Purpose: Evaluation of a new sensor for micrometer-resolution photon-counting CT. Approach: DAC-sweeps are performed using a commercial x-ray tube and are compared to simulations. An edge-scan using a 250 µm tungsten wafer without any interaction logic is also performed, as well as single interaction readout of the energy spectrum that is compared to simulations. Results: The edge-scan shows a line spread function with a full width at half maximum of 11.6 µm and a 5% modulation transfer function at 850 lp/cm. Conclusions: Fair agreement with simulations indicated that employing the interaction can further significantly improve spatial resolution.

Purpose: Current photon-counting detectors are limited to a pixel size of 0.3 mm-1 mm, as decreasing the pixel size further generally introduces degraded dose efficiency and energy resolution from excessive charge sharing. In this work, we present experimental measurements of the first photon-counting detector prototype designed to leverage the charge sharing to estimate the photon interaction position, where simulations indicate a theoretical resolution of around 1 µm using a similar geometry. The goal of the measurements is to validate our Monte-Carlo simulation for further development. Approach: DAC sweeps are performed with an X-ray beam at specified locations on the sensor front, with the beam at 20 keV and 35 keV, as well as with different sensor biases with the beam at 35 keV. The experimental data are then compared to a Monte Carlo simulation combined with a charge transport model. In this first prototype wire bonds are used, and as such only a few channels are connected. Results: The experimental data agree generally well with the simulated data with the beam close to the electrodes, with the simulated data diverging from the experiments with the beam further away from the electrodes. The induced charge cloud signal exhibits a fairly linear dependency on the beam position, indicating that any estimation techniques will yield more precise position when the photon interacts further away from the electrodes, rather than closer. Conclusions: With the experimental data and the simulations agreeing generally well, together with the same software previously indicating a resolution of around 1 µm, we expect an ultra-high-resolution detector to be in reach, and are encouraged to continue development.

Purpose: Current photon-counting computed tomography detectors are limited to a pixel size of around 0.3 to 0.5 mm due to excessive charge sharing degrading the dose efficiency and energy resolution as the pixels become smaller. In this work, we present measurements of a prototype photon-counting detector that leverages the charge sharing to reach a theoretical sub-pixel resolution in the order of 1 μm. The goal of the study is to validate our Monte-Carlo simulation using measurements, enabling further development. Approach: We measure the channel response at the MAX IV Lab, in the DanMAX beamline, with a 35 keV photon beam, and compare the measurements with a 2D Monte Carlo simulation combined with a charge transport model. Only a few channels on the prototype are connected to keep the number of wire bonds low. Results: The measurements agree generally well with the simulations with the beam close to the electrodes but diverge as the beam is moved further away. The induced charge cloud signals also seem to increase linearly as the beam is moved away from the electrodes. Conclusions: The agreement between measurements and simulations indicates that the Monte-Carlo simulation can accurately model the channel response of the detector with the photon interactions close to the electrodes, which indicates that the unconnected electrodes introduce unwanted effects that need to be further explored. With the same Monte-Carlo simulation previously indicating a resolution of around 1 μm with similar geometry, the results are promising that an ultra-high resolution detector is not far in the future.

Objective. An ultra-fine-pitch deep silicon detector has been developed for clinical photon-counting computed tomography (CT). With a small pixel size of 14 × 650 μm2, it has shown potential to reach micrometre spatial resolution in previous simulation studies. A detector prototype with such geometry has been manufactured, and we report on the first experimental evaluation of its count-rate performance. Approach. The measurement was carried out at MAX IV synchrotron laboratory with 35 keV monochromatic x-rays. By inserting tungsten attenuators of 50, 75, 100, 150, 200, 225, 325 μ m-thicknesses into the beam, the response of the detector to fluence rates from 3.3 × 107 to 1.3 × 1011 mm−2 s−1 was characterized. Main results. The measurement result showed that the detector exhibited count rate linearity up to 6.66 × 108 mm−2 s−1 with 13% count loss and was still functional at count rate up to 2.9 × 1010 mm−2 s−1. A semi-nonparalyzable dead-time model was fitted to the count-rate behaviour of the detector, showing great agreement with the measured data, with an estimated nonparalyzable dead time of 2.9 ns. Significance. This is the first experimental evaluation of the count-rate performance for a deep silicon detector with such small pixel geometry. The results suggest that this type of detector shows the potential to be used at fluence rates encountered in clinical CT with little count loss due to pile-up.

Purpose: When implementing differential phase-contrast imaging with current CT detectors the limited pixel size forces one to scan an analyser-grating in front of the detector to resolve the interference pattern. This simulation study compares the approach of using an analyser-grating and the approach of using a high-resolution detector to directly resolve the interference pattern by applying the non-prewhitening observer on dose-matched simulated CT scan reconstructions of the two approaches. Approach: A phantom with two concentric cylinders is used and 1000 CT scans are generated with the inner cylinder present and not present. The non-prewhitening observer is applied to the samples and the ROC curve is extracted, together with the results of a 2AFC test, the CNR, and the detectability. Results: The high-resolution approach shows a 20% increase in the phase AUC compared to the grating-based approach, with a similar increase in the 2AFC test score. The image CNR of the phase shows an increase of 134%, with the detectability increasing by 138% when compared to the grating based approach. Conclusions: An ultra-high-resolution detector, capable of directly resolving the interference pattern of differential phase-contrast imaging could change medical imaging CT as we know it. The implementation in a clinical CT would be simpler, could lower cost, and increase the dose-efficiency due to the obviation of the G2-grating, and at the same time provide an additional two diagnostic signals.