This work examines the use of a detectability index to control an Automatic Exposure Control (AEC) system for an
amorphous-Selenium digital mammography detector. The default AEC mode for the system was evaluated using
homogeneous poly(methyl methacrylate) (PMMA) plates of thickness 20, 40, 60 and 70 mm to find the tube potential
and anode/filter settings selected by the system. Detectability index (d') using a non-prewhitened model observer with
eye filter (NPWE) was calculated for these beam qualities as a function of air kerma at the detector. AEC settings were
calculated that gave constant d' as a function of beam quality for a homogeneous background; a target d' was used that
ensured the system passed the achievable image quality criterion for the 0.1 mm diameter disc in the European
Guidelines. Threshold gold thickness was measured using the CDMAM test object as a function of beam quality for the
AEC mode, which held pixel value (PV) constant, and for the constant d' mode. Threshold gold thickness for the 0.1 mm
disc increased by a factor of 2.18 for the constant PV mode, while constant d' mode held threshold gold thickness
constant to within 7% and signal-difference-to-noise-ratio (SdNR) constant to within 5%. The constant d' settings
derived for homogeneous images were then applied to a phantom with a structured background. Threshold gold
thickness for the 0.13 mm disc increased by a factor of 1.90 for the constant PV mode, while constant d' mode held
threshold gold thickness constant within 38% for 0.13 mm disk.
This work compares the detector performances of the recent Kodak Min-R EV 190/Min-R EV and current Kodak Min-R 2190/Min-R 2000 mammography screen-film combinations with the Kodak CR 850M system using the new EHR-M and standard HR plates. Basic image quality parameters (MTF, NNPS and DQE) were evaluated according to ISO 9236-3 conditions (i.e. 28 kV; Mo/Mo; HVL = 0.64 mm eq. Al) at an entrance air kerma level of 60 μGy. Compared with the Min-R 2000, the Kodak Min-R EV screen-film system has a higher contrast and an intrinsically lower noise level, leading to a better DQE. Due to a lower noise level, the new EHR-M plate improves the DQE of the CR system, in comparison with the use of the standard HR plate (30 % improvement) in a mammography cassette. Compared with the CR plates, screen-film systems still permit to resolve finer details and have a significantly higher DQE for all spatial frequencies.