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A method for geometric calibration of edge-on detectors in a CT-gantry
KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging. (SHL, Medical Imaging)ORCID iD: 0000-0002-8626-495X
KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging. (SHL, Medical Imaging)ORCID iD: 0000-0002-3039-9791
2016 (English)In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 43, no 11, 6165-6174 p.Article in journal (Refereed) Published
Abstract [en]

Purpose: Photon-counting edge-on detectors are currently being considered for use

in clinical computed tomography (CT) systems. A method for geometric calibration

of edge-on detectors mounted in a CT gantry has been developed and evaluated.

The method is complementary to the geometrical calibration methods developed for

CT systems using flat-panel detectors and takes the extra dimension of the edge-on

detectors (along the direction of the x-rays) into account.

Methods: The method uses projection images of a simple phantom together with

geometrical arguments to accurately estimate the orientation and relative position of

the edge-on detectors. Both computer simulations and experimental measurements

were used to verify the method.

Results: It is experimentally demonstrated that the method can determine the orientation

of the detector with an accuracy of 0.08 degrees. The method is also shown

to be insensitive to errors in the modeled parameters used in the algorithm.

Conclusions: The presented method can accurately determine the orientation and

relative position of edge-on detectors mounted in a CT-gantry and can be used to

evaluate the detector mounting and to produce an accurate forward model of the

imaging system. Also, the method has potential to reduce the dimensionality of the

geometric calibration of the full CT system since the direction of the x-rays with

respect to the detector is measured.

Place, publisher, year, edition, pages
2016. Vol. 43, no 11, 6165-6174 p.
National Category
Medical Equipment Engineering
Research subject
Medical Technology
Identifiers
URN: urn:nbn:se:kth:diva-195018DOI: 10.1118/1.4964792ISI: 000387007500038ScopusID: 2-s2.0-84994032306OAI: oai:DiVA.org:kth-195018DiVA: diva2:1043888
Note

QCR 20161103

Available from: 2016-11-01 Created: 2016-11-01 Last updated: 2016-12-14Bibliographically approved
In thesis
1. Methods of image acquisition and calibration for x-ray computed tomography
Open this publication in new window or tab >>Methods of image acquisition and calibration for x-ray computed tomography
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

X-ray computed tomography (CT) is a common medical imaging device for acquiring high-resolution 3D images of the interior of the human body. The images are formed by mathematical reconstruction from hundreds of planar x-ray images that have been acquired during less then a second.

Photon-counting spectral detectors are seen by many as the next big step in the development of medical CT. The potential benefits include: quantitative CT, ultra-low dose imaging and optimal contrast-to-noise performance. The current aim for the research pursued by the Physics of Medical Imaging Group at KTH is to develop, and commercialize, a photon-counting spectral detector using silicon wafers in edge-on geometry. With the introduction of a new detector comes many challenges, some of which this Thesis aims to address.

Efficient calibration schemes will be an essential part of the realization of photon-counting spectral detectors in clinical CT. In the first part of the Thesis, three calibration methods are presented: two methods for calibration of the energy thresholds on multi-bin spectral detectors and one method for geometric calibration of edge-on detectors that are mounted in a CT gantry.

The CT image acquisition produces large amounts of data that have to be transported out of the system, preferably in real-time. Already today, fewer samples are acquired when operating at very high rotation speeds due to bandwidth limitations. For photon-counting spectral detectors, the amount of data will be even larger due to the additional energy information and the generally smaller pixels, and it is therefore desirable to minimize the number of angular samples acquired per revolution. In the second part of the Thesis, two methods for relaxing the angular sampling requirement are presented. The first method uses the built-in redundancy of multi-layer detectors to increase the angular sampling rate via a temporal offset between the detector layers. The second method uses decimation in the view (angular) direction as a means for compression of CT sinogram data. The compression can be performed on the CT gantry and thus lower the required bandwidth of the data transfer.

Although the overall aim of this work has been to develop methods that facilitate the introduction of photon-counting spectral detectors for medical CT, the presented methods are also applicable in the broader context of calibration of x-ray detectors and CT image acquisition.

Abstract [sv]

Datortomografi (CT) är en vanligt förekommande medicinsk avbildningsteknik som används för att ta högupplösta 3D bilder av människans inre. Bilderna rekonstrueras matematiskt från hundratals 2D röntgenbilder som har tagits under mindre än en sekund.

Introduktionen av spektrala fotonräknande röntgendetektorer anses vara nästa stora steg i utvecklingen av medicinsk CT. De potentiella fördelarna innefattar: kvantitativ CT, avbildning vid ultra-låg dos och optimalt kontrast-brus förhållande. Målet för det arbete som utförs av gruppen för Medicinsk Bildfysik på KTH är att utveckla och kommersialisera en spektral fotonräknande detektor baserad på kiselskivor som är monterade ”edge-on” (med kanten pekandes mot röntgenkällan). Den här avhandlingen adresserar några utav de utmaningar som följer införandet av denna nya typ av detektor.

Tillgången till effektiva kalibreringstekniker kommer att vara nödvändig för realisationen av spektrala fotonräknande detektorer i medicinsk CT. I den första delen av avhandlingen presenteras tre kalibreringsmetoder, varav två relaterar till kalibrering av energitrösklarna på spektrala röntgendetektorer och en relaterar till geometrisk kalibrering av ”edge-on” detektorer monterade i en CT scanner.

Bildtagningen i CT producerar stora mängder data som måste transporteras ut ur systemet, gärna i realtid. Redan idag tvingas man ofta ha färre mätpunkter när man använder höga rotationshastigheter på grund av begränsningar i utläsningens bandbredd eller mätelektronikens hastighet. För spektrala fotonräknande detektorer kommer mängden data att öka på grund av den extra energiinformationen och de generellt mindre pixlarna. Därför är det önskvärt att minimera antalet mätpunkter i vinkelled per varv. I den andra delen av avhandlingen presenteras två metoder som minskar kravet på antalet mätpunkter per varv. Den första metoden använder den inbyggda redundansen hos detektorer med flera lager för att ökan antalet mätpunkter i vinkelled genom att förskjuta mätpunkterna för dom olika lagerna i tiden. Den andra metoden använder decimering i vinkelled för att komprimera CT data. Kompressionen kan utföras på CT scannern och kan användas för att minska kravet på datautläsningens bandbredd.

Det övergripande målet för arbetet som utgör avhandling har varit att utveckla metoder som möjliggör introduktionen av spektrala fotonräknande detektorer för medicinsk CT. De presenterade metoderna är emellertid även användbara i den mer generella kontexten av kalibrering av röntgendetektorer och bildtagning i CT.

Place, publisher, year, edition, pages
Stockholm: Kungliga Tekniska högskolan, 2016. 36 p.
Series
TRITA-FYS, ISSN 0280-316X ; 2016:60
National Category
Medical Equipment Engineering
Research subject
Medical Technology
Identifiers
urn:nbn:se:kth:diva-195024 (URN)978-91-7729-191-6 (ISBN)
Public defence
2016-12-09, M2, Brinellvägen 64, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20161104

Available from: 2016-11-04 Created: 2016-11-01 Last updated: 2016-11-04Bibliographically approved

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