Ghosts were found to persist up to 30 min or longer, depending upon the exposure and frame time. Finally, the contrast of objects as a function of time following an exposure was measured in order to examine long-term image persistence (“ghosts”). Image lag was also analyzed in terms of the temporal-frequency-dependent transfer function derived from the radiation response, allowing a quantitative description of system components contributing to lag. First-frame lag (i.e., the relative residual signal in the first frame following readout of an exposure) was ∼2–10%, depending upon incident exposure and was spatially nonuniform to a slight degree across the FPI second-, third-, and fourth-frame lag were ∼0.7%, 0.4%, and 0.3%, respectively (at 25% sensor saturation). Image lag was analyzed as a function of frame time and incident exposure. As part of the basic characterization of the FPI, pixel dark signal and noise (magnitude, temporal stability, and spatial uniformity) as well as radiation response (signal size, linearity, gain, and reciprocity) were also measured. Specifically, issues of image lag (including the magnitude, spatial uniformity, temporal-frequency characteristics, and dependence upon exposure and frame time) and long-term image persistence (“ghosts”) were investigated. Spatial and temporal imaging characteristics of an amorphous silicon flat-panel imager (FPI) were investigated in terms relevant to the application of such devices in cone-beam computed tomography (CBCT) and other x-ray imaging modalities, including general radiography, fluoroscopy, mammography, radiotherapy portal imaging, and nondestructive testing.
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