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New Efficient Detector for Radiation Therapy Imaging using Gas Electron Multipliers
Stockholm University, Faculty of Science, Medical Radiation Physics (together with KI).
2006 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Currently film is being replaced by electronic detectors for portal imaging in radiation therapy. This development offers obvious advantages such as on-line quality assurance and digital images that can easily be accessed, processed and communicated. In spite of the improvements, the image quality has not been significantly enhanced, partly since the quantum efficiency compared to film is essentially the same, and the new electronic devices also suffer from sensitivity to the harsh radiation environment. In this thesis we propose a third generation electronic portal imaging device with increased quantum efficiency and potentially higher image quality.

Due to the parallel readout capability it is much faster than current devices, providing at least 200 frames per second (fps), and would even allow for a quality assurance and adaptive actions after each accelerator pulse. The new detector is also sensitive over a broader range of energies (10 keV - 50 MeV) and can be used to obtain diagnostic images immediately prior to the treatment without repositioning the patient. The imaging could be in the form of portal imaging or computed tomography. The new detector is based on a sandwich design containing several layers of Gas Electron Multipliers (GEMs) in combination with, or integrated with, perforated converter plates. The charge created by the ionizing radiation is drifted to the bottom of the assembly where a tailored readout system collects and digitizes the charge. The new readout system is further designed in such a way that no sensitive electronics is placed in the radiation beam and the detector is expected to be radiation resistant since it consists mainly of kapton, copper and gas.

A single GEM detector was responding linearly when tested with a 50 MV photon beam at a fluence rate of ~1010 photons mm-2 s-1 during 3-5 μs long pulses, but also with x-ray energies of 10-50 keV at a fluence rate of up to ~108 photons mm-2 s-1. The electron transmission of a 100 μm thick Cu plate with an optical transparency of ~46% was found to be ~15.4%, i.e. the effective hole transmission for the electrons was about one third of the hole area. A low effective GEM gain is enough to compensate for the losses in converters of this dimension. A prototype for the dedicated electronic readout system was designed with 50 x 100 pixels at a pitch of 1.27 mm x 1.27 mm. X-ray images were achieved with a single GEM layer and also in a double GEM setup with a converter plate interleaved. To verify the readout speed a Newton pendulum was imaged at a frame rate of 70 fps and alpha particles were imaged in 188 fps. The experimental studies indicates that the existing prototype can be developed as a competitive alternative for imaging in radiation therapy.

Place, publisher, year, edition, pages
Stockholm: Medicinsk strålningsfysik (tills m KI) , 2006. , 49 p.
Keyword [en]
Portal imaging, Gas Electron Multiplier, Gas detector
National Category
Medical Laboratory and Measurements Technologies
Identifiers
URN: urn:nbn:se:su:diva-857ISBN: 91-7155-218-9 (print)OAI: oai:DiVA.org:su-857DiVA: diva2:200503
Public defence
2006-03-17, sal FB53, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00
Opponent
Supervisors
Available from: 2006-02-17 Created: 2006-02-17Bibliographically approved
List of papers
1. Amplification and conditioning properties of GEM and CAT detector for beam monitoring
Open this publication in new window or tab >>Amplification and conditioning properties of GEM and CAT detector for beam monitoring
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1999 In: Proceeding of International Workshop on Micro Pattern Gaseous Detectors, OrsayArticle in journal (Refereed) Published
Identifiers
urn:nbn:se:su:diva-25772 (URN)
Note
Part of urn:nbn:se:su:diva-857Available from: 2006-02-17 Created: 2006-02-17Bibliographically approved
2. Evaluation of a GEM and CAT-based detector for radiation therapy beam monitoring
Open this publication in new window or tab >>Evaluation of a GEM and CAT-based detector for radiation therapy beam monitoring
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2000 In: Nuclear Instruments and Methods in Physics Research A, Vol. 454, 136-141 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:su:diva-25773 (URN)
Note
Part of urn:nbn:se:su:diva-857Available from: 2006-02-17 Created: 2006-02-17Bibliographically approved
3. A novel portal imaging device for advanced radiation therapy
Open this publication in new window or tab >>A novel portal imaging device for advanced radiation therapy
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2001 In: IEEE Transactions on Nuclear Science, Vol. 48, no 4, Part 2, 1496-1502 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:su:diva-25774 (URN)
Note
Part of urn:nbn:se:su:diva-857Available from: 2006-02-17 Created: 2006-02-17Bibliographically approved
4. Study of hole-type gas multiplication structures for portal imaging and other high counting rate applications
Open this publication in new window or tab >>Study of hole-type gas multiplication structures for portal imaging and other high counting rate applications
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2003 In: IEEE Transactions on Nuclear Science, Vol. 50, no 4, Part 1, 809-819 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:su:diva-25775 (URN)
Note
Part of urn:nbn:se:su:diva-857Available from: 2006-02-17 Created: 2006-02-17Bibliographically approved
5. A radiation tolerant electronic readout system for portal imaging
Open this publication in new window or tab >>A radiation tolerant electronic readout system for portal imaging
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2004 In: Nuclear Instruments and Methods in Physics Research A, Vol. 525, 308-312 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:su:diva-25776 (URN)
Note
Part of urn:nbn:se:su:diva-857Available from: 2006-02-17 Created: 2006-02-17Bibliographically approved

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