[en] The detective quantum efficiency (DQE) of solid state detectors using cadmium tungstate scintillation crystal for x-ray computed tomography (CT) and various factors, which affect the DQE, were estimated by calculations using analytic model and Monte Carlo method. The scattered photon intensity and the cross talk ratio were simulated using MCNP code, and their reduction by collimators and spacers was calculated for their various sizes. The effects of collimators and spacers on the geometric efficiency were also taken into consideration. The absorption efficiency of cadmium tungstate crystal and the light col ection efficiency onto photodiodes surface for various surface conditions of crystal and reflectors were also calculated using DETECT code. As the results, there was no significant effect of cross talk and variations of the collection efficiency were very large and strongly affected the detector sensitivity. The absorption efficiency increased and the light collection efficiency decreased as the crystal height increased. Therefore, the critical height existed, that made the product of both efficiencies maximum. These estimation methods and results can be used to design or to evaluate performances of detection system for computed tomography