Edgar Kraft

First results from a hybrid prototype CT scanner for exploring benefits of quantum-counting in clinical CT

μWe introduce a novel hybrid prototype scanner built to explore benefits of the quantum-counting technique in the context of clinical CT. The scanner is equipped with two measurement systems. One is a CdTe-based counting detector with 22cm field-of-view. Its revised ASIC architecture allows configuration of the counter thresholds of the 225m small sub-pixels in chess patterns, enabling data acquisition in four energy bins or studying high-flux scenarios with pile-up trigger. The other one is a conventional GOS-based energy-integrating detector from a clinical CT scanner. The integration of both detection technologies in one CT scanner provides two major advantages. It allows direct comparison of image quality and contrast reproduction as well as instantaneous quantification of the relative dose usage and material separation performance achievable with counting techniques. In addition, data from the conventional detector can be used as complementary information during reconstruction of the images from the counting device. In this paper we present CT images acquired with the hybrid prototype scanner, illustrate its underlying conceptual methods, and provide first experimental results quantifying clinical benefits of quantum-counting CT.

A research prototype system for quantum-counting clinical CT

Recent publications emphasize the benefits of quantum-counting applied to the field of Computed Tomography (CT). We present a research prototype scanner with a CdTe-based quantum-counting detector and 20 cm field-of-view (FOV). As of today there is no direct converter material on the market able to operate reliably in the harsh high-flux regime of clinical CT scanners. Nevertheless, we investigate the CT imaging performance that could be expected with high-flux capable material. Therefore we chose pixel sizes of 0.05 mm2, a good compromise between high-flux counting ability and energy resolution. Every pixel is equipped with two energy threshold counters, enabling contrast-optimization and dual-energy scans. We present a first quantitative analysis of contrast measurements, in which we limit ourselves to a low-flux scenario. Using an Iodine-based contrast agent, we find 17% contrast enhancement at 120 kVp, compared to energy-integrating CT. In addition, the general dual-energy capability was confirmed in first measurements. We conclude our work by demonstrating good agreement of measurement results and detailed CT-system simulations.

Quantum-counting CT in the regime of count-rate paralysis: introduction of the pile-up trigger method

The application of quantum-counting detectors in clinical Computed Tomography (CT) is challenged by extreme X-ray fluxes provided by modern high-power X-ray tubes. Scanning of small objects or sub-optimal patient positioning may lead to situations where those fluxes impinge on the detector without attenuation. Even in operation modes optimized for high-rate applications, with small pixels and high bias voltage, CdTe/CdZnTe detectors deliver pulses in the range of several nanoseconds. This can result in severe pulse pile-up causing detector paralysis and ambiguous detector signals. To overcome this problem we introduce the pile-up trigger, a novel method that provides unambiguous detector signals in rate regimes where classical rising-edge counters run into count-rate paralysis. We present detailed CT image simulations assuming ideal sensor material not suffering from polarization effects at high X-ray fluxes. This way we demonstrate the general feasibility of the pile-up trigger method and quantify resulting imaging properties such as contrasts, image noise and dual-energy performance in the high-flux regime of clinical CT devices.

Experimental evaluation of the pile-up trigger method in a revised quantum-counting CT detector

The application of quantum-counting detectors in clinical Computed Tomography (CT) is challenged by very large Xray photon fluxes present in modern systems. Situations with sub-optimal patient positioning or scanning of small objects can cause unattenuated exposure of parts of the detector. The typical pulse durations in CdTe/CdZnTe sensor range in the order of several nanoseconds, even if the detector design is optimized for high-rate applications by using high sensor depletion voltages and small pixel sizes. This can lead to severe pile-up of the pulses, resulting in count efficiency degradation or even ambiguous detector signals. The recently introduced pile-up trigger method solves this problem by combining the signal of a photon counting channel with a signal indicative of the level of pile-up. Latter is obtained with a photon-counting channel operated at threshold energies beyond the maximum energy of the incident photon spectrum so that its signal arises purely from pulse pile-up. We present an experimental evaluation of the pile-up trigger method in a revised quantum-counting CT detector and compare our results to simulations of the method with idealized detector properties.