iaea human health series publications - SEDIM

8.5.2. Detector response and noise8.5.2.1. Scope— Objective: To measure the response and noise characteristics of the image acquisition system under standardradiation exposure conditions; to detect temporal changes in these quantities.— Minimum frequency: Annually and after service to the detector.8.5.2.2. Instrumentation(1) PMMA slab(s) of a total thickness of 45 mm used for AEC testing and either the aluminium or the PMMAcontrast object used for SDNR measurements (Fig. 23).(2) Acquisition or radiologist workstation with ROI capability or QC software for image analysis. Alternatively,images can be downloaded to another computer and analysed using image analysis software.(3) Detector baseline performance chart (Chart 4 in Annex II).8.5.2.3. MethodologyCR systems should be set to the modes listed in Table 11, and the applicable EIs should be recorded asdescribed in the table. 16Note: These tests should be performed on flat fielded 16 but unenhanced (‘for processing’) images(i.e. without peripheral or resolution enhancement). For example, GE Fineview TM must be turned off.(1) Create a ‘patient’ study with an appropriate name.(2) Place the 45 mm thick PMMA slab on the tabletop with its long edge aligned with the chest wall edge of thebreast support. Use collimation that allows the full detector area to be exposed.(3) Place the contrast object on the upper surface of the PMMA.(4) Lower the compression plate so that it is in contact with the test object.(5) In manual mode (with the radiographic grid in place), obtain four images with different exposures accordingto the technique factors indicated in the detector baseline performance chart (Chart 4 in Annex II). Theseimages will be used to characterize the detector response. For ease of analysis, ensure that the laterality (leftor right breast) chosen for each image is the same, so that the chest wall edge will appear on the same side ofthe monitor in all images. All measured values are to be recorded on the detector performance chart (Chart 5in Annex II).(6) For CR systems, wait one minute before processing.(a)Systems with ROI capability (usually precludes CR systems)(1) For each image, with the image displayed so that the contrast object is clearly visible (see Fig. 24), place anROI of approximately 80 mm 2 (~10 mm in diameter) over, and entirely contained within, the contrast object.Record the MPV on the detector performance chart (Chart 5 in Annex II) and label this value A; it will be usedto calculate the SDNR.(2) In a region outside, but immediately adjacent to, the contrast object, record the MPV and standard deviationwithin an ROI of similar size to that used above as B and C, respectively. For CR systems, which frequentlydo not apply any equalization procedure, non-uniformity in the direction parallel to the chest wall couldproduce significant variations of the SDNR values, depending on the position chosen for the contrast object(if it is not embedded in the PMMA) and the measurement ROIs. This could be misinterpreted as a fluctuationof the mammography unit.16Not applicable to CR images.95