iaea human health series publications - SEDIM

(2) Stack the 20 and 25 mm PMMA slabs, or use a 45 mm full field uniform phantom on the breast supportplatform. Place the contrast disc or square on the upper surface 11 such that the contrast object lies on the centreline of the breast support, approximately 40 mm from the chest wall edge, as shown in Fig. 23. Ensure that thefront edge of the PMMA slab extends slightly (a few mm) beyond the chest wall edge of the breast support.Use collimation that allows the full detector area to be exposed.(3) Place the 8 mm spacer on top of the phantom.(4) Lower the compression plate and apply the compression force typically used clinically (e.g. 80 N) 12 .(5) If the AEC detector location can be adjusted, it should be placed within 1 cm of the chest wall.(6) Choose the exposure mode used clinically. On initial commissioning, all available automatic modes should betested. For systems that are not fully automatic, set the AEC or kV, target and filter to the appropriatetechnique for the equivalent breast thickness in Table 12.(7) Make one exposure and record the kV, target, filter and mAs AEC on the compression–AEC evaluation chart(Chart 2 in Annex II) and on the detector performance chart (Chart 5 in Annex II), where they are used todetermine the reference technique. These exposure factors are required to determine the incident air kerma(see Section 8.7.2), which in turn is used to calculate the MGD.(8) Leaving the contrast disc or square in the same location, make exposures using the AEC, using 20 mm and70 mm total thickness of PMMA. Each time, place the appropriate spacer above the uppermost slab (seeTable 11). 13(9) Either display the images in the ‘for processing’ format on a workstation equipped with analytical tools (ROI,mean, standard deviation) or download the ‘for processing’ images to a separate computer for analysis.(10) View the unprocessed images taken at the three thicknesses with the image displayed so that the contrastobject is clearly visible (see Fig. 24).(11) According to the contrast object used, place a circular or square ROI approximately 0.8 cm 2 in area (~1 cm indiameter) over and entirely contained within the contrast object area. For the aluminium square, an ROI of45 mm 2 (7.5 mm in diameter) can be used. Use the same size ROI (or as close to it as possible) each time (forfurther information on the choice of ROI size, see Ref. [49]). For CR systems, which might not apply any flatfielding, non-uniformity in the direction parallel to the chest wall could produce significant variations of theSDNR values, depending on the position chosen for the contrast object (if it is not embedded in the PMMA)and the distance between the measurement ROIs. This could be misinterpreted as a fluctuation of themammography unit.(12) Measure the MPV, recording it as A.(13) Choose an ROI located just beside the contrast object, and measure the background MPV, B, and thebackground standard deviation, C.(14) Enter the results into the compression–AEC evaluation chart (Chart 2 in Annex II) and the radiographerbaselines and summary chart (Chart 3 in Annex II), if applicable.(15) Calculate the SDNR as:SDNR = |A–B|/C (1)(16) For a system equipped with multiple dose modes, at commissioning the above procedure should be repeatedfor each of the applicable modes, recording the SDNR in each case.(17) For the 45 and 70 mm exposures, determine the exposure time either by direct measurement or by dividing themAs required for the exposure by the mAs from the manufacturer’s technical information for the system.11For consistency of positioning and to avoid loss, it is convenient to mount the contrast object on the surface of the 20 mmthick slab.12The actual force should be similar to the typical value used clinically, but the same value should be used for all testing. Notethat in some systems and in some modes of operation, the compressed breast thickness is utilized in an automated algorithm todetermine the technique factors; this thickness is, in turn, dependent on the degree of compression applied.13Because the aluminium square is thin, if it is used it can be placed directly on the 20 mm slab and additional slabs can besuccessively placed over it during the test.89