Abstract

Direct conversion flat panel x-ray imaging detector is presently one of the important tools in medical diagnosis of a patient. It provides an excellent image quality, portability, and dose utilization. Amorphous selenium based direct conversion detector with an active matrix array has been in the focus of researchers for the last two decades and extensive work and improvement have been done on this. There are several parameters of an x-ray imaging detector through which the imaging performance of a detector could be measured. The most important measure is the frequency, f , dependent detective quantum efficiency, DQE(f ). In this thesis, we have proposed a parallel cascaded linear system model for calculating DQE(f ) by considering the effects of K-fluorescence reabsorption, the range of primary photo electrons, charge carrier trapping, aperture function, noise aliasing, and addition of electronic noise. DQE (Detective Quantum Efficiency) depends significantly on the transport properties (mobility-lifetime product) and the creation of K-fluorescent x-ray photons. The DQE model is applied to fluoroscopic and mammographic detectors and is validated with the recent published experimental data. It has been found that the DQE( f ) can be improved by ensuring that the carrier with the higher mobility-lifetime product is drifted towards the pixel electrode, i.e., the bottom electrode of the detector. A simplified zero spatial frequency, DQE(0), is also proposed in this thesis. There exists an optimum detector thickness that maximizes the DQE under charge carrier trapping. Although the model is applied to Amorphous Selenium (a-Se) and Mercuric Iodide (HgI 2 ) based imaging detectors, it can also be applied to analyze the DQE(f ) performance of the imaging detectors based on other photoconductive materials like CdZnTe, PbI2 etc.