A Novel Amorphous Selenium Avalanche Detector Structure for Low Dose Medical X-Ray Imaging

A novel amorphous selenium (a-Se) avalanche detector structure for low dose direct-conversion flat-panel X-ray detector is proposed. The proposed structure contains blocking layers to reduce carrier injection from metal electrodes and hole trapping layer to separate X-ray absorption layer from avalanche gain region. The feasibility of the structure for avalanche gain with negligible avalanche noise is studied by using the semiconductor module of COMSOL multiphysics together with a cascaded linear system. The model considers carrier injection from electrodes and charge carrier transport through various layers of multilayer a-Se structure in order to analyze the transient and steady-state electric field distribution across the detector. A cascaded linear system model that includes reabsorption of K-fluorescent X-rays, carrier trapping in bulk and trapping layer, and avalanche multiplication of charge carrier is used to calculate the frequency-dependent detective quantum efficiency [DQE(f)] and modulation transfer function (MTF) of the proposed structure. The avalanche gain enhances the signal strength and improves the DQE(f) by overcoming the effect of electronic noise at low X-ray doses. The structure is applied for breast tomosynthesis and observed that the proposed structure offers the required avalanche gain to ensure quantum noise limited operation at reduced exposures.