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Methods of image acquisition and calibration for x-ray computed tomography
KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging. (SHL, Medical Imaging)
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: urn:nbn:se:kth:diva-195024ISBN: 978-91-7729-191-6OAI: oai:DiVA.org:kth-195024DiVA: diva2:1044400
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
List of papers
1. Energy calibration by simultaneous model-fitting to threshold scans for several kVp
Open this publication in new window or tab >>Energy calibration by simultaneous model-fitting to threshold scans for several kVp
2016 (English)Article in journal (Other academic) Submitted
Abstract [en]

 Accurate calibration of the energy thresholds on spectral x-ray detectors used for computed tomography is essential for avoiding ring artifacts and bias when using basis material decomposition. Previously, it has been proposed to calibrate the energy thresholds by fitting simulated data to a threshold scan of a polychromatic spectrum from a medical x-ray tube. Here, we suggest that the energy calibration is performed by fitting simulated data to threshold scans acquired at several different kVp settings simultaneously. The method is evaluated experimentally by its capacity to correctly position the noise floor and the k-edge of tungsten. It is shown that using several kVp settings largely improves the accuracy of the energy calibration, particularly in the low energy range. Since the method only relies on the spectra from the x-ray tube (without any specific filtration), it is ideal for the use in a clinical computed tomography system. It is also shown that the method can operate with a fairly low number of detected photons, whichis promising in terms of calibration time.

National Category
Accelerator Physics and Instrumentation
Research subject
Medical Technology
Identifiers
urn:nbn:se:kth:diva-195017 (URN)
Note

QCR 20161103

Available from: 2016-11-01 Created: 2016-11-01 Last updated: 2016-11-22Bibliographically approved
2. Relative calibration of energy thresholds on multi-bin spectral x-ray detectors
Open this publication in new window or tab >>Relative calibration of energy thresholds on multi-bin spectral x-ray detectors
2016 (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. 840, 1-4 p.Article in journal (Refereed) Published
Abstract [en]

Accurate and reliable energy calibration of spectral x-ray detectors used in medical imaging is essential for avoiding ring artifacts in the reconstructed images (computed tomography) and for performing accurate material basis decomposition. A simple and accurate method for relative calibration of the energy thresholds on a multi-bin spectral x-ray detector is presented. The method obtains the linear relations between all energy thresholds in a channel by scanning the thresholds with respect to each other during x-ray illumination. The method does not rely on a model of the detector's response function and does not require any identifiable features in the x-ray spectrum. Applying the same method, the offset between the thresholds can be determined also without external stimuli by utilizing the electronic noise as a source. The simplicity and accuracy of the method makes it suitable for implementation in clinical multi-bin spectral x-ray imaging systems.

Place, publisher, year, edition, pages
Elsevier, 2016
Keyword
Computed tomography Energy calibration Multi-bin Photon-counting Spectral X-ray detectors
National Category
Medical Equipment Engineering
Research subject
Medical Technology
Identifiers
urn:nbn:se:kth:diva-195022 (URN)10.1016/j.nima.2016.09.045 (DOI)2-s2.0-84991812993 (ScopusID)
Note

QC 20161101

Available from: 2016-11-01 Created: 2016-11-01 Last updated: 2016-11-03Bibliographically approved
3. A method for geometric calibration of edge-on detectors in a CT-gantry
Open this publication in new window or tab >>A method for geometric calibration of edge-on detectors in a CT-gantry
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.

National Category
Medical Equipment Engineering
Research subject
Medical Technology
Identifiers
urn:nbn:se:kth:diva-195018 (URN)10.1118/1.4964792 (DOI)
Note

QCR 20161103

Available from: 2016-11-01 Created: 2016-11-01 Last updated: 2016-11-03Bibliographically approved
4. Angular oversampling with temporally offset layers on multilayer detectors in computed tomography
Open this publication in new window or tab >>Angular oversampling with temporally offset layers on multilayer detectors in computed tomography
2016 (English)In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 43, no 6, 2877-2883 p.Article in journal (Refereed) Published
Abstract [en]

Purpose: Todays computed tomography (CT) scanners operate at an increasingly high rotation speed in order to reduce motion artifacts and to fulfill the requirements of dynamic acquisition, e.g., perfusion and cardiac imaging, with lower angular sampling rate as a consequence. In this paper, a simple method for obtaining angular oversampling when using multilayer detectors in continuous rotation CT is presented. Methods: By introducing temporal offsets between the measurement periods of the different layers on a multilayer detector, the angular sampling rate can be increased by a factor equal to the number of layers on the detector. The increased angular sampling rate reduces the risk of producing aliasing artifacts in the image. A simulation of a detector with two layers is performed to prove the concept. Results: The simulation study shows that aliasing artifacts from insufficient angular sampling are reduced by the proposed method. Specifically, when imaging a single point blurred by a 2D Gaussian kernel, the method is shown to reduce the strength of the aliasing artifacts by approximately an order of magnitude. Conclusions: The presented oversampling method is easy to implement in todays multilayer detectors and has the potential to reduce aliasing artifacts in the reconstructed images.

Place, publisher, year, edition, pages
AAPM - American Association of Physicists in Medicine, 2016
Keyword
computed tomography, double-layer, edge-on, multilayer detector, oversampling
National Category
Medical Image Processing
Identifiers
urn:nbn:se:kth:diva-194541 (URN)10.1118/1.4948505 (DOI)2-s2.0-84969761004 (ScopusID)
Note

QC 20161031

Available from: 2016-10-31 Created: 2016-10-31 Last updated: 2016-11-03Bibliographically approved
5. Compression of CT sinogram data by decimation in the view direction
Open this publication in new window or tab >>Compression of CT sinogram data by decimation in the view direction
(English)Article in journal (Other academic) Submitted
Abstract [en]

Purpose: In clinical computed tomography (CT), the image data is acquired during continuous

rotation. If the time during which the signal is integrated (the frame time) is too long, the data

is blurred in the view direction. This can be overcome by having a high angular sampling rate,

but for systems with limited bandwidth, the increased amount of data can be a problem. In this

paper, we evaluate the benefit of maintaining a high angular sampling rate on the CT gantry

and performing a decimation (digital low-pass filtration followed by a downsampling) in the view

direction before the bottleneck of the data transfer chain.

Methods: A theoretical evaluation of the effects of the decimation is presented and the

implementation of the digital filter is discussed. The compression scheme is evaluated on image

data of a CATPHANR 504 phantom.

Results: From a resolution point-of-view, it is beneficial to sample at more than twice the

Nyquist frequency (the minimum rate required for not introducing aliasing). At this sampling

rate, a decimation step can reduce the amount of data by a factor of two without loosing any of

the signal. It is demonstrated that a 2:1 compression can be achieved without compromising the

spatial resolution or increasing the noise.

Conclusions: In continuous rotation CT, the angular sampling rate must be high in order

not to suffer from blur in the view direction. From a sampling point-of-view, the sampling rate

is higher than necessary and the signal can be decimated (low-pass filtered and downsampled)

without losing any signal. The proposed compression scheme can be implemented on the CT

gantry and thus reduce the bandwidth requirements on the data transfer.

National Category
Medical Equipment Engineering
Research subject
Medical Technology
Identifiers
urn:nbn:se:kth:diva-195021 (URN)
Note

QCR 20161101

Available from: 2016-11-01 Created: 2016-11-01 Last updated: 2016-11-03Bibliographically approved

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