A ghost story: Spatio-temporal response characteristics of an indirect-detection flat-panel imager

Spatial and temporal imaging characteristics of an amorphous silicon flat-p anel imager (FPI) were investigated in terms relevant to the application of such devices in cone-beam computed tomography (CBCT) and other x-ray imagi ng modalities, including general radiography, fluoroscopy, tomogramography, radiotherapy portal imaging, and nondestructive testing. Specifically, iss ues of image lag (including the magnitude, spatial uniformity, temporal-fre quency characteristics, and dependence upon exposure and frame time) and lo ng-term image persistence (''ghosts'') were investigated. As part of the ba sic characterization of the FPI, pixel dark signal and noise (magnitude, te mporal stability, and spatial uniformity) as well as radiation response (si gnal size, linearity, gain, and reciprocity) were also measured. Image lag was analyzed as a function of frame time and incident exposure. First-frame lag (i.e., the relative residual signal in the first frame following reado ut of an exposure) was similar to 2-10%, depending upon incident exposure a nd was spatially nonuniform to a slight degree across the FPI; second-, thi rd-, and fourth-frame lag were similar to 0.7%, 0.4%, and 0.3%, respectivel y (at 25% sensor saturation). Image lag was also analyzed in terms of the t emporal-frequency-dependent transfer function derived from the radiation re sponse, allowing a quantitative description of system components contributi ng to lag. Finally, the contrast of objects as a function of time following an exposure was measured in order to examine long-term image persistence ( ''ghosts''). Ghosts were found to persist up to 30 min or longer, depending upon the exposure and frame time. Two means of reducing the apparent contr ast of ghost images were tested: (i) rapid scanning of the EPI at maximum f rame rate, and (ii) flood-field exposure of the FPI; neither was entirely s atisfactory. These results pose important considerations for application of FPIs in CBCT as well as other x-ray imaging modalities. For example in CBC T, the magnitude of image lag is such that significant artifacts in tomogra phic reconstructions may result if strategies are not adopted either to red uce or correct the lag between successive projections (e.g., rapid scanning between projections or iterative correction algorithms, respectively). Sim ilarly, long-term image persistence may necessitate frequent recalibration of offset corrections. (C) 1999 American Association of Physicists in Medic ine. [S0094-2405 (99)00608-2].