Marcus Zuber

Charge Summing in Spectroscopic X-Ray Detectors With High-Z Sensors

The spectroscopic performance of photon counting detectors is limited by the effects of charge sharing between neighboring pixels and the emission of characteristic X-rays. For these reasons, an event can be either missed or counted more than once. These effects become more and more of a concern when pixel pitches are reduced, and for the technology available so far, this meant that there would always be a trade-off between a high spatial and a high spectral resolution. In this work, we present first measurements obtained with the new Medipix3RX ASIC, which features a network of charge summing circuits establishing a communication between pixels which helps to mitigate these effects. Combined with cadmium telluride sensors, we show that this new technology is successful at improving a detectors spectroscopic capabilities even at pixel pitches as small as 55 μm. At this pitch, we measure an energy response function similar to that observed for a pixel pitch of 165 μm in the absence of a charge summing circuitry. This amounts to an effective reduction of the pixel area by at least one order of magnitude at a comparable energy response. Additionally, we present synchrotron measurements at high X-ray fluxes, where significant pulse pile-up occurs, and provide first experimental evidence for a net benefit when balancing spectroscopic performance and high flux tolerance in charge summing mode.

Review of hybrid pixel detector readout ASICs for spectroscopic X-ray imaging

Semiconductor detector readout chips with pulse processing electronics have made possible spectroscopic X-ray imaging, bringing an improvement in the overall image quality and, in the case of medical imaging, a reduction in the X-ray dose delivered to the patient. In this contribution we review the state of the art in semiconductor-detector readout ASICs for spectroscopic X-ray imaging with emphasis on hybrid pixel detector technology. We discuss how some of the key challenges of the technology (such as dealing with high fluxes, maintaining spectral fidelity, power consumption density) are addressed by the various ASICs. In order to understand the fundamental limits of the technology, the physics of the interaction of radiation with the semiconductor detector and the process of signal induction in the input electrodes of the readout circuit are described. Simulations of the process of signal induction are presented that reveal the importance of making use of the small pixel effect to minimize the impact of the slow motion of holes and hole trapping in the induced signal in high-Z sensor materials. This can contribute to preserve fidelity in the measured spectrum with relatively short values of the shaper peaking time. Simulations also show, on the other hand, the distortion in the energy spectrum due to charge sharing and fluorescence photons when the pixel pitch is decreased. However, using recent measurements from the Medipix3 ASIC, we demonstrate that the spectroscopic information contained in the incoming photon beam can be recovered by the implementation in hardware of an algorithm whereby the signal from a single photon is reconstructed and allocated to the pixel with the largest deposition.

Performance of a Medipix3RX Spectroscopic Pixel Detector With a High Resistivity Gallium Arsenide Sensor

High resistivity gallium arsenide is considered a suitable sensor material for spectroscopic X-ray imaging detectors. These sensors typically have thicknesses between a few hundred μm and 1 mm to ensure a high photon detection efficiency. However, for small pixel sizes down to several tens of μm, an effect called charge sharing reduces a detectors spectroscopic performance. The recently developed Medipix3RX readout chip overcomes this limitation by implementing a charge summing circuit, which allows the reconstruction of the full energy information of a photon interaction in a single pixel. In this work, we present the characterization of the first Medipix3RX detector assembly with a 500 μm thick high resistivity, chromium compensated gallium arsenide sensor. We analyze its properties and demonstrate the functionality of the charge summing mode by means of energy response functions recorded at a synchrotron. Furthermore, the imaging properties of the detector, in terms of its modulation transfer functions and signal-to-noise ratios, are investigated. After more than one decade of attempts to establish gallium arsenide as a sensor material for photon counting detectors, our results represent a breakthrough in obtaining detector-grade material. The sensor we introduce is therefore suitable for high resolution X-ray imaging applications.

How spectroscopic x-ray imaging benefits from inter-pixel communication.

Spectroscopic x-ray imaging based on pixellated semiconductor detectors can be sensitive to charge sharing and K-fluorescence, depending on the sensor material used, its thickness and the pixel pitch employed. As a consequence, spectroscopic resolution is partially lost. In this paper, we study a new detector ASIC, the Medipix3RX, that offers a novel feature called charge summing, which is established by making adjacent pixels communicate with each other. Consequently, single photon interactions resulting in multiple hits are almost completely avoided. We investigate this charge summing mode with respect to those of its imaging properties that are of interest in medical physics and benchmark them against the case without charge summing. In particular, we review its influence on spectroscopic resolution and find that the low energy bias normally present when recording energy spectra is dramatically reduced. Furthermore, we show that charge summing provides a modulation transfer function which is almost independent of the energy threshold setting, which is in contrast to approaches common so far. We demonstrate that this property is directly linked to the detective quantum efficiency, which is found to increase by a factor of three or more when the energy threshold approaches the photon energy and when using charge summing. As a consequence, the contrast-to-noise ratio is found to double at elevated threshold levels and the dynamic range increases for a given counter depth. All these effects are shown to lead to an improved ability to perform material discrimination in spectroscopic CT, using iodine and gadolinium contrast agents. Hence, when compared to conventional photon counting detectors, these benefits carry the potential of substantially reducing the imaging dose a patient is exposed to during diagnostic CT examinations.

Count rate linearity and spectral response of the Medipix3RX chip coupled to a 300μm silicon sensor under high flux conditions

For clinical X-ray imaging, the detector performance under high flux conditions is very important, with typical flux rates for modern CT systems reaching 109 photons s−1 mm−2 in the direct beam. In addition, for spectral imaging a good energy resolution under these conditions is needed. This poses difficulties, since pulse pileup in the pixel electronics does not only affect the count rate, leading to a deviation from the otherwise linear behavior, but also degrades the spectral response of the detector, making k-edge subtraction and other contrast enhancement techniques less efficient. In this paper, we investigate the count rate capabilities and the energy response of the Medipix3RX chip under high flux conditions using 10 keV monochromatic photons.

A comparison of various strategies to equalize the lower energy thresholds of a CdTe Medipix2 hexa detector for X-ray imaging applications

Technological advances have made possible the development of pixelized photon counting semiconductor detectors, many of which are used in X-ray imaging to resolve the spectral composition of the incident photons. Here, in a so-called Hexa detector, we employ a 3 × 2 array of Medipix2 MXR readout chips, bump bonded to a cadmium telluride sensor of 1 mm thickness with a pixel pitch of 165 μm. Each pixel in this assembly offers two variable energy thresholds, which enables counting of only those photons within an energy range of interest. As manufacturing tolerances cause deviations in each of the pixels responses, the two thresholds can be calibrated for every pixel to render their response to radiation more homogeneous. In this work, we compare various methods that we chose to equalize the lower thresholds: a) the noise edge of the detector electronics, the characteristic X-rays from b) silver and c) tantalum foils as well as flat fields obtained at d) 40 and e) 120 kVp. It will be shown that the energy dependence in the resulting adjustment bit maps are only small, whereas the question as to which strategy to choose (peak position vs. image homogeneity) will have a greater influence on the resulting corrections. Additionally, we observed a decrease in the mean adjustment values with increasing distance from the central axis of the Hexa detector under study.

On the origin and nature of the grating interferometric dark-field contrast obtained with low-brilliance x-ray sources.

The x-ray dark-field contrast accessible via grating interferometry is sensitive to features at length scales well below what is resolvable by a detector system. It is commonly explained as arising from small-angle x-ray scattering (SAXS), and can be implemented both at synchrotron beamlines and with low-brilliance sources such as x-ray tubes. Here, we demonstrate that for tube based setups the underlying process of image formation can be fundamentally different. For focal spots or detector pixels that comprise multiple grating periods, we show that dark-field images contain a strong artificial and system-specific component not arising from SAXS. Based on experiments carried out with a nanofocus x-ray tube and the example of an excised rat lung, we demonstrate that the dark-field contrast observed for porous media transforms into a differential phase contrast for large geometric magnifications. Using a photon counting detector with an adjustable point spread function, we confirm that a dark-field image can indeed be formed by an intra-pixel differential phase contrast that cannot be resolved as such due to a dephasing between the periodicities of the absorption grating and the Talbot carpet. Our findings are further corroborated by a link between the strength of this pseudo-dark-field contrast and our x-ray tubes focal spot size in a three-grating setup. These results must not be ignored when measurements are intended to be reproducible across systems.

An investigation into the temporal stability of CdTe-based photon counting detectors during spectral micro-CT acquisitions

Spectroscopic micro-computed tomography (CT) using photon counting detectors is a technology that promises to deliver material-specific images in pre-clinical research while suppressing noise related to dark current. Inherent to such applications is the need for a high spatial resolution, which can only be achieved with small focal spot sizes in the micrometre range. This limits the achievable x-ray fluxes and implies long acquisitions easily exceeding one hour, during which it is paramount to maintain a constant detector response. During the past years, the Medipix3RX detector has become a popular platform for research into low-flux and multi-energy CT, as it hosts a number of elaborate features that may serve as an inspiration for future systems when studied carefully. In this contribution, we illustrate our findings on how to reduce image artefacts in CT reconstructions using such a detector with a cadmium telluride sensor. We find that maintaining a constant temperature is a prerequisite to guarantee energy threshold stability. More importantly, we identify transient sensor leakage currents as a significant source to artefact formation. We demonstrate an improvement in image quality when allowing the chip to compensate for abnormally high leakge currents. The reduction of spectral resolution reported earlier for such settings is found to have no detrimental effect on material decomposition. Quite the contrary, the improved handling of transient leakage currents allows to even increase contrast-to-noise ratios in material decomposed images due to a reduced artefact level.

Imaging properties of high aspect ratio absorption gratings for use in preclinical x-ray grating interferometry.

X-ray grating interferometry is one among various methods that allow extracting the so-called phase and visibility contrasts in addition to the well-known transmission images. Crucial to achieving a high image quality are the absorption gratings employed. Here, we present an in-depth analysis of how the grating type and lamella heights influence the final images. Benchmarking gratings of two different designs, we show that a frequently used proxy for image quality, a gratings so-called visibility, is insufficient to predict contrast-to-noise ratios (CNRs). Presenting scans from an excised rat lung, we demonstrate that the CNRs obtained for transmission and visibility images anti-correlate. This is explained by the stronger attenuation implied by gratings that are engineered to provide high visibilities by means of an increased lamella height. We show that even the visibility contrast can suffer from this effect when the associated reduced photon flux on the detector is not outweighed by a corresponding gain in visibility. Resulting in an inevitable trade-off between the quality of the two contrasts, the question of how an optimal grating should be designed can hence only be answered in terms of Pareto optimality.

Saturation effects of CdTe photon counting detectors under high photon fluxes

Pixelated photon counting semiconductor detectors enable to resolve the spectral composition of the incident photons in X-ray imaging, provided that a detector is operated in the absence of pulse pile-up and sensor polarisation. The purpose of this study was to examine the imaging properties of Medipix2 MXR arrays under high photon fluxes, which do not meet this requirement. The detectors studied feature a 1 mm thick cadmium telluride sensor and pixel pitches of 110 μm and 165 μm. We show that the critical photon flux, at which the detectors linear response breaks down, can be shifted towards higher values by increasing the IKrum current in the charge sensitive preamplifier, which corresponds to reducing the pulse shaping time. The negative impact of this procedure on the energy response function, which was measured using an 241Am source, seems only moderate. Furthermore, we describe various kinds of image artefacts due to sensor defects. While bulks of non-counting pixels always appear at the same position and shrink with increased IKrum values, non-counting single pixels randomly show up at different positions during image acquisition and their number rises with an increased photon flux. Additionally, we demonstrate that the critical flux varies among the pixels, leading to large-scale image inhomogeneities at high photon fluxes.