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A Segmented Silicon Strip Detector for Photon-Counting Spectral Computed Tomography
KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
2012 (English)Doctoral thesis, comprehensive summary (Other academic) [Artistic work]
Abstract [en]

Spectral computed tomography with energy-resolving detectors has a potential to improve the detectability of images and correspondingly reduce the radiation dose to patients by extracting and properly using the energy information in the broad x-ray spectrum. A silicon photon-counting detector has been developed for spectral CT and it has successfully solved the problem of high photon flux in clinical CT applications by adopting the segmented detector structure and operating the detector in edge-on geometry. The detector was evaluated by both the simulation and measurements.

The effects of energy loss and charge sharing on the energy response of this segmented silicon strip detector with different pixel sizes were investigated by Monte Carlo simulation and a comparison to pixelated CdTe detectors is presented. The validity of spherical approximations of initial charge cloud shape in silicon detectors was evaluated and a more accurate statistical model has been proposed.

A photon-counting energy-resolving application specific integrated circuit (ASIC) developed for spectral CT was characterized extensively by electrical pulses, pulsed laser and real x-ray photons from both the synchrotron and an x-ray tube. It has been demonstrated that the ASIC performs as designed. A noise level of 1.09 keV RMS has been measured and a threshold dispersion of 0.89 keV RMS has been determined. The count rate performance of the ASIC in terms of count loss and energy resolution was evaluated by real x-rays and promising results have been obtained.

The segmented silicon strip detector was evaluated using synchrotron radiation. An energy resolution of 16.1% has been determined with 22 keV photons in the lowest flux limit, which deteriorates to 21.5% at an input count rate of 100 Mcps mm−2. The fraction of charge shared events has been estimated and found to be 11.1% for 22 keV and 15.3% for 30 keV. A lower fraction of charge shared events and an improved energy resolution can be expected by applying a higher bias voltage to the detector.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. , viii, 43 p.
Series
Trita-FYS, ISSN 0280-316X ; 2012:88
Keyword [en]
photon counting, spectral computed tomography, silicon strip detector, ASIC, energy resolution, cadmium telluride, charge sharing, Monte Carlo simulation, synchrotron
National Category
Electrical Engineering, Electronic Engineering, Information Engineering Medical Equipment Engineering Medical Laboratory and Measurements Technologies Other Engineering and Technologies not elsewhere specified Radiology, Nuclear Medicine and Medical Imaging
Identifiers
URN: urn:nbn:se:kth:diva-105614ISBN: 978-91-7501-589-7 (print)OAI: oai:DiVA.org:kth-105614DiVA: diva2:571537
Public defence
2012-12-14, FA32, AlbaNova University Center, KTH, Roslagstullsbacken 21, Stockholm, 13:15 (English)
Opponent
Supervisors
Note

QC 20121123

Available from: 2012-11-23 Created: 2012-11-23 Last updated: 2012-11-23Bibliographically approved
List of papers
1. Evaluation of Energy Loss and Charge Sharing in Cadmium Telluride Detectors for Photon-Counting Computed Tomography
Open this publication in new window or tab >>Evaluation of Energy Loss and Charge Sharing in Cadmium Telluride Detectors for Photon-Counting Computed Tomography
2011 (English)In: IEEE Transactions on Nuclear Science, ISSN 0018-9499, E-ISSN 1558-1578, Vol. 58, no 3, 614-625 p.Article in journal (Refereed) Published
Abstract [en]

We present estimates of energy loss and charge sharing for a pixelated cadmium telluride (CdTe) detector used for photon-counting spectral computed tomography (CT). In a photon-counting pixelated CdTe detector, several physical effects lead to detected events with reduced energies, including Compton scattering, fluorescence emission, charge diffusion, trapping of charge carriers and slow-hole-motion-induced incomplete charge collection. Charge sharing is the result of the lost energy being collected by adjacent pixels. We simulated the photon transport and the charge-collection process with a Monte Carlo-based simulation and evaluated these effects on the detector performance. The trapping effect and poor hole collection have been studied together using an analytical model. We also investigated the detector response under the influence of only the fluorescence effect. We conclude that the charge sharing effects should be taken into account when the pixel is smaller than 1 mm(2). A straightforward way to decrease the double counting of X-rays from events with charge sharing is to increase the electronic threshold. However, increasing the threshold comes at the cost of losing low-energy events, which is undesirable, at least in applications such as pediatric imaging.

Keyword
Cadmium telluride, charge sharing, computed tomography, energy loss, Monte Carlo simulation, photon counting, signal generation
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-35618 (URN)10.1109/TNS.2011.2122267 (DOI)000291655900005 ()2-s2.0-79959378612 (Scopus ID)
Note
QC 20110705Available from: 2011-07-05 Created: 2011-07-04 Last updated: 2017-12-11Bibliographically approved
2. Validity of spherical approximations of initial charge cloud shape in silicon detectors
Open this publication in new window or tab >>Validity of spherical approximations of initial charge cloud shape in silicon detectors
2011 (English)In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 648, no SUPPL. 1, 190-193 p.Article in journal (Refereed) Published
Abstract [en]

Spherical approximation has been used extensively in low-energy X-ray imaging to represent the initial charge cloud produced by photon interactions in silicon detectors, mainly because of its simplicity. However, for high-energy X-rays, where the initial charge distribution is as important as the diffusion process, the spherical approximation will not result in a realistic detector response. In this paper, we present a bubble-line model that simulates the initial charge cloud in silicon detectors for photons in the energy range of medical imaging. An initial charge cloud can be generated by sampling the center of gravity and the track size from statistical distributions derived from Monte Carlo generated tracks and by distributing a certain proportion of photon energy into a bubble (68%) and a line portion uniformly. The simulations of detector response demonstrate that the new model simulates the detector response accurately and corresponds well to Monte Carlo simulation.

Keyword
Spherical approximation; Statistical model; Monte Carlo simulation; Signal induction; Silicon
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-47662 (URN)10.1016/j.nima.2010.11.173 (DOI)000305376900049 ()2-s2.0-79960835983 (Scopus ID)
Note
QC 20111115Available from: 2011-11-11 Created: 2011-11-11 Last updated: 2017-12-08Bibliographically approved
3. Preliminary evaluation of a silicon strip detector for photon-counting spectral CT
Open this publication in new window or tab >>Preliminary evaluation of a silicon strip detector for photon-counting spectral CT
Show others...
2012 (English)In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 677, 45-51 p.Article in journal (Refereed) Published
Abstract [en]

An edge-on silicon strip detector designed for photon-counting spectral computed tomography (CT) is presented. Progress on the development of an application specific integrated circuit (ASIC) to process the pulses and sort them into energy bins is reported upon. The ASIC and detector are evaluated in terms of electronic noise, energy resolution, count rate linearity under high-frequency periodic pulses, threshold variation and gain. The high-frequency periodic pulses are injected both by means of an external pulse generator and a pulsed laser illuminating the silicon diode. The pulsed laser system has similar to 100 ps pulse width and thus generates near instantaneous pulses in the diode, thus mimicking real X-ray conversions. The evaluation shows a low thermal noise level of 0.77 key RMS, an energy resolution of 1.5 keV RMS when electron-hole pairs are generated in the detector diode by the laser injection. The test results furthermore indicate a good energy-discriminating capability of the detector with the thresholds spread out, assigning the external pulses to higher and higher energy bins as the pulse intensity is increased.

Keyword
Photon counting, Silicon strip detector, ASIC, Spectral computed tomography, Laser test
National Category
Atom and Molecular Physics and Optics Medical Engineering
Identifiers
urn:nbn:se:kth:diva-96427 (URN)10.1016/j.nima.2012.02.034 (DOI)000303790500010 ()2-s2.0-84858743996 (Scopus ID)
Note
QC 20120607Available from: 2012-06-07 Created: 2012-06-04 Last updated: 2017-12-07Bibliographically approved
4. Evaluation of a Second-Generation Ultra-Fast Energy-Resolved ASIC for Photon-Counting Spectral CT
Open this publication in new window or tab >>Evaluation of a Second-Generation Ultra-Fast Energy-Resolved ASIC for Photon-Counting Spectral CT
Show others...
2013 (English)In: IEEE Transactions on Nuclear Science, ISSN 0018-9499, E-ISSN 1558-1578, Vol. 60, no 1, 437-445 p.Article in journal (Refereed) Published
Abstract [en]

A second-generation ultra-fast energy-resolved application specific integrated circuit (ASIC) has been developed for photon-counting spectral computed tomography (CT). The energy resolution, threshold dispersion and gain of the ASIC were characterized with synchrotron radiation at Diamond Light Source. The standard deviation of threshold offsets at zero keV is 0.89 keV. An RMS energy resolution of 1.09 keV has been demonstrated for 15 keV photon energy at a count rate of 40 kcps, and it deteriorates at a rate of 0.29 keV/Mcps with the increase of output cout rate. The count rate performance of the ASIC has also been evaluated with 120 kV polychromatic x-rays produced by a tungsten anode tube and the results are presented.

Keyword
Photon counting, spectral computed tomography, silicon strip detector, ASIC, synchrotron
National Category
Engineering and Technology
Research subject
SRA - E-Science (SeRC)
Identifiers
urn:nbn:se:kth:diva-105611 (URN)10.1109/TNS.2012.2228276 (DOI)000314973200032 ()2-s2.0-84873729895 (Scopus ID)
Funder
Swedish e‐Science Research Center
Note

QC 20130319. Updated from accepted to published.

Available from: 2012-11-22 Created: 2012-11-22 Last updated: 2017-12-07Bibliographically approved
5. Energy resolution of a segmented silicon strip detector for photon-counting spectral CT
Open this publication in new window or tab >>Energy resolution of a segmented silicon strip detector for photon-counting spectral CT
Show others...
2013 (English)In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 715, 11-17 p.Article in journal (Refereed) Published
Abstract [en]

We investigated the energy resolution of a segmented silicon strip detector for photon-counting spectral computed tomography (CT). The detector response to different monochromatic photon energies and various photon fluxes was characterized at the Elettra synchrotron. An RMS energy resolution of 1.50 keV has been demonstrated for 22 keV photons at zero flux, and it deteriorated as a function of input count rate at a rate of 5.13 eV mm2 /Mcps. The charge sharing effect has been evaluated. The results show that around 11.1% of the interacting photons experience charge sharing for 22 keV photons and 15.3% for 30 keV.

Keyword
Photon counting, silicon strip detector, spectral computed tomography, energy resolution, synchrotron
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-105613 (URN)10.1016/j.nima.2013.02.030 (DOI)000319252300002 ()2-s2.0-84876258566 (Scopus ID)
Note

Updated from "Submitted" to "Published" QC 20130710

Available from: 2012-11-22 Created: 2012-11-22 Last updated: 2017-12-07Bibliographically approved

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