Assessment of the spectral performance of hybrid photon counting x‐ray detectors

Abstract

Purpose: Hybrid Photon Counting (HPC) detectors profoundly improved x-ray diffraction experiments at third generation synchrotron facilities. Enabling the simultaneous measurement of x-ray intensities in multiple energy bins, they also have many potential applications in the field of medical imaging. A prerequisite for this is a clean spectral response. To quantify how efficiently HPC detectors are able to assign photons to the correct energy bin, a quantity called Spectral Efficiency (SE) is introduced. This figure of merit measures the number of x-rays with correctly assigned energy normalized to the number of incoming photons. Methods: A prototype HPC detector has been used to perform precision measurements of x-ray spectra at the BESSY synchrotron. The detector consists of a novel ASIC with pixels of 75 9 75 lm 2 size and a 750 lm thick CdTe sensor. The experimental data are complemented by the results of a Monte-Carlo (MC) simulation, which not only includes the physical detection process but also pulse pile-up at high photon fluxes. The spectra and the measured photon flux are used to infer the Spectral Efficiency. Results: In the energy range from 10 to 60 keV, both the Quantum Efficiency and the Spectral Efficiency were precisely measured and simulated. Good agreement between simulation and experiment has been achieved. For the small pixels of the prototype detector, a SE between 15% and 77% has been determined. The MC simulation is used to predict the SE for various pixel sizes at different photon fluxes. For a typical flux of 5•10 7 photons/mm 2 /s used in human Computed Tomography (CT), the highest SE is achieved for pixel sizes in the range between 150 9 150 lm 2 and 300 9 300 lm 2 . Conclusions: The Spectral Efficiency turns out to be a useful figure of merit to quantify the spectral performance of HPC detectors. It allows a quantitative comparison of detectors with different sensor and ASIC configurations over a broad range of x-ray energies and fluxes. The maximization of the SE is a prerequisite for a successful usage of HPC detectors in the field of medical imaging.

The Effects of Spectral X-Ray Photon Counting Detector Parameters on Detector Performance: Thickness and Pitch

“…Similarly, the work presented here has considered the case of a monoenergetic field in a way that is consistent with approaches to physical x-CSI detector assessment undertaken by other groups [20] [30], as it is easier to assess spectral performance in physical systems using monoenergetic or discretised sources such as a synchrotron or radioisotopes. Clinical applications usually involve the use of a polychromatic beam, however spectral metrics will have an impact on final system performance, when it comes to energy dependent tasks such as material decomposition.…”

“…The varying approaches are often difficult to compare directly due to differences in the parameters varied, assumptions built in (notably whether pileup is included) and the metrics against which they are assessed. These differences partly stem from whether the studies are focusing on improving ASIC designs [20] or demonstrating the potential benefits of x-CSI to a particular clinical task [21]. In order to facilitate more consistent comparisons between the major detector parameters and their effects on detector performance this a single framework which allows all of these parameters to be varied and compared to a set list of metrics is needed.…”

Investigation of spectral performance for single‐scan contrast‐enhanced breast CT using photon‐counting technology: A phantom study

“…On the one hand, the smaller detector elements offer a higher spatial resolution, while on the other hand, more cross-talksuch as charge sharing and fluorescent x raysoccurs in pcBCT acquisitions compared to the whole-body CT; as a consequence, the energy resolution is reduced in pcBCT. 54 In addition, the spectral separation required for pcBCT is more demanding, since the x-ray spectrum used is narrow compared to the whole-body scanner (60 kVp vs 150 kVp). Despite these challenges in pcBCT, the spectral performance of the pcBCT and the whole-body scanner is similar.…”

Grating-based spectral X-ray dark-field imaging for correlation with structural size measures

“…They have the ability to measure the incoming X-ray intensities in several energy intervals simultaneously, meaning that the signal is divided into different photon spectra 27 . By that, the energy information is not lost 28 . Therefore, it is possible to image a sample with several different photon spectra within one measurement.…”