The aim of this study was to evaluate YAlO3: Ce (YAP: Ce) and Y+Al+O+: Ce (YAG: Ce) powder phosphors for use in medical imaging radiation detectors. Both materials are of very short emission decay time being promising for real time medical imaging. YAP: Ce and YAG: Ce powders were used to prepare various screens (19-166 mg/cm2). The absolute luminescence efficiency (emitted light flux over incident x-ray exposure), the intrinsic x-ray to light conversion efficiency and the spectral compatibility with various optical sensors were experimentally determined in a wide range of x-ray energies (22-140 kVp). In addition parameters related to image quality such as the modulation transfer function and the detective quantum efficiency were examined. Theoretical models, describing radiation and light transfer, was employed to fit experimental data and to estimate values of optical parameters. Absolute efficiency was found to decrease with x-ray energy. Highest efficiency was obtained for the 88 mg/cm2 YAP: Ce and for the 107 mg/cm2 YAG: Ce screens. The imaging performance of both materials was found lower to currently employed phosphors. However due to their very fast response and their high spectral compatibility to optical sensors they may be considered for use in real time digital imagers.