A flat-panel detector based on a photoconductive detector with an active matrix array provides superior x-ray images. Amorphous selenium (a-Se) is currently the best choice of photoconductor for clinical x-ray image detectors.   Spatial resolution in terms of modulation transfer function (MTF) is an important metric to examine the image quality of a detector. Based on current understanding for diagnostic energy x-rays, the dominant mechanisms responsible for the loss of resolution are: charge carrier trapping, range of primary photoelectrons, and reabsorption of K-fluorescent x-ray photons. Any trapped carrier in the photoconductor layer induces charges not only on the corresponding pixel but also on the neighbouring pixels, and consequently there is a lateral spread of the signal. The absorption of x-ray in the photoconductor first creates a high energy photoelectron that creates hundreds of electron-hole pairs (EHP) along its arbitrary path. This creates an EHP cloud within a volume and destroys image resolution. Some of the K-fluorescent x-ray photons may be reabsorbed at different points within the detector which creates the loss of resolution.   In this thesis, a semi-analytical model for calculating MTF due to the reabsorption of K-fluorescent x-ray photons is proposed. A complete MTF model is proposed and it is evaluated as a function of material properties (carrier lifetime), detector thickness, x-ray photon energy and operating field applied. The analytical MTF model is compared with the published experimental data on various x-ray detectors and shows very good agreement. The overall MTF model can be applied to other direct-conversion detectors such as HgI 2 , PbO and CdZnTe x-ray image detectors as well