Previously two different amorphous selenium (a-Se) photodetector designs were proposed for indirect conversion medical imaging using polyimide (PI) as blocking contacts: a lateral comb structure where a PI layer covers both electrodes and a vertical structure where a PI layer is placed between the positively biased electrode and a-Se. These detectors have raised interest due to their simple structure, ease of fabrication, very low dark current at high electric fields, and high quantum efficiency. In this study we examine the transient behavior of these detectors for fluoroscopic or tomographic applications. The photoresponse to a short light pulse (10-100 μs) is examined in both structures. The presence of charge accumulation at the PI/a-Se interface is investigated by observing the photoresponse of consecutive light pulses under different electric fields. This work demonstrates the promise of low-cost a-Se devices for use in indirect conversion large area digital medical X-ray imaging applications such as real-time fluoroscopy and computed tomography.