Image quality of direct conversion detectors for mammography and radiography: a theoretical comparison

Abstract

Direct conversion detectors have the potential to provide very high resolution and high detective quantum efficiency (DQE). Selection of a material that is appropriate for the task is dictated by the material properties. A linear cascaded systems analysis of DQE is used to predict the performance of several detector materials such as amorphous Se, CdZnTe, and PbI₂. A model is used to predict the spatial frequency-dependent <i>DQE</i>(<i>f</i>) for each material. This model includes: (1) x-ray absorption, (2) K fluorescence, (3) conversion gain, and (4) incomplete charge collection. A depth-dependent approach is used to account for gain variations and charge transport characteristics that change throughout the detector. In the model a parallel cascade, and non-elementary stages are used to model the effect of K-fluorescence reabsorption followed by incomplete charge collection. The <i>DQE</i>(<i>f</i>) is determined across an x-ray energy range of 10 to 100 keV for each material under typical bias conditions ranging from 0.1 V/&mu;m to 10 V/&mu;m. K-fluorescence escape and reabsorption blurring can cause marked reductions in the <i>DQE</i>(<i>f</i>). It is further reduced by incomplete charge collection which can theoretically decrease the <i>DQE</i>(<i>f</i>) by as much as 50% in extreme situations. This model will help determine key factors that will influence material selection for direct conversion x-ray systems.