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Characterisation and application of photon counting X-ray detector systems
Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media. (STC)
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis concerns the development and characterisation of X-ray imaging systems based on single photon processing. “Colour” X-ray imaging opens up new perspectives within the fields of medical X-ray diagnosis and also in industrial X-ray quality control. The difference in absorption for different “colours” can be used to discern materials in the object. For instance, this information might be used to identify diseases such as brittle-bone disease. The “colour” of the X-rays can be identified if the detector system can process each X-ray photon individually. Such a detector system is called a “single photon processing” system or, less precise, a “photon counting system”.

With modern technology it is possible to construct photon counting detector systems that can resolve details to a level of approximately 50 µm. However with such small pixels a problem will occur. In a semiconductor detector each absorbed X-ray photon creates a cloud of charge which contributes to the image. For high photon energies the size of the charge cloud is comparable to 50 µm and might be distributed between several pixels in the image. Charge sharing is a key problem since, not only is the resolution degenerated, but it also destroys the “colour” information in the image.

This thesis presents characterisation and simulations to provide a detailed understanding of the physical processes concerning charge sharing in detectors from the MEDIPIX collaboration. Charge summing schemes utilising pixel to pixel communications are proposed. Charge sharing can also be suppressed by introducing 3D-detector structures. In the next generation of the MEDIPIX system, Medipix3, charge summing will be implemented. This system, equipped with a 3D-silicon detector, or a thin planar high-Z detector of good quality, has the potential to become a commercial product for medical imaging. This would be beneficial to the public health within the entire European Union.

Abstract [sv]

Denna avhandling berör utveckling och karaktärisering av fotonräknande röntgensystem. ”Färgröntgen” öppnar nya perspektiv för medicinsk röntgendiagnostik och även för materialröntgen inom industrin. Skillnaden i absorption av olika ”färger” kan användas för att särskilja olika material i ett objekt. Färginformationen kan till exempel användas i sjukvården för att identifiera benskörhet. Färgen på röntgenfotonen kan identifieras om detektorsystemet kan detektera varje foton individuellt. Sådana detektorsystem kallas ”fotonräknande” system.

Med modern teknik är det möjligt att konstruera fotonräknande detektorsystem som kan urskilja detaljer ner till en upplösning på circa 50 µm. Med så små pixlar kommer ett problem att uppstå. I en halvledardetektor ger varje absorberad foton upphov till ett laddningsmoln som bidrar till den erhållna bilden. För höga fotonenergier är storleken på laddningsmolnet jämförbar med 50 µm och molnet kan därför fördelas över flera pixlar i bilden. Laddningsdelning är ett centralt problem delvis på grund av att bildens upplösning försämras, men framför allt för att färginformationen i bilden förstörs.

Denna avhandling presenterar karaktärisering och simulering för att ge en mer detaljerad förståelse för fysikaliska processer som bidrar till laddningsdelning i detektorer från MEDIPIX-projekter. Designstrategier för summering av laddning genom kommunikation från pixel till pixel föreslås. Laddningsdelning kan också begränsas genom att introducera detektorkonstruktioner i 3D-struktur. I nästa generation av MEDIPIX-systemet, Medipix3, kommer summering av laddning att vara implementerat. Detta system, utrustat med en 3D-detektor i kisel, eller en tunn plan detektor av högabsorberande material med god kvalitet, har potentialen att kunna kommersialiseras för medicinska röntgensystem. Detta skulle bidra till bättre folkhälsa inom hela Europeiska Unionen.

Place, publisher, year, edition, pages
Sundsvall: Mid Sweden Univ , 2007.
Series
Mid Sweden University doctoral thesis, ISSN 1652-893X ; 26
Keywords [en]
Monte Carlo simulation, Three-dimensional, X-ray flouriscence, Charge transport, Semiconductor materials, Image sensors, CdTe, Photon counting, Synchrotron radiation, Material recognition, X-ray, Pixel Detector, Silicon, Charge sharing, Imaging, Medipix, Spectroscopy, Dental diagnosis, Image quality, Energy weighting
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
URN: urn:nbn:se:miun:diva-38ISBN: 978-91-85317-55-4 (print)OAI: oai:DiVA.org:miun-38DiVA, id: diva2:1988
Public defence
2007-06-01, O102, Kornboden, Holmgatan 10, Sundsvall, 10:15 (English)
Opponent
Supervisors
Available from: 2007-11-23 Created: 2007-11-23 Last updated: 2011-02-06Bibliographically approved
List of papers
1.
The record could not be found. The reason may be that the record is no longer available or you may have typed in a wrong id in the address field.
2. Spectroscopy applications for the Medipix photon counting X-ray system
Open this publication in new window or tab >>Spectroscopy applications for the Medipix photon counting X-ray system
2004 (English)In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 531, no 1-2, p. 251-257Article in journal (Refereed) Published
Abstract [en]

Since the Medipix system is a photon counting system, it is possible to use it as a spectroscopic device, which is sensitive to the energy of the incoming photons. The system can, for example, be used to achieve the spectrum of an unknown source. Since it is an imaging system, it can be useful for energy dependent scattering experiments. The factors limiting the spectroscopic resolution are source and channel noise and detector non-uniformity. A well-known method for correction non-uniformity is to use mono energetic source and a threshold adjustment. However, since the threshold calibration is energy dependent, its use for sources with wider energy range is limited. We have shown an easy method to create threshold calibration masks for different thresholds that include both detector and channel stationary influences with analysis of histograms of series of images. The model for histograms was created assuming a standard dental X-ray source, which allows finding mask-creating parameters like threshold range (Vth and Vthadj). This procedure can be performed for each energy of interest. Based on these arrays, mask files that narrowed the threshold distribution close to the theoretical limit, were prepared. The limit of the spectroscopic resolution for the system was measured by analysing histograms for series of flat images under identical conditions.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE BV, 2004
Keywords
X-ray, Imaging; Pixel detector; Medipix, Spectroscopy
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:miun:diva-363 (URN)10.1016/j.nima.2004.06.013 (DOI)000224233400036 ()1676 (Local ID)1676 (Archive number)1676 (OAI)
Conference
5th International Workshop on Radiation Imaging Detectors, Sep 07-11, 2003, Riga, latvia
Note
5th International Workshop on Radiation Imaging Detectors, Sep 07-11, 2003, Riga, LatviaAvailable from: 2008-12-11 Created: 2008-12-11 Last updated: 2017-12-13Bibliographically approved
3. Material recognition with the Medipix photon counting colour X-ray system.
Open this publication in new window or tab >>Material recognition with the Medipix photon counting colour X-ray system.
2004 (English)In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 531, no 1-2, p. 265-269Article in journal (Refereed) Published
Abstract [en]

An energy sensitive imaging system like Medipixl has proved to be promising in distinguishing different materials in an X-ray image of an object. We propose a general method utilising X-ray energy information for materialrecognition. For objects where the thickness of the materials is unknown, a convenient material parameter to identify is K = α12, which is the ratio of the logarithms of the measured transmissions In(t1)/In(t2). If a database of the parameter K for different materials and energies is created, this method can be used for material recognition independent of the thickness of the materials. Series of images of an object consisting of aluminium and silicon were taken with different energy thresholds. The X-ray absorption for silicon and aluminium is very similar for the range 40-60 keV and only differs for lower energies. The results show that it is possible to distinguish between aluminium and silicon on images achieved by Medipixl using a standard dental source. By decreasing the spatial resolution a better contrast between the materials was achieved. The resolution of contrasts shown by the histograms was close to the limit of the system due to the statistical noise of the signal.

National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:miun:diva-364 (URN)10.1016/j.nima.2004.06.015 (DOI)000224233400038 ()1686 (Local ID)1686 (Archive number)1686 (OAI)
Conference
5th International Workshop on Radiation Imaging Detectors, Sep 07-11, 2003, Riga, Latvia
Note
5th International Workshop on Radiation Imaging Detectors, Sep 07-11, 2003, Riga, LatviaAvailable from: 2008-12-11 Created: 2008-12-11 Last updated: 2017-12-13Bibliographically approved
4. Energy dependence in dental imaging with Medipix2
Open this publication in new window or tab >>Energy dependence in dental imaging with Medipix2
2005 (English)In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 546, no 1-2, p. 19-23Article in journal (Refereed) Published
Abstract [en]

The possibility to achieve better contrast in dental images by selecting specific X-ray energies is investigated. By applying both the low and the high photon counting energy threshold of the Medipix2 chip only a narrow interval of the spectrum of a 60 keV standard dental source used is selected. The relative contrast between the parts of the tooth with the highest respectively lowest absorption increases 18 % when the energy spread of the X-rays is narrowed from the full spectrum to a 4 keV window centered around 36 keV. The small tail of high energy photons from the full source spectrum lowers the achieved relative contrast significantly. This result can be used in development of X-ray sources or in consideration of energy weighting.

Keywords
Dental diagnosis; X-ray; Image quality; Pixel detector; Medipix, Energy weighting
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:miun:diva-365 (URN)10.1016/j.nima.2005.03.035 (DOI)000230288200004 ()2261 (Local ID)2261 (Archive number)2261 (OAI)
Conference
6th International Workshop on Radiation Imaging Detectors, Jul 25-29, 2004, Glasgow, Scotland
Note
6th International Workshop on Radiation Imaging Detectors, Jul 25-29, 2004, Glasgow, ScotlandAvailable from: 2008-12-11 Created: 2008-12-11 Last updated: 2017-12-13Bibliographically approved
5. Spectral response of Pixellated Semiconductor X-ray Detectors
Open this publication in new window or tab >>Spectral response of Pixellated Semiconductor X-ray Detectors
2005 (English)In: 2005 IEEE Nuclear Science Symposium Conference Record, Vols 1-5, IEEE , 2005, Vol. 5, p. 2967-2970, article id 1596954Conference paper, Published paper (Refereed)
Abstract [en]

X-ray imaging with energy resolution can be performed using a detector matrix bonded to a photon counting CMOS readout circuit as the MEDIPIX2 chip. In previous experiments it has been shown that charge sharing between neighboring pixels plays an important role in the formation of the image and especially for the spectral information in the image. Charge sharing is caused both by the localization of the initial energy deposition and by diffusion during the transport of the charge to the readout electrode. In this work we have studied different factors that can effect the energy resolution in pixellated X-ray imaging detectors. Results are compared to experimental data.

Place, publisher, year, edition, pages
IEEE, 2005
Series
IEEE NUCLEAR SCIENCE SYMPOSIUM - CONFERENCE RECORD, ISSN 1082-3654
Keywords
Medipix, Charge transport, Semiconductor materials, X-ray fluorescence, X-ray image sensors
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:miun:diva-366 (URN)10.1109/NSSMIC.2005.1596954 (DOI)000241851904120 ()2-s2.0-33846640180 (Scopus ID)4199 (Local ID)0-7803-9221-3 (ISBN)4199 (Archive number)4199 (OAI)
Conference
Nuclear Science Symposium/Medical Imaging Conference, Oct 23-29, 2005, Fajardo, PR
Available from: 2008-12-11 Created: 2008-12-11 Last updated: 2016-10-05Bibliographically approved
6. Characterization of a pixellated CdTe detector with single-photon processing readout
Open this publication in new window or tab >>Characterization of a pixellated CdTe detector with single-photon processing readout
Show others...
2006 (English)In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 563, no 563, p. 128-132Article in journal (Refereed) Published
Abstract [en]

A 1mm thick pixellated CdTe detector bonded to the MEDIPIX2 [1] readout chip has been characterized using a monoenergetic microbeam at the ESRF. This is an extension of the tests previously reported in [2]. The results show that a full energy peak can be obtained when a narrow beam is focused in the centre of the pixel. There is also evidence of significant charge diffusion and fluorescence. The results indicate that the charge sharing is the most important problem and will cause loss of the energy information in an imaging application. The second problem is the fluorescence which limits the number of counts in the full energy peak even for hits in the centre of the pixel.

Keywords
Elektroteknik, elektronik och fotonik
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:miun:diva-367 (URN)10.1016/j.nima.2006.01.076 (DOI)000238764700031 ()2-s2.0-33745009060 (Scopus ID)
Conference
7th International Workshop on Radiation Imaging Detectors, Jul 04-07, 2005, Grenoble, France
Note

Correct author list in Erratum to "Characterisation of a pixellated CdTe detector with Single Photon Processing Readout", Nuclear Instruments and Methods in Physics Research A (2007), doi:10.1016/j.nima.2007.01.096. 7th International Workshop on Radiation Imaging Detectors, Jul 04-07, 2005, Grenoble, France

Erratum to: “Characterization of a pixellated CdTe detector with single-photon processing readout”: [Nucl. Instr. and Meth. A 563 (2006) 128–132]

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment

Volume 574, Issue 1, 21 April 2007, Pages 205

DOI: 10.1016/j.nima.2006.01.096

ISI: 000246159200029

ScopusID: 2-s2.0-33947690351

Available from: 2008-12-11 Created: 2008-12-11 Last updated: 2017-12-13Bibliographically approved
7. Characterisation of the charge sharing in pixellated Si detectors with single-photon processing readout
Open this publication in new window or tab >>Characterisation of the charge sharing in pixellated Si detectors with single-photon processing readout
Show others...
2006 (English)In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. A 563, no 1, p. 133-136Article in journal (Refereed) Published
Abstract [en]

Pixellated silicon detectors with a thickness of 300 µm and 700 µm bonded to the MEDIPIX2 readout chip have been characterised using a monoenergetic microbeam at the ESRF. The spectral response when a 10 x 10 µm2 wide 40 keV beam is centred on a single pixel is achieved. When the beam is scanned over the pixel, the charge sharing will increase when the beam approaches the border of the pixel. The experimental results have been verified by charge transport simulations and X-ray scattering simulations. Agreement between measurements and simulations can be achieved if a wider beam is assumed in the simulations. Widening of the absorption profile can to a large extent be explained by backscattering of lower energy photons by the tin/led bump-bounds below the detector. Widening of the detected beam is also an effect of angular alignment problems, especially on the 700 µm detector. Since the angel between the depth and a half pixel is only 2.2º, alignment of thick pixellated silicon detectors will be a problem to consider when designing X-ray imaging setups.

Keywords
Charge sharing, X-ray, Medipix, Pixel detector, Monte Carlo simulation, Synchrotron radiation
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:miun:diva-368 (URN)10.1016/j.nima.2006.01.077 (DOI)000238764700032 ()2-s2.0-33744969359 (Scopus ID)3758 (Local ID)3758 (Archive number)3758 (OAI)
Conference
7th International Workshop on Radiation Imaging Detectors, Jul 04-07, 2005, Grenoble, France
Note

7th International Workshop on Radiation Imaging Detectors, Jul 04-07, 2005, Grenoble, France

Available from: 2008-12-11 Created: 2008-12-11 Last updated: 2017-12-13Bibliographically approved
8. Spectral performance of a pixellated X-ray imaging detector with suppressed charge sharing
Open this publication in new window or tab >>Spectral performance of a pixellated X-ray imaging detector with suppressed charge sharing
2007 (English)In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. A 576, no 1, p. 248-250Article in journal (Refereed) Published
Abstract [en]

The ability to do X-ray imaging with energy resolution on a small pixel detector is severely limited by the charge sharing in the detector. Different schemes to overcome the problem have been proposed. Previous studies using synchrotron radiation have shown that, for a 300 µm thick Si detector, almost no charge sharing occurs for photons hitting the centre of each pixel. In this study we have used slits to focus the beam from a standard X-ray unit on the centre of a pixel on a MEDIPIX detector. The attenuation of the spectrum were measured for a number of samples of different materials with K-edge energy in the range of 30 to 50 keV. The measurements were done by scanning an energy window through the spectrum. Requirements for new X-ray imaging systems with true energy resolution are discussed based on these measurements.

Keywords
X-ray spectroscopy, Pixel detector, Medipix, Charge sharing, Material recognition
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:miun:diva-369 (URN)10.1016/j.nima.2007.01.095 (DOI)000247330000053 ()2-s2.0-34248162990 (Scopus ID)4246 (Local ID)4246 (Archive number)4246 (OAI)
Conference
8th International Workshop on Radiation Imaging Detectors, Jul 02-06, 2006, Pisa, Italy
Note

8th International Workshop on Radiation Imaging Detectors, Jul 02-06, 2006, Pisa, Italy

Available from: 2007-11-23 Created: 2007-11-23 Last updated: 2017-12-13Bibliographically approved
9. Charge sharing suppression using pixel-to-pixel communication in photon counting X-ray imaging systems
Open this publication in new window or tab >>Charge sharing suppression using pixel-to-pixel communication in photon counting X-ray imaging systems
2007 (English)In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 576, no 1, p. 243-247Article in journal (Refereed) Published
Abstract [en]

In planar silicon detector structures the charge sharing between pixels is one limiting factor for colour X-ray imaging using integrated photon counting pixel detectors. 3D detector structures have been proposed as one solution to this problem. However, there are also readout system solutions to the problem i.e. introducing pixel to pixel communication and distributed charge summing in the readout electronics. In this work different charge summing schemes are evaluated using Monte Carlo simulation techniques. The increase in electronic noise introduced by the charge summing is one of the most severe problems. A proper selection of summing scheme is necessary to obtain an efficient system.

Keywords
Charge sharing, photon counting, imaging
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:miun:diva-370 (URN)10.1016/j.nima.2007.01.160 (DOI)000247330000052 ()2-s2.0-34248184587 (Scopus ID)4247 (Local ID)4247 (Archive number)4247 (OAI)
Conference
8th International Workshop on Radiation Imaging Detectors, Jul 02-06, 2006, Pisa, italy
Note

8th International Workshop on Radiation Imaging Detectors, JUL 02-06, 2006, Pisa, Italy

Available from: 2008-12-11 Created: 2008-12-11 Last updated: 2017-12-13Bibliographically approved

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