By Vvon Engineering Team | Published | Updated
How to select mammography and tomosynthesis test objects for an approved quality-control programme, including image data, analysis, dose and periodic physics checks.
Breast imaging is unforgiving of small losses in image quality in a way that most radiography is not. The features that matter are micro-calcifications a few hundred micrometres across and low contrast masses that sit only slightly above the surrounding tissue. A detector that has drifted, a filter that has been changed, an automatic exposure control that is reading a different thickness than it thinks: none of those produce an image that looks broken. They produce an image that looks fine and no longer shows the smallest detail it used to.
A mammography quality programme combines visual assessment, quantitative measurements, dose evaluation and equipment checks. Define the applicable protocol and responsibilities with the medical physicist before choosing the phantom set.
Mammography test objects answer four separate questions and it is worth keeping them apart, because departments routinely buy one object and expect it to cover all four.
Routine and periodic tests have different purposes. Their frequencies, limits and actions must follow the applicable requirements, the system manufacturer and the approved physics programme; weekly or monthly phantom scoring alone is not a complete mammography QC schedule.
For a department that will do one test per week or per month, the routine objects are the right purchase. TOR MAS is supplied with a 7 cm attenuator stack and covers sensitometry, resolution limit, low and high contrast detail detectability and simulated micro-calcifications, so a single score can be recorded at each session. TOR MAX is the same idea with two resolution limit patterns rather than one, which is useful where the system is being compared against another unit and the single pattern is not discriminating.
Neither of those produces an image that looks like a breast, and that is deliberate: a geometric target is easier to score consistently. Where a more natural looking image is wanted alongside the geometric one, TOR MAM supplements them. One half carries filaments, micro-particles and low contrast details, and the other simulates breast tissue with micro-calcifications, so a radiographer can see what a real detail looks like at the current performance level rather than only counting discs.
Automated analysis such as AutoPIA can help standardise supported phantom measurements and trend results. Confirm the phantom, modality and image-format compatibility, then validate the analysis and baseline with the physicist. Automation does not replace investigation of artefacts or expert review of abnormal results.
Commissioning and annual work needs a full set. DMAM2 is built to the fourth edition European guidelines and brings together a threshold contrast detail plate, filaments, a graded set of PMMA plates, mesh and grid targets, a steel straight edge, a resolution pattern and the filters needed for homogeneity and ghosting tests. Once the department has that set, DMAM2 Gold is available separately as just the gold contrast detail plate, carrying 78 details in six sizes across a wide contrast range, which is what a routine threshold contrast score is read from without unpacking the whole case.
For the physical measurements, PIXMAM is a stack of finely toleranced PMMA plates, one of which carries an encapsulated aluminium foil, used for automatic exposure control checks, detector homogeneity, signal to noise ratio and contrast to noise ratio. The graded stack is the important part. Presenting the system with a series of thicknesses is the only way to find out whether the exposure control behaves across the range of patients it will actually see, rather than at one convenient thickness.
Quantitative signal and noise tests need the image type, region selection and analysis method specified by the QC protocol. Access to suitable for-processing or unprocessed data is often important. Confirm export and analysis capability at acceptance; display-processed images should not be substituted where the test requires another data type.
A tomosynthesis system reconstructs a stack of planes from projections taken across a limited angular range. That introduces two things a two dimensional phantom cannot test: how well the system resolves in the depth direction, and whether the reconstruction is geometrically faithful.
PIXMAM 3D is the routine answer. It adds a plate holding twenty five embedded spheres for the Z resolution test to the standard PIXMAM stack, so geometric distortion and image resolution can be checked alongside the usual physical measurements. For commissioning and annual work, TOMO IQ is the complete set, designed to the EUREF breast tomosynthesis protocol, combining the graded PMMA stack and Z resolution plate with a TORMAM phantom, an NPS aluminium filter, an MTF steel edge and x-ray rulers.
Book adequate protected physics time for the required two-dimensional and tomosynthesis tests. The scope and duration depend on the system and protocol, so include them in the operating plan before purchasing the phantom set.
Several tests in a breast imaging programme have nothing to do with image quality targets, and they are the ones most often missing from a Bangladeshi department's equipment list.
Budget storage and transport protection for shared test objects. Inspect plates and targets for scratches or other damage and consult the manufacturer about their continued suitability or replacement; a damaged object can introduce artefacts into the QC result.
Assign routine tests to appropriately trained staff and the specialist survey to a qualified medical physicist, with defined escalation and cover. Do not assume a fixed number of minutes or a one-day survey for every system. Confirm the protocol, competence and time allocation locally.
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