iaea human health series publications - SEDIM

8.5.5. Detector uniformity and artefact evaluation8.5.5.1. ScopeHere, a qualitative approach is described; more quantitative methods for assessing local variations in detectorperformance are also available [50].— Objective: To assess the degree and source of artefacts visualized in full field digital mammograms orphantom images; to ensure that the flat field image is uniform in terms of signal level and noise; to establishthe imaging technique and viewing parameters to be used for the monthly full field artefact check.— Minimum frequency: Annually and after service to the X ray tube head or detector, or modifications to theimage acquisition or correction software.8.5.5.2. Instrumentation(1) A 45 mm thick slab of PMMA or a sheet of aluminium, 2–3 mm thick, large enough to cover the entiredetector. 18 For magnification artefact evaluation, a 25 mm PMMA slab or a 1–2 mm aluminium sheet couldbe used. It should be ensured that the aluminium sheet is not scratched. There must be no radiographicallyvisible structure in the phantom. 19(2) Acquisition or radiologist workstation. Alternatively, images may be downloaded to another computer andanalysed using image analysis software.8.5.5.3. Methodology(1) Create a ‘patient’ study with an appropriate name.(2) Place the uniform phantom in the X ray field, either on the tabletop or suspended from the tube assembly, sothat the front edge extends beyond the chest wall edge of the breast support plate and also extends beyondeach indicated edge of the image field. Leave the grid on for large focal spot exposures and the mostfrequently used compression paddle in place for all images. Use collimation that allows the full detector areato be imaged.(3) Lower the compression paddle so that it is in contact with the test object, or to a height of 45 mm above thebreast support.(4) Use the exposure mode for a clinical image of a breast of equivalent thickness. Turn off any imageenhancements or post-processing options.(5) Expose the image.(6) For CR systems, use the settings listed in Table 11.(7) Enter the imaging parameters into the artefact evaluation/flat field uniformity chart (Chart 8 in Annex II).(8) Repeat steps 5–7 for all target–filter combinations used in clinical practice. Use a manual (or AEC, ifpossible) exposure technique that results in a reasonably exposed image. One image (at a typical clinical kV)is required for each target–filter combination used clinically. Record the technique factors in the radiographerbaselines and summary chart (Chart 3 in Annex II) for use by the radiographers in their monthly full fieldartefact tests. 20(9) Install the magnification stand (if used clinically).18Alternatively, provision could be made to position a smaller phantom closer to the X ray source to cover the entire imagefield. If this is done, care must be taken to align the plane of the phantom parallel to that of the detector. Some manufacturers supply apurpose built block of PMMA that attaches to the collimator.19In practice, it is probable that over time any material will become scratched through handling. In the case of moderatescratching, the physicist should record the presence of such scratches so that they are not confused with problems associated with theimaging system. If defects become excessive, the material should be replaced.20If the radiographer uses the same test object as the physicist and the Excel spreadsheets provided are used, the parameters willbe entered automatically into Chart 3 in Annex II. If the radiographer uses a different test object than the physicist, different exposurefactors may be required.102