Photon-counting detector technology using directly converting high-Z sensor materials has recently gained popularity in medical imaging due to its capability to reduce patient dose, increase spatial resolution and provide single shot multienergy information. However, in medical imaging applications, the novel technology is not yet widely used due to technical challenges in manufacturing gap-less detectors with large areas. Here, a nearly gap-less, large-area, multi-energy photoncounting detector prototype is presented which was built with existing ASIC and sensor technology. It features an active area of 8x8 cm<sup>2</sup>, a 1 mm thick cadmium telluride (CdTe) sensor, four independent energy thresholds and a pixel size of 150 μm. Single and multi-threshold imaging performance of the detector is evaluated by assessing various metrics relevant for conventional (polychromatic) and spectral imaging applications. A high detective quantum efficiency (DQE(0)=0.98), a low dark noise threshold (6.5 keV) and a high count rate capability (up to 3x10<sup>8</sup> counts/s/mm<sup>2</sup>) indicate that the detector is optimally suited for conventional medical X-ray imaging, especially for low dose applications. Spectral performance was assessed by acquiring spectra from fluorescence samples, and the results show a high accuracy of energy peak positions (< 1 keV), precise energy resolution (within a few keV) and decent peak-to-background ratios. Spectral absorption measurements of water and iodinated contrast agent, as well as spectral X-ray radiographs of a human hand phantom, decomposed into bone and soft tissue basis images, demonstrate the multi-energy performance of the detector.
State-of-the-art X-ray breast imaging (BI) modalities such as digital breast tomosynthesis (DBT), contrast enhanced spectral mammography (CESM) and breast CT (BCT) impose demanding requirements on digital X-ray detectors. This work studies the imaging performance of a GaAs two-threshold photon-counting detector (PCD) prototype for BI relevant X-ray spectra. The prototype has a 75 μm pixel size, two calibrated energy thresholds from 8 to 60 keV, 8 x 4 cm<sup>2</sup> area and a 0.5 mm thick GaAs sensor. The X-ray spectra used were 28 and 35 kVp with 2 mm Al filtration from a W-target tube emulating RQA-M2. The main imaging metrics probed include modulation transfer function (MTF) and detective quantum efficiency (DQE). Air kerma spanned three orders of magnitude from 370 nGy to 330 μGy. Furthermore, the detector’s linearity, lag and ghosting were also tested. For 28 kVp, the GaAs PCD exhibits 85% and 48% DQE for 0 and 5 lp/mm respectively, independent of the applied dose. MTF ranges from 98% to 53% for 1 and 6.667 lp/mm (Nyquist limit). Excellent linearity, zero lag and ghosting were observed. GaAs PCD technology is an ideal candidate for BI detector panels. Its stable temporal behavior, inherent zero readout noise and excellent DQE independent of the applied X-ray dose, improve on the combined advantages of current CsI/CMOS and a-Se/a-Si BI detectors. In addition, the multiple energy thresholds can enable spectral single-shot methods without motion blur.