Effect of detector element size on signal detectability in digital mammography

The purpose of this work was to evaluate the effect of detector element size on detection and discrimination of small objects in digital mammograms in the presence of quantum noise, stochastic breast structure {P (f) = K/f(3))} and system (electronic) noise. Theoretical analysis was done using the Fourier domain approximation of Albert and Maidment (Medical Physics, Vol. 27, pp 2417-2434, 2000), including averaging over random but known signal locations and aliasing. Realistic CsI-based indirect detection system operating parameters were used with 4 detector element sizes (25, 50, 75 and 100 microns). Detailed results depended on selected input exposure and system noise level (10 mR and 100 photons/del equivalent photon input were used). Results will be described using size thresholds based on human decision performance estimates - typical threshold SNRs were about 6 to 7 at 95% decision accuracy. The selected tasks and approximate threshold sizes were: searching for a microcalcification in a 1 cm(2) cluster (140 microns), two-alternative forced-choice microcalcification discrimination of shape (220 microns) and edge gradient (290 microns), and searching for a spiculation (cylinder model) around the perimeter of a known mass. MC task threshold variations with del size were small (usually less than 5% variation). Spiculation thresholds depended on length and surrounding tissue composition - del size and system noise had little effect. For 15 mm spiculations, the threshold diameters were about 0.5 nim in fatty tissue and 1.3 mm in 50% glandular tissue.