ROI imaging techniques can improve image quality and reduce radiation dose to patient and staff when the optimal combination of filter material and image receptor is used. An investigation was conducted to evaluate the effect of thickness of a-Se (0.5 mm and 1.0 mm) on image contrast, contrast to noise ratio (CNR), and figure of merit (FOM) with or without ROI filters (various thickness' of Gd and Cu) and to compare the results with the corresponding values obtained using a 0.4 mm thick cesium iodide (CsI) image receptor. Simulated x-ray spectra and published attenuation coefficients were used to calculate the x-ray transmission through a broad range of thickness' of various contrast materials for beams of 50 to 100 kVp. The results indicate that a-Se provides substantially better contrast compared to CsI for barium and iodinated contrast media for all cases, especially when the combination of Gd filter and the thinner a-Se is used. Moreover, during ROI procedures, the thicker a-Se generally increases image contrast, CNR, and FOM compared to CsI. Although, the thinner a-Se provides the highest image contrast for all cases, its combination with Cu results in lower CNR and FOM at higher kVp's compared to CsI.