Automatic technique parameter selection on a digital mammography system: an evaluation of SNR and CNR as a function of AGD on a GE Senographe DS

摘要

There are obvious differences in the automatic selection of kVp, anode, and filter between a digital mammography system and the equivalent film/screen system for the same thickness of PMMA absorber. To investigate the reason for these changes, a large number of images were acquired using 2, 4, 6, and 8 cm thick PMMA absorbing slabs, and various combinations of kVp, mAs, anode, and filter. The SNR and CNR were calculated using two different contrast test objects and plotted as a function of AGD. The results can be summarized as follows: 1) For any given AGD with 4, 6, and 8 cm thick PMMA absorbers, SNR_(Rh/Rh) > SNR_(Mo/Mo) > SNR_(Mo/Mo), 2) For any given AGD: At 4 cm thickness, CNR_(Mo/Mo) > CNR_(Mo/Rh) > CNR_(Rh/Rh), At 6 cm thickness, CNR_(Rh/Rh) ～ CNR_(Mo/Rh) > CNR_(Mo/Mo), At 8 cm thickness, CNR_(Rh/Rh) > CNR_(Mo/Rh), 3) For any given absorber thickness and target/filter combination, CNR is approximately proportional to SNR, 4) For any given absorber thickness and target/filter combination, SNR (and hence CNR) is approximately proportional to (AGD)~(0.3) rather than the expected (AGD)~(0.5), 5) CNR measured using 0.1 mm thick aluminium foil as the contrast object is more dependent on choice of kVp than using a 1 mm deep hole in a PMMA slab.