Simulated sensor characterization for virtual clinical trials in mammography and digital breast tomosynthesis

Virtual clinical trials in X-ray breast imaging permit to compare different technical solutions and imaging modalities at reduced costs, involved personnel, reduced times and reduced radiation risks to patients. In this context, the detector characteristics (spatial resolution, noise level and efficiency) play a key role for an appropriate generation of simulated images. The project AGATA proposes to compute images as dose deposit maps in a detector layer of defined materials. Simulated images are then post-processed on the basis of suitable comparison between intrinsic characteristics of real and simulated detectors. With this scope, as first step for the post-processing manipulations, we evaluated the presampled modulation transfer function (MTF), the detector-response function and the noise power spectrum (NPS) of the simulated detectors. Two detectors were simulated: (1) 0.20 mm-thick a-Se direct flat panel with 70 µm pixel pitch and (2) CsI(Tl) indirect flat panel with 100 µm pixel pitch and scintillator layer 0.25 mm thick. In addition, the impact of simulating the de-excitation processes (Auger emission and fluorescence) was explored. Simulated detector characteristics were evaluated for W/Rh spectra between 25 kV and 31 kV. The in-silico platform used a Monte Carlo software based on Geant4 toolkit (vers. 6). First, the simulation and tracking of electrons generated from photoelectric or Compton interactions was shown to have neglectable influence on the pixel values for the explored spectra, with the produced electrons presenting short ranges with respect to the pixel dimension. In the case of the CsI detector, which has fluorescence energies higher than those of the simulated X-ray photons, the deexcitation processes have not noticeable influence on the calculated pixel values. On the other hand, the MTF of the a-Se detector resulted slightly lower when the fluorescence is simulated in the detector materials, due to the dose spread derived from the fluorescence photons, which can travel far from the initial ionization interaction. Regarding the a-Se detector, the noise power spectrum resulted lower with simulated deexcitation.