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Energy Calibration of a Silicon-Strip Detector for Photon-Counting Spectral CT by Direct Usage of the X-ray Tube Spectrum
KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.ORCID iD: 0000-0001-8560-3262
KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
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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.

Place, publisher, year, edition, pages
2015. Vol. 62, no 1, 68-75 p.
Keyword [en]
Calibration, computed tomography, Monte-Carlo simulation, photon-counting, silicon-strip detector, spectral CT
National Category
Other Physics Topics
URN: urn:nbn:se:kth:diva-161964DOI: 10.1109/TNS.2014.2373641ISI: 000349672700008ScopusID: 2-s2.0-84923304829OAI: diva2:800686

QC 20150407

Available from: 2015-04-07 Created: 2015-03-20 Last updated: 2016-09-08Bibliographically approved
In thesis
1. Characterization and Energy Calibration of a Silicon-Strip Detector for Photon-Counting Spectral Computed Tomography
Open this publication in new window or tab >>Characterization and Energy Calibration of a Silicon-Strip Detector for Photon-Counting Spectral Computed Tomography
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.
TRITA-FYS, ISSN 0280-316X ; 2016:56
National Category
Medical Equipment Engineering
Research subject
Medical Technology
urn:nbn:se:kth:diva-192240 (URN)978-91-7729-079-7 (ISBN)
External cooperation:
Public defence
2016-09-30, Kollegiesalen, Brinellvägen 8, KTH, Stockholm, 09:00 (English)

QC 20160908

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

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