【課題】改良されたＸ線検出器システムを提供する。【解決手段】Ｘ線源からのＸ線放射を検出するフォトンカウンティングＸ線検出器（２０）、及び前記Ｘ線検出器における光子相互作用の時間に関する情報と、前記Ｘ線検出器に対する前記Ｘ線源の位置に関する情報とに基づいて、前記Ｘ線検出器に入射する放射線に関する情報を決定する及び／又は取得する同時計数検出システム（６０）を含むＸ線検出器システム（５）を提供する。このようなＸ線検出器システムを含むＸ線イメージングシステム、並びに対応する同時計数検出システム及び対応する方法も提供する。【選択図】図２Ｂ

Purpose: Our purpose is to investigate the timing resolution in edge-on silicon strip detectors for photon-counting spectral computed tomography. Today, the timing for detection of individual x-rays is not measured, but in the future, timing information can be valuable to accurately reconstruct the interactions caused by each primary photon. Approach: We assume a pixel size of 12 x 500 mu m(2) and a detector with double-sided readout with low-noise CMOS electronics for pulse processing for every pixel on each side. Due to the electrode width in relation to the wafer thickness, the induced current signals are largely dominated by charge movement close to the collecting electrodes. By employing double-sided readout electrodes, at least two signals are generated for each interaction. By comparing the timing of the induced current pulses, the time of the interaction can be determined and used to identify interactions that originate from the same incident photon. Using a Monte Carlo simulation of photon interactions in combination with a charge transport model, we evaluate the performance of estimating the time of the interaction for different interaction positions. Results: Our simulations indicate that a time resolution of 1 ns can be achieved with a noise level of 0.5 keV. In a detector with no electronic noise, the corresponding time resolution is similar to 0.1 ns. Conclusions: Time resolution in edge-on silicon strip CT detectors can potentially be used to increase the signal-to-noise-ratio and energy resolution by helping in identifying Compton scattered photons in the detector.

Computed tomography (CT) is a medical imaging modality in which cross-sectional images of the human body are created using x-rays. Commercial CT scanners utilize energy-integrating detectors to measure the x-ray attenuation. However, photon-counting detectors with energy-discriminating abilities have started to emerge. In a photon-counting spectral detector, photons can be counted individually and the photon energy is registered using energy thresholds. In contrast to energy-integrating detectors, which integrate all photon energies during a measurement interval, this allows for an improved detector performance including an increased signal-to-noise ratio, higher spatial resolution, and improved spectral imaging.

One of the current photon-counting systems that is being evaluated for clinical use is the deep silicon detector developed by the Physics of Medical Imaging group at KTH. This Thesis is based on the deep silicon detector concept and focuses on methods to improve the performance of a silicon photon-counting detector for CT and how these might facilitate event reconstruction. In the first part of the Thesis, three different methods to improve the detector performance are presented. One of the methods describes how information about the charge cloud distribution can be used to improve the spatial resolution. With the proposed method, subpixel resolution can be achieved, corresponding to a spatial resolution equivalent of approximately 1 μm in the most accurate dimension. A silicon detector with double-sided readout electrodes is further proposed which enables estimating the time of the photon interaction with high accuracy. The resulting time resolution of approximately 1 ns can potentially be utilized to identify interactions that originate from the same incident photon. With double-sided readout, it is also possible to dramatically improve the spatial resolution in the direction across the silicon wafer thickness. It is also proposed to utilize an adjustable shaping time in the readout electronics to decrease the electronic noise level. This can be used to improve the detector performance with respect to dose efficiency and power consumption.

In the second part of the Thesis, a method to perform event reconstruction is presented. The method consists of a framework of likelihood functions that are used to estimate the incident photon energy and primary interaction position. Based on this framework, the ability of estimating the photon energy and primary interaction position is evaluated for a case in which the incident photons are assumed to be well-separated in time.

In summary, there is potential in increasing the performance with respect to the spatial, temporal, and energy resolution in silicon photon-counting detectors for CT and the results suggest that event reconstruction might be possible in the future.

Datortomografi (CT) är en bildgivande medicinsk teknik som används för att skapa tvärsnittsbilder av människokroppen med hjälp av röntgenstrålning. Kommersiella CT-scannrar använder energiintegrerande detektorer för att mäta röntgenstrålarnas attenuering. Fotonräknande detektorer med spektral upplösningsförmåga har dock börjat introduceras. I en sådan detektor kan fotonerna räknas individuellt och fotonenergin registreras med hjälp av energitrösklar. Jämfört med energiintegrerande detektorer, där energin från de infallande fotonerna integreras under ett visst mätintervall, möjliggör detta en förbättrad detektorprestanda såsom ökat signal-brusförhållande, högre spatiell upplösning samt förbättrad spektral avbildning.

Ett av de fotonräknande system som just nu utvärderas kliniskt är deep silicon-detektorn som utvecklats av gruppen inom Medicinsk Bildfysik vid KTH. Denna avhandling utgår ifrån konceptet för deep silicon-detektorer och studerar metoder för att förbättra prestandan hos en fotonräknande kiseldetektor för CT samt hur dessa skulle kunna möjliggöra rekonstruktion av fotoninteraktioner. I avhandlingens första del presenteras tre olika metoder för att förbättra detektorns prestanda. En av metoderna beskriver hur information om laddningsmolnets fördelning i detektorn kan användas för att förbättra den spatiella upplösningen. Med den föreslagna metoden kan subpixelupplösning uppnås, motsvarande en spatiell upplösning på ungefär 1 μm i den dimension där upplösningen är som högst. För att kunna uppskatta tiden för varje fotoninteraktion med hög noggrannhet föreslås en kiseldetektor med dubbelsidiga avläsningselektroder. Denna konfiguration resulterar i en tidsupplösning på åtminstone 1 ns som potentiellt kan användas för att identifiera interaktioner som härrör från samma infallande foton. Med dubbelsidig avläsning är det också möjligt att förbättra den spatiella upplösningen i riktningen tvärsöver kiselskivans tjocklek. För att påverka den elektroniska brusnivån föreslås användandet av en justerbar pulsbredd i detektorns avläsningselektronik. Detta kan förbättra detektorns doseffektivitet samt energiförbrukning.

I avhandlingens andra del presenteras en metod för att utföra rekonstruktion av fotoninteraktioner. Metoden består av ett ramverk av sannolikhetsfunktioner som används för att estimera den infallande fotonenergin och den primära interaktionspositionen. Baserat på detta ramverk utvärderas förmågan att uppskatta fotonenergin för ett fall där de infallande fotonerna antas vara väl åtskilda i tiden.

Sammanfattningsvis finns det potential i att förbättra prestandan med avseende på spatiell, tids- och energiupplösning i fotonräknande kiseldetektorer för CT och resultaten tyder på att händelserekonstruktion kan vara möjlig i framtiden.

Purpose: Spatial resolution for current scintillator-based computed tomography (CT) detectors is limited by the pixel size of about 1 mm. Direct conversion photon-counting detector prototypes with silicon- or cadmium-based detector materials have lately demonstrated spatial resolution equivalent to about 0.3 mm. We propose a development of the deep silicon photon-counting detector which will enable a resolution of 1  μm, a substantial improvement compared to the state of the art.

Approach: With the deep silicon sensor, it is possible to integrate CMOS electronics and reduce the pixel size at the same time as significant on-sensor data processing capability is introduced. A Gaussian curve can then be fitted to the charge cloud created in each interaction.We evaluate the feasibility of measuring the charge cloud shape of Compton interactions for deep silicon to increase the spatial resolution. By combining a Monte Carlo photon simulation with a charge transport model, we study the charge cloud distributions and induced currents as functions of the interaction position. For a simulated deep silicon detector with a pixel size of 12  μm, we present a method for estimating the interaction position.

Results: Using estimations for electronic noise and a lowest threshold of 0.88 keV, we obtain a spatial resolution equivalent to 1.37  μm in the direction parallel to the silicon wafer and 78.28  μm in the direction orthogonal to the wafer.

Conclusions: We have presented a simulation study of a deep silicon detector with a pixel size of 12  ×  500  μm2 and a method to estimate the x-ray interaction position with ultra-high resolution. Higher spatial resolution can in general be important to detect smaller details in the image. The very high spatial resolution in one dimension could be a path to a practical implementation of phase contrast imaging in CT.

An x-ray imaging system includes an x-ray source and detector. The detector is a photon counting x-ray detector, enabling detection of photon-counting events. The system acquires at least one phase contrast image based on photon-counting events. The detector includes x-ray detector sub-modules, also referred to as wafers, each including detector elements. The sub-modules are oriented in edge-on geometry with their edge directed towards the x-ray source, assuming the x-rays enter through the edge. Each sub-module or wafer has a thickness with two opposite sides of different potentials to enable charge drift towards the side, where the detector elements/pixels, are arranged. The system estimates charge diffusion from a Compton interaction or an interaction through photoeffect related to an incident x-ray photon in a sub-module or wafer of the x-ray detector, and estimates a point of interaction of the x-ray photon sub-module based on the determined estimate of charge diffusion.

One of the existing prototype detector systems for full-field photon-counting CT is a silicon detector developed by our group. Spatial resolution is clinically important to resolve small details and can enable more efficient phase-contrast imaging. However, improving the resolution is difficult as decreasing the pixel size is associated with technical challenges. By integrating CMOS electronics into the silicon sensor, it is possible to reduce the pixel size drastically while also introducing on-sensor data processing capabilities. In this work, we evaluate the feasibility of measuring the charge cloud shape of Compton interactions in a silicon strip detector to increase the spatial resolution. With an incident spectrum of 140 kVp, Compton interactions constitute 66.2% of the detected interactions. By combining a Monte Carlo photon simulation with a charge transport model, we study the charge cloud distributions and induced currents as functions of the interaction position. For a simulated silicon strip detector with a pixel size of 12×500 µm2, we present a method in which the interaction position can be determined. For an ideal case without electronic noise an average absolute error of 0.65 µm is obtained in the direction along the wafer and 13.08 µm in the trans-wafer direction. With simulated electronic noise and a lowest threshold of 0.88 keV the corresponding values are 1.38 µm and 122.83 µm. Our results show that the proposed method has the potential to very significantly increase the spatial resolution in a full-field photon-counting detector for CT. 

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.

Photon-counting silicon strip detectors are attracting interest for use in next generation CT scanners. For silicon detectors, a low noise floor is necessary to obtain a good dose efficiency. A low noise floor can be achieved by having a filter with a long shaping time in the readout electronics. This also increases the pulse length, resulting in a long deadtime and thereby a degraded count-rate performance. However, as the flux typically varies greatly during a CT scan, a high count-rate capability is not required for all projection lines. It would therefore be desirable to use more than one shaping time within a single scan. To evaluate the potential benefit of using more than one shaping time, it is of interest to characterize the relation between the shaping time, the noise, and the resulting pulse shape. In this work we present noise and pulse shape measurements on a photon-counting detector with two different shaping times along with a complementary simulation model of the readout electronics. We show that increasing the shaping time from 28.1 ns to 39.4 ns decreases the noise and increases the signal-to-noise ratio (SNR) with 6.5% at low count rates and we also present pulse shapes for each shaping time as measured at a synchrotron source. Our results demonstrate that the shaping time plays an important role in optimizing the dose efficiency in a photon-counting x-ray detector.