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- Danielsson, Mats, Professor, et al.
(författare)
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Photon-counting x-ray detectors for CT
- 2021
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Ingår i: Physics in Medicine and Biology. - : IOP Publishing. - 0031-9155 .- 1361-6560. ; 66:3, s. 03TR01-
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Forskningsöversikt (refereegranskat)abstract
- The introduction of photon-counting detectors is expected to be the next major breakthrough in clinical x-ray CT. During the last decade, there has been considerable research activity in the field of photon-counting CT, in terms of both hardware development and theoretical understanding of the factors affecting image quality. In this article, we review the recent progress in this field with the intent of highlighting the relationship between detector design considerations and the resulting image quality. We discuss detector design choices such as converter material, pixel size, and readout electronics design, and then elucidate their impact on detector performance in terms of dose efficiency, spatial resolution, and energy resolution. Furthermore, we give an overview of data processing, reconstruction methods and metrics of imaging performance; outline clinical applications; and discuss potential future developments.
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- Sundberg, Christel, et al.
(författare)
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Silicon photon-counting detector for full-field CT usingan ASIC with adjustable shaping time
- 2020
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Ingår i: Journal of Medical Imaging. - : SPIE - International Society for Optical Engineering. - 2329-4302 .- 2329-4310. ; 7:5
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Tidskriftsartikel (refereegranskat)abstract
- Purpose: Photon-counting silicon strip detectors are attracting interest for use in next-generation CT scanners. For CT detectors in a clinical environment, it is desirable to have a low power consumption. However, decreasing the power consumption leads to higher noise. This is particularly detrimental for silicon detectors, which require a low noise floor to obtain a good dose efficiency. The increase in noise can be mitigated using a longer shaping time in the readout electronics. This also results in longer pulses, which requires an increased deadtime, thereby degrading the count-rate performance. However, as the photon flux varies greatly during a typical CT scan, not all projection lines require a high count-rate capability. We propose adjusting the shaping time to counteract the increased noise that results from decreasing the power consumption.Approach: To show the potential of increasing the shaping time to decrease the noise level, synchrotron measurements were performed using a detector prototype with two shaping time settings. From the measurements, a simulation model was developed and used to predict the performance of a future channel design.Results: Based on the synchrotron measurements, we show that increasing the shaping time from 28.1 to 39.4 ns decreases the noise and increases the signal-to-noise ratio with 6.5% at low count rates. With the developed simulation model, we predict that a 50% decrease in power can be attained in a proposed future detector design by increasing the shaping time with a factor of 1.875.Conclusion: Our results show that the shaping time can be an important tool to adapt the pulse length and noise level to the photon flux and thereby optimize the dose efficiency of photon-counting silicon detectors.
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- Brunskog, Rickard, et al.
(författare)
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Experimental Evaluation of a Micron-Resolution CT Detector
- 2024
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Ingår i: Medical Imaging 2024: Physics of Medical Imaging. - : SPIE-Intl Soc Optical Eng.
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Konferensbidrag (refereegranskat)abstract
- 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.
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- Brunskog, Rickard, et al.
(författare)
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First experimental evaluation of a high-resolution deep silicon photon-counting sensor
- 2024
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Ingår i: Journal of Medical Imaging. - : SPIE-Intl Soc Optical Eng. - 2329-4302 .- 2329-4310. ; 11:1
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 7. |
- Daniel, Maxime, et al.
(författare)
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Charge collection efficiency of CdTe detectors : Impact of charge collection time and polarisation
- 2023
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Ingår i: Medical Imaging 2023. - : SPIE-Intl Soc Optical Eng.
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Konferensbidrag (refereegranskat)abstract
- Cadmium telluride (CdTe) is one of the materials used in photon-counting detectors for x-ray computed tomography. One challenge with this material is that it is susceptible to polarisation due to holes being trapped in impurities in the material. This can potentially lead to the buildup of bulk charge in the semiconductor, causing decreased charge collection efficiency and degraded energy resolution. In this work, we develop a simulation model of CdTe detectors with polarisation and use it to study the effect of polarisation on the measured energy spectrum for different charge collection times. To this end, we use a theoretical model of charge buildup to find the critical charge in the detector's bulk above which the detector can be considered completely polarised. We then simulate a 320-by-270-by-1600 μm CdTe detector used in CT clinical imaging, for varying degrees of polarisation (ratio between the actual charge and the critical charge) and charge collection time. Our results show that the measured spectrum gets heavily distorted for large degrees of polarisation or for short charge collection time. We also put these results in context by discussing how they relate to the critical fluence rate and the time of flight of the charge carriers. These results can lead to improved simulation models of CdTe detectors and a better understanding the factors affecting their imaging performance.
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- Jin, Zihui, et al.
(författare)
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First experimental evaluation of count-rate performance for micrometre resolution deep silicon detector
- 2024
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Ingår i: Physics in Medicine and Biology. - : IOP Publishing. - 0031-9155 .- 1361-6560. ; 69:3
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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