iaea human health series publications - SEDIM

7.5. SEMI-ANNUAL TESTS7.5.1. Computed radiography plate sensitivity matching and plate artefacts7.5.1.1. Scope— Objective: To confirm uniformity in the sensitivity of the CR plates used in mammography; to determine thepresence of plate related artefacts.— Minimum frequency: Semi-annually and after service to the CR reader that might affect its efficiency.7.5.1.2. Instrumentation(1) All plates used routinely in the mammography service.(2) Slab of PMMA with uniform thickness, preferably large enough to cover the entire image receptor, such asthat used for the weekly tests described in Section 7.2.3.7.5.1.3. Methodology(1) Select all plates to be evaluated.(2) For each plate, record the plate number and the condition of the cassette latches on the plate sensitivitymatching and plate artefacts chart (Chart 14 in Annex I).(3) Place the PMMA slab on the breast support of the mammography unit, ensuring that the AEC sensors arecovered. It is also necessary to ensure that the sensors are under the central part of the slab.(4) The exposure should preferably be carried out with the AEC. If the equipment does not have an AEC, selectthe technique factors manually (and use the same for all cassettes). These factors should be those clinicallyused to obtain images of a breast corresponding to the test object. Make the exposure and record the factors(AEC mode and settings, anode, filter, kV, mAs) used to make the exposure of each cassette on the datacollection sheet.(5) Process the plate after a constant time delay (say, 30 seconds), to minimize the impact of latent image fading,using the same menu choices and image processing for each plate (see Table 6).(6) Record the EI and mAs used for each cassette. Note the result on Chart 14 in Annex I.(7) Repeat steps 4 through 6 for all plates.(8) Calculate the mean value of the EI and mAs for all plates of the same size.(9) For each plate, determine the difference and the percentage difference between the mAs for that plate and therelevant mean value.(10) For each plate, determine the difference and the percentage difference between the EI for that plate and therelevant mean value.(11) If two plate sizes are used, determine the difference and percentage difference between the mean EIs for thetwo plate sizes.(12) View each image on the acquisition workstation and inspect (using a narrow window setting, as advised bythe medical physicist) for significant artefacts (signs of scratches, scrapes, dents, etc.).(13) Record the presence or absence of significant artefacts on Chart 14 in Annex I.(14) Note the outcome of the task on the monthly, quarterly and semi-annual checklist (Chart 2 in Annex I).7.5.1.4. Interpretation of results and conclusionsThe tolerances for the exposure index for CR plates are given in Table 9. In addition to the specific tolerancesoutlined in Table 9:(1) There should be no significant artefacts due to damage to or deterioration of individual plates.(2) Typically, the mAs used for a particular plate should be within ±5% of the mean value for plates of the samesize.77