Evaluation of a photon counting Medipix3RX cadmium zinc telluride spectral x-ray detector

The recent availability of functional cadmium zinc telluride (CZT)-based spectral x-ray detectors, with effective charge sharing correction capabilities, presents enhanced opportunities for spectral x-ray imaging.1,2 The transition from silicon to a CZT-based detection layer significantly extends the upper limit of the x-ray energy operating range.3,4 This extended energy range encompasses the K absorption edges of several contrast enhancement materials of interest, including: iodine, barium, gadolinium, and gold. There is work underway to explore these materials for improved contrast agents.5–11 This work reports our evaluation of a CZT-based Medipix3RX photon counting detector (PCD).12 Historically, the design of the Medipix3RX was similar to that of its predecessor, the Medipix3, and provided the users with the ability to operate in either of the two mutually exclusive “pixel matrix modes” [fine pitch mode (FM) and spectroscopic mode (SM)] and also to select between the two mutually exclusive “acquisition modes” [single-pixel mode (SPM) and charge summing mode (CSM)].5,11,13–17 The Medipix3RX device our group used throughout all evaluations presented in this work was specifically designed for use in the Medipix All Resolution System (MARS) spectral microcomputed tomography (micro-CT) scanner (MARS Bioimaging Ltd., Christchurch, New Zealand). Considering the detector’s intended integration into the MARS scanner, the manufacturer elected to implement additional design features, including: a unique bump bonding scheme (detector pixel array restricted to 128×128) and software imposed operational mode (CSM by default). These design constraints allowed each pixel’s four charge summed counters to acquire simultaneous and noise correlated images. However, they also confine the detector to one functional setting: SM pixel matrix mode paired with CSM acquisition mode. The SPM acquisition mode (no CSM or option to disable CSM) and the FM pixel matrix mode (detector pixel array size 256×256) were beyond the scope of our investigation for that reason; however, information using these modes are detailed within evaluations, which used other Medipix3RX design modes.5,14 The performance and application of spectral PCDs are generally limited by the negative effects incurred by charge sharing and pulse pileup.15 The application-specific integrated circuit (ASIC) of our group’s Medipix3RX has charge summing circuitry, which is established across every 2×2 cluster of 110-μm pixels, and each of these clusters form a so-called “summing node.” All of the detector’s summing nodes are designed to exhibit the same response to charge sharing events. First, each of the partial pulses detected by the node’s four pixels are summed together. Second, the original incident photon’s full pulse height is allocated to the single pixel (within that 2×2 cluster), which received the largest partial pulse.13,14,16 In CSM, each pixel has four photon counters which are charged summed, and these only tally incident photons with energies greater than their specific energy threshold settings.17 The pulse pileup phenomenon occurs when a single pixel on a PCD experiences the arrival of two incident photons with insufficient time between their depositions for individual processing.15,18 The vendor of our Medipix3RX has expressed that operating the device in circumstances where pixels experience a photon count rate exceeding their recommended maximum count rate of 10 counts/ms/pixel will incite pulse pileup. Earlier research performed with the Medipix3RX device has investigated topics that include: the use of a silicon sensor, CSM, and FM in the spectral x-ray imaging of material K-edges,14 the use of a gadolinium arsenide sensor, CSM, and FM in a spectral CT scanning scenario,5 and the application and effectiveness of CSM in comparison to SPM.14,17,19 Some recent work involving the MARS scanner has focused on the use of its spectral scan data for x-ray beam profile assessments20 and also in material decomposition algorithms.9 However, a systematic evaluation of the Medipix3RX detector in terms of fundamental performance parameters such as linearity, pile up, uniformity, and stability has not been conducted. The main purpose of our study was to analyze the fundamental characteristics of a CZT-based Medipix3RX detector (operated using the SM and CSM modes), which was designed to be used in the MARS spectral micro-CT imaging system. We present six distinct evaluations of the Medipix3RX in this report, each of which was devised to examine a certain performance feature directly applicable to spectral micro-CT imaging. Based on the results of this study, we found that the performance of this device makes it an acceptable candidate for use in spectral micro-CT imaging equipment.12