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Sub-pixel information retrieval from spectral x-ray images
KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.ORCID iD: 0000-0002-5092-8822
Department of Clinical Neuroscience, Karolinska Institutet, SE-17177 Stockholm, Sweden and Department of Neuroradiology, Karolinska University Hospital Solna, SE-17176 Stockholm, Sweden.
KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
(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 [en]
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: urn:nbn:se:kth:diva-187262OAI: oai:DiVA.org:kth-187262DiVA: diva2:929513
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
In thesis
1. Spectral Computed Tomography with a Photon-Counting Silicon-Strip Detector
Open this publication in new window or tab >>Spectral Computed Tomography with a Photon-Counting Silicon-Strip Detector
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
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:nbn:se:kth:diva-187263 (URN)978-91-7595-991-7 (ISBN)
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

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