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Characterization and Energy Calibration of a Silicon-Strip Detector for Photon-Counting Spectral Computed Tomography
KTH, School of Technology and Health (STH), Medical Engineering, Medical Imaging.
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Multibin photon-counting x-ray detectors are promising candidates to be applied in next generation computed tomography (CT), whereby energy information from a broad x-ray spectrum can be extracted and properly used for improving image quality and correspondingly reducing radiation dose. A silicon-strip detector has been developed for spectral CT, which operates in photon-counting mode and allows pulse-height discrimination with 8 adjustable energy bins.

Critical characteristics, energy resolution and count-rate performance, of the detector are evaluated. An absolute energy resolution (E) from 1.5 keV to 1.9 keV with increasing x-ray energy from 40 keV to 120 keV is found. Pulse pileup degrades the energy resolution by 0.4 keV when increasing the input count rate from zero to 100 Mcps mm−2, while charge sharing shows negligible effect. A near linear relationship between the input and output count rates is observed up to 90 Mcps mm−2 in a clinical CT environment. In addition, no saturation effect appears for the maximally achieved photon flux of 485 Mphotons s−1 mm−2 with a count rate loss of 30%.

The detector is energy calibrated in terms of gain and offset with the aid of monoenergetic x rays. The gain variation among channels is below 4%, whereas the variation of offsets is on the order of 1 keV. In order to do the energy calibration in a routinely available way, a method that makes use of the broad x-ray spectrum instead of using monoenergetic x rays is proposed. It is based on a regression analysis that adjusts a modelled spectrum of deposited energies to a measured pulse-height spectrum. Application of this method shows high potential to be applied in an existing CT scanner with an uncertainty of a calibrated threshold between 0.1 and 0.2 keV.

The energy-calibration method is further used in the development of a spectral response model of the detector. This model is used to accurately bin-wise predict the response of each detector channel, which is validated by two application examples. First, the model is used in combination with the inhomogeneity compensation method to eliminate ring artefacts in CT images. Second, the spectral response model is used as the basis of the maximum likelihood approach for projection-based material decomposition. The reconstructed basis images show a good separation between the calcium-like material and the contrast agents, iodine and gadolinium. Additionally, the contrast agent concentrations are reconstructed with more than 94% accuracy.

Place, publisher, year, edition, pages
Stockholm, Sweden: KTH Royal Institute of Technology, 2016. , 46 p.
Series
TRITA-FYS, ISSN 0280-316X ; 2016:56
National Category
Medical Equipment Engineering
Research subject
Medical Technology
Identifiers
URN: urn:nbn:se:kth:diva-192240ISBN: 978-91-7729-079-7OAI: oai:DiVA.org:kth-192240DiVA: diva2:967342
Public defence
2016-09-30, Kollegiesalen, Brinellvägen 8, KTH, Stockholm, 09:00 (English)
Opponent
Supervisors
Note

QC 20160908

Available from: 2016-09-08 Created: 2016-09-08 Last updated: 2016-09-09Bibliographically approved
List of papers
1. A Silicon-Strip Detector for Photon-Counting Spectral CT: Energy Resolution From 40 keV to 120 keV
Open this publication in new window or tab >>A Silicon-Strip Detector for Photon-Counting Spectral CT: Energy Resolution From 40 keV to 120 keV
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2014 (English)In: IEEE Transactions on Nuclear Science, ISSN 0018-9499, E-ISSN 1558-1578, Vol. 61, no 3, 1099-1105 p.Article in journal (Refereed) Published
Abstract [en]

We are developing a segmented silicon-strip detector for spectral computed tomography. The detector operates in photon-counting mode and allows pulse-height discrimination with 8 adjustable energy bins. In this work, we determine the energy resolution of a detector module using monoenergetic x-rays from 40 keV to 120 keV, provided at the European Synchrotron Radiation Facility, Grenoble. For each incident x-ray energy, pulse height spectra at different input photon fluxes are obtained. We investigate changes of the energy resolution due to charge sharing between pixels and pulse pileup. The different incident energies are used to channel-wise calibrate the pulse-height response in terms of signal gain and offset and to probe the homogeneity of the detector module. The detector shows a linear pulse-height response in the energy range from 40 keV to 120 keV. The gain variation among the channels is below 4%, whereas the variation of the offsets is on the order of 1 keV. We find an absolute energy resolution (sigma(E)) that degrades from 1.5 keV to 1.9 keV with increasing x-ray energy from 40 keV to 100 keV. With increasing input count rate, sigma(E) degrades by approximately 4 . 10(-3) keV Mcps(-1) mm(2), which is, within error bars, the same for the different energies. The effect of charge sharing on the width of the response peak is found to be negligible.

Keyword
Computed tomography, energy resolution, photon-counting, silicon strip detector, spectral CT
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-148323 (URN)10.1109/TNS.2014.2300153 (DOI)000337905600006 ()2-s2.0-84903277926 (ScopusID)
Note

QC 20140807

Available from: 2014-08-07 Created: 2014-08-05 Last updated: 2016-09-08Bibliographically approved
2. Energy Calibration of a Silicon-Strip Detector for Photon-Counting Spectral CT by Direct Usage of the X-ray Tube Spectrum
Open this publication in new window or tab >>Energy Calibration of a Silicon-Strip Detector for Photon-Counting Spectral CT by Direct Usage of the X-ray Tube Spectrum
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2015 (English)In: IEEE Transactions on Nuclear Science, ISSN 0018-9499, E-ISSN 1558-1578, Vol. 62, no 1, 68-75 p.Article in journal (Refereed) Published
Abstract [en]

The variation among energy thresholds in a multibin detector for photon-counting spectral CT can lead to ring artefacts in the reconstructed images. Calibration of the energy thresholds can be used to achieve homogeneous threshold settings or to develop compensation methods to reduce the artefacts. We have developed an energy-calibrationmethod for the different comparator thresholds employed in a photon-counting silicon-strip detector. In our case, this corresponds to specifying the linear relation between the threshold positions in units of mV and the actual deposited photon energies in units of keV. This relation is determined by gain and offset values that differ for different detector channels due to variations in the manufacturing process. Typically, the calibration is accomplished by correlating the peak positions of obtained pulse-height spectra to known photon energies, e. g. with the aid of mono-energetic x rays from synchrotron radiation, radioactive isotopes or fluorescence materials. Instead of mono-energetic x rays, the calibrationmethod presented in this papermakes use of a broad x-ray spectrum provided by commercial x-ray tubes. Gain and offset as the calibration parameters are obtained by a regression analysis that adjusts a simulated spectrum of deposited energies to ameasured pulse-height spectrum. Besides the basic photon interactions such as Rayleigh scattering, Compton scattering and photo-electric absorption, the simulation takes into account the effect of pulse pileup, charge sharing and the electronic noise of the detector channels. We verify the method for different detector channels with the aid of a table-top setup, where we find the uncertainty of the keV-value of a calibrated threshold to be between 0.1 and 0.2 keV.

Keyword
Calibration, computed tomography, Monte-Carlo simulation, photon-counting, silicon-strip detector, spectral CT
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-161964 (URN)10.1109/TNS.2014.2373641 (DOI)000349672700008 ()2-s2.0-84923304829 (ScopusID)
Note

QC 20150407

Available from: 2015-04-07 Created: 2015-03-20 Last updated: 2016-09-08Bibliographically approved
3. Spectral response model for a multibin photon-counting spectral computed tomography detector and its applications
Open this publication in new window or tab >>Spectral response model for a multibin photon-counting spectral computed tomography detector and its applications
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2015 (English)In: Journal of Medical Imaging, ISSN 2329-4302, E-ISSN 2329-4310, Vol. 2, no 3, 033502Article in journal (Refereed) Published
Abstract [en]

Variations among detector channels in computed tomography can lead to ring artifacts in the reconstructed images and biased estimates in projection-based material decomposition. Typically, the ring artifacts are corrected by compensation methods based on flat fielding, where transmission measurements are required for a number of material-thickness combinations. Phantoms used in these methods can be rather complex and require an extensive number of transmission measurements. Moreover, material decomposition needs knowledge of the individual response of each detector channel to account for the detector inhomogeneities. For this purpose, we have developed a spectral response model that binwise predicts the response of a multibin photon-counting detector individually for each detector channel. The spectral response model is performed in two steps. The first step employs a forward model to predict the expected numbers of photon counts, taking into account parameters such as the incident x-ray spectrum, absorption efficiency, and energy response of the detector. The second step utilizes a limited number of transmission measurements with a set of flat slabs of two absorber materials to fine-tune the model predictions, resulting in a good correspondence with the physical measurements. To verify the response model, we apply the model in two cases. First, the model is used in combination with a compensation method which requires an extensive number of transmission measurements to determine the necessary parameters. Our spectral response model successfully replaces these measurements by simulations, saving a significant amount of measurement time. Second, the spectral response model is used as the basis of the maximum likelihood approach for projection-based material decomposition. The reconstructed basis images show a good separation between the calcium-like material and the contrast agents, iodine and gadolinium. The contrast agent concentrations are reconstructed with more than 94% accuracy.

Place, publisher, year, edition, pages
SPIE - International Society for Optical Engineering, 2015
Keyword
silicon strip detector, photon-counting computed tomography, forward model, ring artifact, material decomposition
National Category
Radiology, Nuclear Medicine and Medical Imaging
Identifiers
urn:nbn:se:kth:diva-186578 (URN)10.1117/1.JMI.2.3.033502 (DOI)000374234800003 ()
Note

QC 20160513

Available from: 2016-05-13 Created: 2016-05-13 Last updated: 2016-09-08Bibliographically approved
4. Count rate performance of a silicon-strip detector for photon-counting spectral CT
Open this publication in new window or tab >>Count rate performance of a silicon-strip detector for photon-counting spectral CT
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2016 (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. 827, 102-106 p.Article in journal (Refereed) Published
Abstract [en]

A silicon-strip detector is developed for spectral computed tomography. The detector operates in photon counting mode and allows pulse-height discrimination with 8 adjustable energy bins. In this work, we evaluate the count-rate performance of the detector in a clinical CT environment. The output counts of the detector are measured for x-ray tube currents up to 500 mA at 120 kV tube voltage, which produces a maximum photon flux of 485 Mphotons/s/mm(2) for the unattenuated beam. The corresponding maximum count-rate loss of the detector is around 30% and there are no saturation effects. A near linear relationship between the input and output count rates can be observed up to 90 Mcps/mm(2), at which point only 3% of the input counts are lost. This means that the loss in the diagnostically relevant count rate region is negligible. A semi-nonparalyzable dead-time model is used to describe the count-rate performance of the detector, which shows a good agreement with the measured data. The non-paralyzable dead time tau(n) for 150 evaluated detector elements is estimated to be 20.2 +/- 5.2 ns. (C) 2016 Elsevier B.V. All rights reserved.

Place, publisher, year, edition, pages
Elsevier, 2016
Keyword
Spectral CT, Photon-counting detector, Silicon-strip detector, Count-rate performance
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-189065 (URN)10.1016/j.nima.2016.04.087 (DOI)000376888900014 ()2-s2.0-84966709675 (ScopusID)
Note

QC 20160628

Available from: 2016-06-28 Created: 2016-06-27 Last updated: 2016-09-08Bibliographically approved

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