Monte Carlo performance on the x-ray converter thickness in digital mammography using software breast models

In x-ray mammography, some of the components that play significant role to early diagnosis are the x-ray source, the breast composition as well as the composition of the x-ray converter. Various studies have previously investigated separately the influence of breast characteristics and detector configuration on the optimization of mammographic imaging systems. However, it is important to examine the combined effect of both components in improving the signal transfer properties in mammography systems of the mammograms. In the present study, the authors compared and evaluated x-ray converters using software breast models and realistic mammographic spectra in terms of: (a) zero-frequency detective quantum efficiency (DQE) and (b) sensitivity. The impact of x-ray converter thickness on contrast threshold (C(TH)) for observer assessment, based on the Rose model, was demonstrated as well.

Monte Carlo techniques were applied to simulate the x-ray interactions within the software breast phantoms and thereafter within the detective medium. Simulations involved: (a) two mammographic x-ray spectra: 28 kV Mo, 0.030 mm Mo, and 32 kV W, 0.050 mm Rh of different entrance surface air kerma (ESAK: 3-7 mGy), (b) realistic breast models (dense and fatty) and (c) x-ray converter materials most frequently considered in investigations on energy integrating digital mammography detectors: the Gd(2)O(2)S:Tb granular phosphor, the CsI:Tl structured phosphor, and the a-Se photoconductive layer. Detector material thickness was considered to vary in the range from 50 mg∕cm(2) up to 150 mg∕cm(2).

The Monte Carlo study showed that: (a) the x-ray beam becomes less penetrating after passing through dense breasts leading to higher values of zero-frequency DQE of the x-ray imaging converters and improved C(TH) values in all cases considered, (b) W∕Rh target∕filter combination results in improved C(TH) values at higher ESAK values, and (c) a-Se shows higher zero-frequency DQE values than the phosphor-based converters, Gd(2)O(2)S:Tb and CsI:Tl. However, thicker layers of CsI:Tl could be comparable to a-Se layers achieving approximately 27.6% C(TH) improvement at a thickness of 150 mg∕cm(2).

The present Monte Carlo investigation indicates that in the energy range employed in mammography, an upper limit, approximately 100 mg∕cm(2), should be considered in the development of thicker a-Se converters. On the other hand, above this thickness value, CsI:Tl converter could improve its imaging performance.