The concept of composite imaging detectors in which a flat-panel array precedes a second level of detection is presented. In radiotherapy, a dual-energy X-ray projection imager would consist of a pair of stacked, flat-panel imagers. The upper detection of level would consist of a flat-panel array with an overlying X-ray converter and would serve as a diagnostic imager. The lower detection level would consist of a second array with an overlying metal sheet/phosphor screen converter and would serve as a megavoltage imager. Such a composite imager would be of significant assistance in helping to resolve the patient localization problem in megavoltage radiotherapy by facilitating the correlation of patient anatomy obtained with diagnostic images with the position of the megavoltage portal field. In a second realization of a composite imager, a flat-panel array placed directly over the crystal in a SPECT (single photon emission computed tomography) camera, or within the detector ring of a PET (positron emission tomography) scanner, would allow the simultaneous collection of emission and transmission projection data.
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