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Quantification and Maximization of Performance Measures for Photon Counting Spectral Computed Tomography
KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.ORCID iD: 0000-0001-7253-0164
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

During my time as a PhD student at the Physics of Medical Imaging group at KTH, I have taken part in the work of developing a photon counting spectrally resolved silicon detector for clinical computed tomography. This work has largely motivated the direction of my research, and is the main reason for my focus on certain issues. Early in the work, a need to quantify and optimize the performance of a spectrally resolved detector was identified. A large part of my work have thus consisted of reviewing conventional methods used for performance quantification and optimization in computed tomography, and identifying which are best suited for the characterization of a spectrally resolved system. In addition, my work has included comparisons of conventional systems with the detector we are developing. The collected result after a little more than four years of work are four publications and three conference papers.

This compilation thesis consists of five introductory chapters and my four publications. The introductory chapters are not self-contained in the sense that the theory and results from all my published work are included. Rather, they are written with the purpose of being a context in which the papers should be read.

The first two chapters treat the general purpose of the introductory chapters, and the theory of computed tomography including the distinction between conventional, non-spectral, computed tomography, and different practical implementations of spectral computed tomography. The second chapter consists of a review of the conventional methods developed for quantification and optimization of image quality in terms of detectability and signal-to-noise ratio, part of which are included in my published work. In addition, the theory on which the method of material basis decomposition is based on is presented, together with a condensed version of the results from my work on the comparison of two systems with fundamentally different practical solutions for material quantification. In the fourth chapter, previously unpublished measurements on the photon counting spectrally resolved detector we are developing are presented, and compared to Monte Carlo simulations. In the fifth and final chapter, a summary of the appended publications is included.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. , x, 65 p.
Series
TRITA-FYS, ISSN 0280-316X ; 15:08
Keyword [en]
spectral computed tomography, silicon detector, detectability index, photon counting, Hotelling SDNR, material basis decomposition
National Category
Other Physics Topics
Identifiers
URN: urn:nbn:se:kth:diva-160899ISBN: 978-91-7595-465-3 (print)OAI: oai:DiVA.org:kth-160899DiVA: diva2:792116
Public defence
2015-03-27, sal D3, Lindstedtsvägen 5, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20150303

Available from: 2015-03-03 Created: 2015-03-03 Last updated: 2015-03-03Bibliographically approved
List of papers
1. Performance evaluation of a sub-millimetre spectrally resolved CT system on high- and low-frequency imaging tasks: a simulation
Open this publication in new window or tab >>Performance evaluation of a sub-millimetre spectrally resolved CT system on high- and low-frequency imaging tasks: a simulation
2012 (English)In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 57, no 8, 2373-2391 p.Article in journal (Refereed) Published
Abstract [en]

We are developing a photon-counting silicon strip detector with 0.4 x 0.5 mm(2) detector elements for clinical CT applications. Except for the limited detection efficiency of approximately 0.8 for a spectrum of 80 kVp, the largest discrepancies from ideal spectral behaviour have been shown to be Compton interactions in the detector and electronic noise. Using the framework of cascaded system analysis, we reconstruct the 3D MTF and NPS of a silicon strip detector including the influence of scatter and charge sharing inside the detector. We compare the reconstructed noise and signal characteristics with a reconstructed 3D MTF and NPS of an ideal energy-integrating detector system with unity detection efficiency, no scatter or charge sharing inside the detector, unity presampling MTF and 1 x 1 mm(2) detector elements. The comparison is done by calculating the dose-normalized detectability index for some clinically relevant imaging tasks and spectra. This work demonstrates that although the detection efficiency of the silicon detector rapidly drops for the acceleration voltages encountered in clinical computed tomography practice, and despite the high fraction of Compton interactions due to the low atomic number, silicon detectors can perform on a par with ideal energy-integrating detectors for routine imaging tasks containing low-frequency components. For imaging tasks containing high-frequency components, the proposed silicon detector system can perform approximately 1.1-1.3 times better than a fully ideal energy-integrating system.

National Category
Radiology, Nuclear Medicine and Medical Imaging
Identifiers
urn:nbn:se:kth:diva-94040 (URN)10.1088/0031-9155/57/8/2373 (DOI)000302567100018 ()2-s2.0-84859355603 (Scopus ID)
Note
QC 20120507Available from: 2012-05-07 Created: 2012-05-07 Last updated: 2017-12-07Bibliographically approved
2. Eliminated risk of iodine contrast cancellation with multibin spectral CT
Open this publication in new window or tab >>Eliminated risk of iodine contrast cancellation with multibin spectral CT
Show others...
2013 (English)In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 58, no 14, N201-N209 p.Article in journal (Refereed) Published
Abstract [en]

This note compares the extent of contrast cancellation induced by iodinated contrast agents in energy integrating and photon counting multibin CT images. The contrast between a hypodense target and soft tissue is modeled for the two systems for a range of iodine concentrations and tube voltages. In energy integrating systems, we show that the contrast vanishes for low concentrations of iodine whereas the same effect is not seen in multibin systems. We conclude that it is the ability of multibin systems to apply weighting schemes post-acquisition that allows the operator to eliminate the risk of contrast cancellation between iodinated targets and the background.

Place, publisher, year, edition, pages
Institute of Physics (IOP), 2013
Keyword
Integrating systems, Iodinated contrast agents, Iodine concentration, Low concentrations, Photon counting, Soft tissue, Tube voltages, Weighting scheme
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-125554 (URN)10.1088/0031-9155/58/14/N201 (DOI)000321243600002 ()2-s2.0-84879993238 (Scopus ID)
Note

QC 20130812

Available from: 2013-08-12 Created: 2013-08-09 Last updated: 2017-12-06Bibliographically approved
3. Theoretical comparison of a dual energy system and photon counting silicon detector used for material quantification in spectral CT
Open this publication in new window or tab >>Theoretical comparison of a dual energy system and photon counting silicon detector used for material quantification in spectral CT
2015 (English)In: IEEE Transactions on Medical Imaging, ISSN 0278-0062, E-ISSN 1558-254X, Vol. 34, no 3, 796-806 p.Article in journal (Refereed) Published
Abstract [en]

Any method using dual energy computed tomography (CT) has to make prior assumptions in order to quantify k-edge contrast agents. This work estimates the mean square error (MSE) in contrast agent quantification employing a method based on assigning each reconstructed voxel a ratio of soft tissue and fat using dual energy CT. The results are compared to the MSE using a photon counting silicon detector with multiple bins. The square root of the MSEs of the quantifications of iodine and gadolinium for an object consisting of soft tissue and fat using the silicon detector and dual energy CT range from below 2% and 1% of the contrast agent content for 100 ${rm mg}/{rm cm}^{3}$ of iodine and gadolinium, up to approximately 10% and 13%, and 6% and 4%, for 5 ${rm mg}/{rm cm}^{3}$ of iodine and gadolinium, respectively. When adding bone with a voxel volume fraction of 2.2%, the square root of the MSEs of the quantifications of iodine and gadolinium using dual energy CT increases to 25% and 6%, respectively, for 5 ${rm mg}/{rm cm}^{3}$ of contrast agent. In conclusion, results indicate that the noise levels of the material quantification using the silicon detector are higher than the noise levels using a dual energy CT when the composition of the object is known. However, using a dual energy CT increases the risk of model specification error and subsequently a large bias in contrast agent quantification, a problem which does not exist when using a multi-bin CT where the number of energy bins is larger than two.

National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-160898 (URN)10.1109/TMI.2014.2362795 (DOI)000350870700012 ()2-s2.0-84923870081 (Scopus ID)
Note

QC 20150303

Available from: 2015-03-03 Created: 2015-03-03 Last updated: 2017-12-04Bibliographically approved
4. Optimal frequency-based weighting for spectral x-ray projection imaging
Open this publication in new window or tab >>Optimal frequency-based weighting for spectral x-ray projection imaging
2015 (English)In: IEEE Transactions on Medical Imaging, ISSN 0278-0062, E-ISSN 1558-254X, Vol. 34, no 3, 779-787 p.Article in journal (Refereed) Published
Abstract [en]

The purpose of this work is to derive a weighting scheme that maximizes the frequency-dependent ideal observer signal-difference-to-noise ratio, commonly referred to as detectability index or Hotelling-SDNR, for spectral X-ray projection imaging. Starting from basic statistical decision theory, optimal frequency-dependent weights are derived for a multiple-bin system and the Hotelling-SDNR calculated. A 28% increase in detectability index is found for high frequency objects when applying optimal frequency-dependent weights instead of pixel-based weights to a simplified model of a silicon detector, decreasing towards 0% for low frequency objects. Simulation results indicate a potentially large increase in detectability for high-frequency object imaging using silicon detectors, thus meriting further evaluations on a real system.

National Category
Other Physics Topics
Research subject
Medical Technology
Identifiers
urn:nbn:se:kth:diva-160897 (URN)10.1109/TMI.2014.2360932 (DOI)000350870700010 ()2-s2.0-84923873993 (Scopus ID)
Note

QC 20150303

Available from: 2015-03-03 Created: 2015-03-03 Last updated: 2017-12-04Bibliographically approved

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