We discuss how the frequency dependent detective quantum efficiency [DQE(f)] in a well-designed amorphous silicon flat panel detector is affected by several phenomena that reduce the DQE in other types of medical imaging detectors. The detector examined employs a CsI(Tl) scintillator and is designed for general diagnostic x-ray imaging applications. We consider DQE degradation due to incomplete x-ray absorption, secondary quantum noise, Swank factor, Lubberts effect, spatial variation in gain, noise aliasing, and additive system noise. The influences of detector design parameters on the frequency- and exposure-dependent DQE are also examined. We find that the DQE does not depend directly on MTF and that DQE is independent of exposure within the detector's operating range, except at the lowest exposures. Likewise the signal per absorbed x-ray, which contains the fill factor as one of several multiplicative components, does not affect DQE except at the lowest exposures. A methodology for determining DQE(f) from measurements of MTF(f), noise power spectrum (NPS), average signal, and x-ray exposure is presented. We find that it is important to incorporate several corrections in the NPS measurement procedure in order to obtain accurate results. These include corrections for lag, non-linearity, response variation from pixel to pixel, and use of a finite number of flat-field images. MTF, NPS, and DQE results are presented for a 41 X 41-cm2 flat panel detector designed for radiographic applications.