Monte Carlo simulation of a CsI-based flat-panel imager for mammography

The development of new digital mammography techniques such as dual-energy imaging, tomosynthesis and CT mammography often requires investigating optimal camera design parameters and imaging techniques. One tool that is useful for this purpose is Monte Carlo simulation. This paper presents a methodology for generating simulated images from a CsI-based, flat-panel imager by using the Geant 3 Monte Carlo code to model x-ray transport and absorption within the CsI scintillator, and the DETECT-II code to track optical photon spread within a columnar model of the CsI scintillator. The Monte Carlo modeling of x-ray transport and absorption within the CsI was validated by comparing to previously published values for the probability of a K-shell interaction, the fluorescent yield, the probability of a K-fluorescent emission, and the escape fraction describing the probability of a K x-ray escaping the scintillator. To validate the combined (Geant coupled with DETECT-II) Monte Carlo approach to form simulated images, comparison of modulation transfer functions (MTFs) and system sensitivity (electrons/mR/pixel) obtained from simulations were compared to empirical measurements obtained with different x-ray spectra and imagers with varying CsI thicknesses. By varying the absorption and reflective properties of the columnar CsI used in the DETECT-II code, good agreement between simulated MTFs and system sensitivity and empirically measured values were observed.