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Spectral Computed Tomography with a Photon-Counting Silicon-Strip Detector
KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging. (Physics of Medical Imaging)ORCID iD: 0000-0002-5092-8822
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Computed tomography (CT) is a widely used medical imaging modality. By rotating an x-ray tube and an x-ray detector around the patient, a CT scanner is able to measure the x-ray transmission from all directions and form an image of the patient’s interior. CT scanners in clinical use today all use energy-integrating detectors, which measure the total incident energy for each measurement interval. A photon-counting detector, on the other hand, counts the number of incoming photons and can in addition measure the energy of each photon by comparing it to a number of energy thresholds. Using photon- counting detectors in computed tomography could lead to improved signal-to-noise ratio, higher spatial resolution and improved spectral imaging which allows better visualization of contrast agents and more reliable quantitative measurements. In this Thesis, the feasibility of using a photon-counting silicon-strip detector for CT is investigated. In the first part of the Thesis, the necessary performance requirements on such a detector is investigated in two different areas: the detector element homogeneity and the capability of handling high photon fluence rates. A metric of inhomogeneity is proposed and used in a simulation study to evaluate different inhomogeneity compensation methods. Also, the photon fluence rate incident on the detector in a scanner in clinical use today is investigated for different patient sizes through dose rate measurements together with simulations of transmission through patient im- ages. In the second part, a prototype detector module is used to demonstrate new applications enabled by the energy resolution of the detector. The ability to generate material-specific images of contrast agents with iodine and gadolinium is demonstrated. Furthermore, it is shown theoretically and ex- perimentally that interfaces in the image can be visualized by imaging the so-called nonlinear partial volume effect. The results suggest that the studied silicon-strip detector is a promising candidate for photon-counting CT.  

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2016. , 43 p.
Series
TRITA-FYS, ISSN 0280-316X ; 2016:20
Keyword [en]
Photon-counting, silicon-strip detector, spectral computed tomography, ring artifacts, fluence rate, basis material decomposition, sub-pixel information
National Category
Other Physics Topics Medical Equipment Engineering
Research subject
Physics
Identifiers
URN: urn:nbn:se:kth:diva-187263ISBN: 978-91-7595-991-7 (print)OAI: oai:DiVA.org:kth-187263DiVA: diva2:929515
Public defence
2016-06-14, FR4, Roslagstullsbacken 21, AlbaNova Universitetscentrum, Stockholm, 09:00 (English)
Opponent
Supervisors
Available from: 2016-05-20 Created: 2016-05-18 Last updated: 2016-05-20Bibliographically approved
List of papers
1. A Framework for Evaluating Threshold Variation Compensation Methods in Photon Counting Spectral CT
Open this publication in new window or tab >>A Framework for Evaluating Threshold Variation Compensation Methods in Photon Counting Spectral CT
2012 (English)In: IEEE Transactions on Medical Imaging, ISSN 0278-0062, E-ISSN 1558-254X, Vol. 31, no 10, 1861-1874 p.Article in journal (Refereed) Published
Abstract [en]

One of the challenges in the development of photon counting spectral computed tomography (CT) detectors is that the location of the energy thresholds tends to vary among detector elements. If not compensated for, this threshold variation leads to ring artifacts in the reconstructed images. In this paper, a framework is presented for the systematic comparison of different methods of compensating for inhomogeneities among detector elements in photon counting CT with multiple energy bins. Furthermore, we propose the use of an affine minimum mean square error estimator, calibrated against transmission measurements on different combinations of two materials, for inhomogeneity compensation. Using the framework developed here, this method is compared to two other compensation schemes, flatfielding using an air scan and signal-to-thickness calibration using a step wedge calibrator, in a simulation study. The results show that for all but the lowest studied level of threshold spread, the proposed method is superior to signal-to-thickness calibration, which in turn is superior to flatfielding. We also demonstrate that the effects of threshold variation can be countered to a large extent by substructuring each detector element into depth segments.

Keyword
Calibration, computed tomography (CT), homogeneity requirements, photon counting, ring artifacts, spectral computed tomography (CT)
National Category
Radiology, Nuclear Medicine and Medical Imaging
Identifiers
urn:nbn:se:kth:diva-105664 (URN)10.1109/TMI.2012.2204274 (DOI)000310149700003 ()2-s2.0-84867086955 (Scopus ID)
Note

QC 20121123

Available from: 2012-11-23 Created: 2012-11-23 Last updated: 2017-12-07Bibliographically approved
2. Upper limits of the photon fluence rate on CT detectors: case study on a commercial scanner
Open this publication in new window or tab >>Upper limits of the photon fluence rate on CT detectors: case study on a commercial scanner
Show others...
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Purpose: The highest photon fluence rate that a CT detector must be able to measure is animportant parameter. We calculate the maximum transmitted fluence rate in a commercial CT scanner as a function of patient size for standard head, chest and abdomen protocols.Method: We scanned an anthropomorphic phantom (Kyoto Kagaku PBU-60) with the reference CT protocols provided by AAPM on a GE LightSpeed VCT scanner and noted the tube currentapplied with the tube current modulation (TCM) system. By rescaling this tube current usingpublished measurements on the tube current modulation of a GE scanner we could estimate the tube current that these protocols would have resulted in for other patient sizes. An ECG gatedchest protocol was also simulated. Using measured dose rate profiles along the bowtie filters, wesimulated imaging of anonymized patient images with a range of sizes on a GE VCT scanner andcalculated the maximum transmitted fluence rate. In addition, the 99th and the 95th percentilesof the transmitted fluence rate distribution behind the patient are calculated and the effect of omitting projection lines passing just below the skin line is investigated.Results: The highest transmitted fluence rates on the detector for the AAPM reference protocolswith centered patients are found for head and chest images of small patients, with a maximumof 7.1 · 107 mm−2 s−1 for head and 9.6 · 107 mm−2 s−1 for chest. Miscentering the head by 50 mm downwards increases the maximum transmitted fluence rate to 3.9 · 108 mm−2 s−1 . The ECG gatedchest protocol gives fluence rates up to 2.3 · 108 − 2.4 · 108 mm−2 s−1 depending on miscentering.Conclusion: The fluence rate on a CT detector reaches 1 · 108 − 4 · 108 mm−2 s−1 in standardimaging protocols, with the highest rates occurring for ECG gated chest and miscentered headscans. These results will be useful to developers of CT detectors, in particular photon countingdetectors.

Keyword
fluence rate, count rate requirements, count rate problem, photon counting CT
National Category
Other Physics Topics Medical Equipment Engineering
Research subject
Physics; Medical Technology
Identifiers
urn:nbn:se:kth:diva-187261 (URN)10.1118/1.4954008 (DOI)
Note

QC 20160524

Available from: 2016-05-18 Created: 2016-05-18 Last updated: 2016-08-16Bibliographically approved
3. Energy-resolved CT imaging with a photon-counting silicon-strip detector
Open this publication in new window or tab >>Energy-resolved CT imaging with a photon-counting silicon-strip detector
Show others...
2014 (English)In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 59, no 22, 6709-6727 p.Article in journal (Refereed) Published
Abstract [en]

Photon-counting detectors are promising candidates for use in the next generation of x-ray computed tomography (CT) scanners. Among the foreseen benefits are higher spatial resolution, better trade-off between noise and dose and energy discriminating capabilities. Silicon is an attractive detector material because of its low cost, mature manufacturing process and high hole mobility. However, it is sometimes overlooked for CT applications because of its low absorption efficiency and high fraction of Compton scatter. The purpose of this work is to demonstrate that silicon is a feasible material for CT detectors by showing energy-resolved CT images acquired with an 80 kVp x-ray tube spectrum using a photon-counting silicon-strip detector with eight energy thresholds developed in our group. We use a single detector module, consisting of a linear array of 50 0.5 x 0.4 mm detector elements, to image a phantom in a table-top lab setup. The phantom consists of a plastic cylinder with circular inserts containing water, fat and aqueous solutions of calcium, iodine and gadolinium, in different concentrations. By using basis material decomposition we obtain water, calcium, iodine and gadolinium basis images and demonstrate that these basis images can be used to separate the different materials in the inserts. We also show results showing that the detector has potential for quantitative measurements of substance concentrations.

Keyword
photon counting, spectral CT, material decomposition, siliconstrip detector
National Category
Radiology, Nuclear Medicine and Medical Imaging
Identifiers
urn:nbn:se:kth:diva-157026 (URN)000344091000005 ()2-s2.0-84908587151 (Scopus ID)
Note

QC 20141205

Available from: 2014-12-05 Created: 2014-12-04 Last updated: 2017-05-19Bibliographically approved
4. Sub-pixel information retrieval from spectral x-ray images
Open this publication in new window or tab >>Sub-pixel information retrieval from spectral x-ray images
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The detector pixel size can be a severe limitation in projection x-ray imaging of fine details inthe human body, but developing higher resolution detectors is technically challenging. Wedemonstrate a novel method of using spectral x-ray measurements, from an energy-resolvingdetector or from multiple acquisitions with different beam quality, to obtain information aboutthe spatial distribution of the linear attenuation coefficient on a length scale smaller than onepixel. The method builds on the fact that the linear attenuation coefficient of all materials inthe human body can be expressed as linear combinations of a small number of basis functions.However, an interface parallel to the x-ray beam has a unique spectral responose which makesit distinguishable from homogeneous materials.To demonstrate the method experimentally, a 120 mm polyethylene phantom with a 6 mmiodine-filled hole in its centre was imaged in a projection geometry using a photon-countingsilicon-strip detector with eight energy bins. X-ray transmission measurements of differentthicknesses of polyethylene and iodine were used to calibrate a forward model describing thedetector response for different objects in the beam. Using the proposed method, an imagespecific to the spectral response of an iodine-polyethylene interface was generated. Theresults show that the borders of the iodine insert are highlighted in the resulting image, ingood agreement with simulations.Our study demonstrates that spectral x-ray measurements can be used to distinguish betweensharp and gradual transitions in an x-ray image. The method may potentially be used forimproving visualization of blood vessel boundaries in stroke care.

Keyword
x-ray imaging, sub-pixel information, spectral x-ray imaging, photon-counting detector
National Category
Other Physics Topics Medical Equipment Engineering
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-187262 (URN)
Funder
VINNOVA, 2014-03800Stockholm County Council, 20140712
Note

QC 20160524

Available from: 2016-05-18 Created: 2016-05-18 Last updated: 2016-05-24Bibliographically approved

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