are connected directly to the generator circuitry. Measurements of the half-value layer (HVL) of the x-ray beam will detect any gross problems with kV output, but for this to be effective as an alternative to measurement of kV, it is necessary to have more stringent criteria for the value of HVL and its consistency over time.
Recommendation: It is recommended that a measurement of HVL be used as an assessment of beam quality. KV should not be measured as a routine practice, but only by a service engineer using appropriately calibrated equipment at installation and when the generator is serviced.
Tube output, linearity, output rate, and reproducibility:
Objective: This test ensures that tube x-ray output rate, linearity, and reproducibility meet MQSA requirements over a range of clinically relevant settings of kV, x-ray target, and beam filter.
Pass/fail criteria: For generator linearity, the output (mR/mAs)measured across a range of mAs settings was required in DMIST to remain within 10% of the mean tube output and increase monotonically with increased kV settings. The exposure output rate at 28 kV for those systems with Mo/Mo target filter combinations was required to be at least 800 mR, 7 mGy air kerma, as specified by MQSA for film/screen systems. Output reproducibility requires the COV for four successive exposures to be less than 0.05.
DMIST results: There were five failures of tube output linearity in 143 testing instances. Two of the four failures were attributable to measuring the output at low mAs settings, well below the manufacturer’s current recommended range of operation. One of the errors is consistent with an operator data transcription error. None of the measurements of tube output, output rate, and output reproducibility indicated a failure.
Effectiveness: Modern x-ray generators used in FFDM systems are universally of high-frequency design and incorporate internal feedback and correction circuitry that maintain virtually constant kV and mA during exposures. Exposure time is also controlled electronically and is highly reliable. X-ray tube output was found not to vary over long time periods.
Recommendation: DMIST believes that it is still worthwhile to include the measurement of x-ray tube output under different tube target/filter/kV combinations as part of a routine QC program as an overall performance check, and also because these data are necessary for computing estimated mean glandular dose. However, all current mammographic x-ray sources easily meet the requirement for output rate, and performance of that test is not recommended. Testing mR/mAs and HVL will provide a warning of generator and spectral problems and will prompt diagnostic testing. It is not recommended to directly evaluate output linearity and exposure linearity. Image noise tests will provide a surrogate test for problems related to linearity and/or reproducibility.
Detector linearity and reproducibility:
Objective: To evaluate the linearity of the detector response, the ratio of mean pixel value (MPV) to measured entrance exposure is tested for constancy.
Pass/fail criteria: The acceptance criterion for detector linearity is that at any point over a range of mAs, with other technical parameters constant, this value does not vary from its mean by more than 10%. For CR plates, sensitivity is deemed to pass if the S-number is within 15% of the target value. The limit on the COV for detector reproducibility measurements is 0.05.
Effectiveness: Tests of linearity and reproducibility are helpful for characterizing detector response, but once the mammography unit is installed are of marginal utility.
Recommendation: DMIST suggests that it should not be necessary to measure short-term detector reproducibility and linearity as part of routine QC. Instead, detector linearity and reproducibility should be measured only as a diagnostic tool, when irregularities are observed: shift of measured MPV or “S-numbers” on signal measurements obtained from the technologist’s weekly uniform phantom image.
Logging of this information could be automated and unacceptable deviations could be used to trigger a warning message, prompting investigation of whether the deviation arose from the x-ray generation system or from the detector.
Half-value layer:
Objective: This test evaluates the effective energy of the x-ray beam. The HVL of the x-ray beam should be high enough to avoid excessive dose to the breast, while not so high that subject contrast is reduced to an unacceptable degree. The test also ensures that the x-ray beam quality is consistent with the target, filter, and kV selected, and enables the calculation of mean glandular dose.
Pass/fail criteria: At a given kV setting in the mammographic kilovoltage range (below 50 kVp), the measured HVL with the compression paddle in place must be within the range set out in the ACR Quality Control Manual.7 For the upper limit, additional values of the constant, (c) have been defined. For W/Mo target/filter combination, c=0.28 and for W/ Al, c=0.32.
DMIST results: Of 396 measurements of HVL, no failures were recorded. The half-value layer showed little variation for any of the units.
Effectiveness: If the HVL for film/screen units is excessive, subject contrast will be reduced. For FFDM, with the capability of contrast manipulation, minor changes in HVL will have much less impact on image contrast than with SFM systems. Nevertheless, if kV is not measured routinely, HVL provides a check for variations in beam quality. In addition, knowledge of the HVL is required to estimate mean glandular dose.
Recommendation: This test should be performed annually. The HVL should be evaluated for each filter and for at least one kV that is typical of clinical operating techniques. DMIST recommend that HVL tables should be provided by the manufacturer for each FFDM model to facilitate dose calculations and to allow verification of correct HVL. These tables should specify the expected HVL under typical target/filter/kV combinations for clinical use. After initial testing, if the HVL is compliant with the manufacturer’s specification, the measured value should be adopted as the reference value and changes from that value tracked. The currently permitted range of HVL is very wide, and designed to accommodate a range of equipment designs; sensitivity to changes in HVL will be easier to detect if there is an established operating point. In the DMIST measurements, it was found that HVL varied typically by no more than 3%. A requirement that HVL be constant within 6% seems to be reasonable.
Focal spot:
Objective: This test ensures that the spatial distribution of x-ray emission from the focal spot in contact or magnification mode does not unduly degrade spatial resolution of the image.
Pass/fail criteria: The limiting effective spatial resolution in line pairs/mm for a bar pattern, placed 4.5 cm above the breast support table, was measured on a mammographic SFM receptor placed on the breast support surface. The MQSA film/screen criteria were employed; the minimum required limiting resolution was; 11line pairs/mm with the pattern bars perpendicular to the anode-cathode axis and 13line pairs/mm with the pattern bars parallel to the anode-cathode axis. Where magnification capability was available, systems were tested according to the same criteria using the small focal spot with the resolution pattern placed 4.5 cm above the magnification stand.
DMIST results: All but 5 of 246 measurements met the MQSA requirements. Of those that failed, three were taken using the magnification stand, and were just outside the limits. There was no measurable reduction of overall spatial resolution on mammographic units which failed this test.
Effectiveness: For contact mammography, the effective resolution provided by the focal spot is considerably higher than the resolution limit imposed by the digital detector and therefore has little influence on the overall MTF of the system. A test of system MTF is considered to be a more sensitive, objective, and relevant measurement of resolution, and is capable of detecting problems with the focal spot, synchronization errors, and other factors affecting system resolution. As more facilities switch to digital imaging, the film and processors needed to make this test will become unavailable.
Recommendation: The separate measurement of focal spot resolution using film/screen images need not be performed, except as a diagnostic test to evaluate resolution problems detected using the MTF test.
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