iaea human health series publications - SEDIM

(b)Testing of density control (if applicable) and/or dose selection modeSome digital units and all CR systems are used with X ray units that have a variable ‘density control’. Indigital mammography, the purpose of the density control is primarily to allow control of the image noise level.Small changes are not normally perceptible. It is desirable that use of the density control will allow the radiationdose to be increased by 25–100%, and similarly decreased by 25–50%, from the ‘0’ or ‘normal’ position in severalsteps.(1) Place a total thickness of 45 mm of PMMA (plus the spacer) on the breast support as described above.(2) For CR systems, to test the +/– density control, make exposures using the AEC. Use two density settingsbelow the ‘0’ or ‘normal’ position, one at ‘0’ and two above it, with each setting attempting to give about a±25% variation in mAs. A total of five exposures is required for this test.(3) Record the density setting and the resulting mAs used to image the PMMA each time.(4) For DX systems, make exposures in all applicable dose modes and determine their effect on the exposurefactors.(5) Record the dose mode setting and the resulting mAs used to image the PMMA each time.(6) It is not necessary to process the image after each exposure — the digital image itself will not be used. Theobjective is to ensure that the density setting adjusts the mAs in a reasonable manner.(7) Provide a copy of the radiographer baselines and summary chart (Chart 3 in Annex II) to the radiographer atthe facility, to be used as a baseline for the radiographer’s tests.8.4.2.4. Interpretation of results and conclusionsTolerances:(a)Thickness behaviour(1) The SDNR values for images of 20, 45 and 70 mm of PMMA should exceed the acceptable values given inTable 13 for the aluminium contrast object and those given in Table 14 for the PMMA contrast object.(2) SDNR performance must be achieved within the current dose limitations (see Table 21).(3) For contact mode mammography, exposure time should not exceed 2.0 s for 45 mm and 4.0 s for 70 mm ofPMMA. This does not apply to scanning systems where the dwell time at any location is much shorter thanthe time taken to image the entire breast.For each thickness of PMMA, acceptable and achievable SDNR values are provided in Tables 13 and 14.These values must be regarded as provisional at this stage, and updates, as available, will be provided in the futureat http://humanhealth.iaea.org. Regardless of what the ultimate SDNR values are, it is expected that a DX systemshould be able to match the achievable SDNR using a dose that is well within the current dose tolerances (seeTable 21). Some CR systems may not be able to reach the achievable SDNR values without exceeding these dosetolerances. In general, greater importance should be given to achieving adequate image quality than to lowering thedose.Caution should be exercised in interpreting the results of SDNR measurements. For all systems, it is expectedthat a higher SDNR corresponds to better image quality, provided the image sharpness is unchanged. Thus anincrease in radiation exposure will reduce quantum noise, resulting in an increase in SDNR and better image qualityif MTF is unchanged. However, an increase in SDNR may be due to a deteriorating MTF, which would result in anoverall reduction in image quality. Thus SDNR is not on its own a reliable measure of image quality but is an easyto measure parameter that is a sensitive indicator of changes that relate to image quality. Therefore any change inSDNR needs to be investigated to understand its cause.90