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Upper limits of the photon fluence rate on CT detectors: case study on a commercial scanner
KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.ORCID iD: 0000-0002-5092-8822
Unit of X-ray Physics, Section of Imaging Physics Solna, Department of Medical Physics,Karolinska University Hospital.
Unit of X-ray Physics, Section of Imaging Physics Solna, Department of Medical Physics,Karolinska University Hospital.
Unit of X-ray Physics, Section of Imaging Physics Solna, Department of Medical Physics, Karolinska University Hospital.
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(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 [en]
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: urn:nbn:se:kth:diva-187261DOI: 10.1118/1.4954008OAI: oai:DiVA.org:kth-187261DiVA: diva2:929512
Note

QC 20160524

Available from: 2016-05-18 Created: 2016-05-18 Last updated: 2016-08-16Bibliographically 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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