The use of detectability indices as a means of Automatic Exposure Control for a digital mammography system

This work examines the use of a detectability index to control an Automatic Exposure Control (AEC) system for an amorphous-Selenium detector. The default AEC for the system was evaluated using homogeneous PMMA plates 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 was calculated for these beam qualities as function of air kerma at the detector. AEC settings were calculated that gave constant d’ as function of beam quality for homogeneous background; a target d’ was used that ensured the system passed the achievable image quality criterion for the 0.1 mm diameter disc of European Guidelines. Threshold Gold Thickness (TGT) was measured with CDMAM as function of beam quality for AEC mode, which held pixel value (PV) constant, and for constant d’ mode. TGT for the 0.1mm disc increased of 2.18 for constant PV mode, while constant d’ mode held TGT constant within 7% and signal-difference-to-noise-ratio constant within 5%. The constant d’ settings for homogeneous images were then applied to a phantom with structured background. TGT for 0.13mm disc increased of 1.90 for constant PV mode, while constant d’ mode held TGT constant within 38%.