Detective quantum efficiency of direct, flat panel x-ray imaging detectors for fluoroscopy

Our goal is to develop a large area, flat panel solid-state detector for fluoroscopy. The detector employs a layer of photoconductor to convert incident x-rays directly to a charge image, which is then read out in real-time using a two dimensional array of thin film transistors (TFTs), or "active matrix". In order to guide the design of an optimum fluoroscopic flat-panel detector, a cascaded linear systems model was developed, from which the spatial frequency dependent detective quantum efficiency (DQE(f)) can be obtained. Then DQE(f) was calculated as a function of different detector design parameters, e.g. pixel fill-factor, x-ray exposure, Swank factor, electronic noise, and the calculation was performed for three different x-ray photoconductors: amorphous selenium (a-Se), cadmium zinc telluride (CZT), and lead iodide (PbI₂). A critical comparison was made of the advantages and disadvantages of each photoconductor. The results showed that the DQE(0) of all direct detectors has a linear dependence on the pixel fill-factor. For an a-Se layer with an electric field of 10 V/μm, DQE(f) is significantly degraded by the electronic noise of the detector, especially at very low x-ray exposure rates (e.g. 0.1 μR/frame). With CZT and PbI₂, the detector is more tolerant of electronic noise because of the larger number of charge generated for each absorbed x-ray. We have applied our cascaded linear systems model of the direct, flat-panel detector to fluoroscopy. The theoretical predictions of DQE(f) for different detector parameters, e.g. the type of x-ray photoconductor, fill-factor, and electronic noise, provide a guideline for an optimum detector design for fluoroscopy.