Amorphous silicon image sensor arrays are being developed for X-ray imaging and document scanning. The arrays have a two-dimensional structure with each pixel containing a sensor and a TFT, both fabricated with a-Si:H by photolithography on 12 x 13 " glass substrates. The design and characteristics of a recent complete X-ray imaging system is described. Measurements show that the sensor can detect a photocurrent as low as 1 fA, with imager integration times up to 250 s. These results are largely determined by the sensor and TFT leakage currents. New information about the TFT transient off-current is obtained.