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Publications (10 of 28) Show all publications
Persson, M., Holmin, S., Karlsson, S., Bornefalk, H. & Danielsson, M. (2018). Subpixel x-ray imaging with an energy-resolving detector. Journal of Medical Imaging, 5(1), Article ID 013507.
Open this publication in new window or tab >>Subpixel x-ray imaging with an energy-resolving detector
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2018 (English)In: Journal of Medical Imaging, ISSN 2329-4302, E-ISSN 2329-4310, Vol. 5, no 1, article id 013507Article in journal (Refereed) Published
Abstract [en]

The detector pixel size can be a severe limitation in x-ray imaging of fine details in the human body. We demonstrate a method of using spectral x-ray measurements to image the spatial distribution of the linear attenuation coefficient on a length scale smaller than one pixel, based on the fact that interfaces parallel to the x-ray beam have a unique spectral response, which distinguishes them from homogeneous materials. We evaluate the method in a simulation study by simulating projection imaging of the border of an iodine insert with 200 mg/ml I in a soft tissue phantom. The results show that the projected iodine profile can be recovered with an RMS resolution of 5% to 34% of the pixel size, using an ideal energy-resolving detector. We also validate this method in an experimental study by imaging an iodine insert in a polyethylene phantom using a photon-counting silicon-strip detector. The results show that abrupt and gradual transitions can be distinguished based on the transmitted x-ray spectrum, in good agreement with simulations. The demonstrated method may potentially be used for improving visualization of blood vessel boundaries, e.g., in acute stroke care.

Place, publisher, year, edition, pages
SPIE - International Society for Optical Engineering, 2018
Keywords
x-ray imaging, subpixel information, spectral x-ray imaging, photon-counting detector, partial volume effect
National Category
Radiology, Nuclear Medicine and Medical Imaging
Identifiers
urn:nbn:se:kth:diva-226818 (URN)10.1117/1.JMI.5.1.013507 (DOI)000429258000030 ()29564367 (PubMedID)2-s2.0-85044647808 (Scopus ID)
Funder
VINNOVA, 2014-03800Stockholm County Council, 20140712
Note

QC 20180503

Available from: 2018-05-03 Created: 2018-05-03 Last updated: 2018-05-03Bibliographically approved
Persson, M. & Grönberg, F. (2017). Bias-variance tradeoff in anticorrelated noise reduction for spectral CT. Paper presented at 4th International Conference on Image Formation in X-Ray Computed Tomography, JUL 18-22, 2016, Bamberg, GERMANY. Medical physics (Lancaster), 44(9), E242-E254
Open this publication in new window or tab >>Bias-variance tradeoff in anticorrelated noise reduction for spectral CT
2017 (English)In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 44, no 9, p. E242-E254Article in journal (Refereed) Published
Abstract [en]

Purpose: In spectral CT, basis material decomposition is commonly used to generate a set of basis images showing the material composition at each point in the field of view. The noise in these images typically contains anticorrelations between the different basis images, which leads to increased noise in each basis image. These anticorrelations can be removed by changing the basis functions used in the material decomposition, but the resulting basis images can then no longer be used for quantitative measurements. Recent studies have demonstrated that reconstruction methods which take the anticorrelations into account give reduced noise in the reconstructed image. The purpose of this work is to analyze an analytically solvable denoising model problem and investigate its effect on the noise level and bias in the image as a function of spatial frequency. Method: A denoising problem with a quadratic regularization term is studied as a mathematically tractable model for such a reconstruction method. An analytic formula for the resulting image in the spatial frequency domain is presented, and this formula is applied to a simple mathematical phantom consisting of an iodinated contrast agent insert embedded in soft tissue. We study the effect of the denoising on the image in terms of its transfer function and the visual appearance, the noise power spectrum and the Fourier component correlation coefficient of the resulting image, and compare the result to a denoising problem which does not model the anticorrelations in the image. Results: Including the anticorrelations in the noise model of the denoising method gives 3-40% lower noise standard deviation in the soft-tissue image while leaving the iodine standard deviation nearly unchanged (0-1% difference). It also gives a sharper edge-spread function. The studied denoising method preserves the noise level and the anticorrelated structure at low spatial frequencies but suppresses the noise and removes the anticorrelations at higher spatial frequencies. Cross-talk between images gives rise to artifacts at high spatial frequencies. Conclusions: Modeling anticorrelations in a denoising problem can decrease the noise level in the basis images by removing anticorrelations at high spatial frequencies while leaving low spatial frequencies unchanged. In this way, basis image cross-talk does not lead to low spatial frequency bias but it may cause artifacts at edges in the image. This theoretical insight will be useful for researchers analyzing and designing reconstruction algorithms for spectral CT.

Place, publisher, year, edition, pages
WILEY, 2017
Keywords
anticorrelated noise, basis material decomposition, computed tomography, denoising, spectral CT
National Category
Medical Image Processing
Identifiers
urn:nbn:se:kth:diva-217065 (URN)10.1002/mp.12322 (DOI)000411804500014 ()28901607 (PubMedID)2-s2.0-85029324082 (Scopus ID)
Conference
4th International Conference on Image Formation in X-Ray Computed Tomography, JUL 18-22, 2016, Bamberg, GERMANY
Note

QC 20171122

Available from: 2017-11-22 Created: 2017-11-22 Last updated: 2017-11-22Bibliographically approved
Persson, M. & Adler, J. (2017). Spectral CT reconstruction with anti-correlated noise model and joint prior. In: : . Paper presented at Proceedings of the 14th International Meeting on Fully Three-Dimensional Image Reconstruction in Radiology and Nuclear Medicine (pp. 580-585).
Open this publication in new window or tab >>Spectral CT reconstruction with anti-correlated noise model and joint prior
2017 (English)Conference paper, Published paper (Other academic)
Abstract [en]

Spectral CT allows reconstructing a set of material selective basis images which can be used for material quantification. These basis images can be reconstructed independently of each other or treated as a joint reconstruction problem. In this work, we investigate the effect of two ways of introducing coupling between the basis images: using an anti-correlated noise model and regularizing the basis images with a joint prior. We simulate imaging of a FORBILD Head phantom with an ideal photon-counting detector and reconstruct the resulting basis sinograms with and without these two kinds of coupling. The results show that the anti-correlated noise model gives better spatial resolution than the uncorrelated noise model at the same noise level, but also introduces artifacts. If anti-correlations are introduced also in the prior, these artifacts are reduced and the resolution is improved further.

National Category
Medical Image Processing
Identifiers
urn:nbn:se:kth:diva-250643 (URN)10.12059/Fully3D.2017-11-3203027 (DOI)
Conference
Proceedings of the 14th International Meeting on Fully Three-Dimensional Image Reconstruction in Radiology and Nuclear Medicine
Note

QC 20190520

Available from: 2019-05-01 Created: 2019-05-01 Last updated: 2019-05-20Bibliographically approved
Persson, M. (2016). Spectral Computed Tomography with a Photon-Counting Silicon-Strip Detector. (Doctoral dissertation). Stockholm: KTH Royal Institute of Technology
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. p. 43
Series
TRITA-FYS, ISSN 0280-316X ; 2016:20
Keywords
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
Liu, X., Persson, M., Bornefalk, H., Karlsson, S., Xu, C., Danielsson, M. & Huber, B. (2016). Spectral response model for a multibin photon-counting spectral computed tomography detector and its applications (vol 2, 033502, 2015). Journal of Medical Imaging, 3(4), Article ID 049801.
Open this publication in new window or tab >>Spectral response model for a multibin photon-counting spectral computed tomography detector and its applications (vol 2, 033502, 2015)
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2016 (English)In: Journal of Medical Imaging, ISSN 2329-4302, E-ISSN 2329-4310, Vol. 3, no 4, article id 049801Article in journal (Refereed) Published
Place, publisher, year, edition, pages
SPIE - International Society for Optical Engineering, 2016
National Category
Radiology, Nuclear Medicine and Medical Imaging
Identifiers
urn:nbn:se:kth:diva-200772 (URN)10.1117/1.JMI.3.4.049801 (DOI)000391124500025 ()
Note

20170206

Available from: 2017-02-06 Created: 2017-02-02 Last updated: 2017-06-22Bibliographically approved
Persson, M., Bujila, R., Nowik, P., Andersson, H., Kull, L., Andersson, J., . . . Danielsson, M. (2016). Upper limits of the photon fluence rate on CT detectors: Case study on a commercial scanner. Medical physics (Lancaster), 43(7), 4398-4411
Open this publication in new window or tab >>Upper limits of the photon fluence rate on CT detectors: Case study on a commercial scanner
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2016 (English)In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 43, no 7, p. 4398-4411Article in journal (Refereed) Published
Abstract [en]

Purpose: The highest photon fluence rate that a computed tomography (CT) detector must be able to measure is an important parameter. The authors calculate the maximum transmitted fluence rate in a commercial CT scanner as a function of patient size for standard head, chest, and abdomen protocols. Methods: The authors 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 current applied with the tube current modulation (TCM) system. By rescaling this tube current using published measurements on the tube current modulation of a GE scanner [N. Keat, "CT scanner automatic exposure control systems," MHRA Evaluation Report 05016, ImPACT, London, UK, 2005], the authors could estimate the tube current that these protocols would have resulted in for other patient sizes. An ECG gated chest protocol was also simulated. Using measured dose rate profiles along the bowtie filters, the authors simulated imaging of anonymized patient images with a range of sizes on a GE VCT scanner and calculated the maximum transmitted fluence rate. In addition, the 99th and the 95th percentiles of 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 protocols with centered patients are found for head images and for intermediate-sized chest images, both with a maximum of 3.4 . 10(8) mm(-2) s-1, at 949 mm distance from the source. Miscentering the head by 50 mm downward increases the maximum transmitted fluence rate to 5.7 . 10(8) mm(-2) s(-1). The ECG gated chest protocol gives fluence rates up to 2.3 . 10(8)-3.6 . 10(8) mm(-2) s(-1) depending on miscentering. Conclusions: The fluence rate on a CT detector reaches 3 . 10(8)-6 . 10(8) mm(-2) s(-1) in standard imaging protocols, with the highest rates occurring for ECG gated chest and miscentered head scans. These results will be useful to developers of CT detectors, in particular photon counting detectors. (C) 2016 American Association of Physicists in Medicine.

Place, publisher, year, edition, pages
AMER ASSOC PHYSICISTS MEDICINE AMER INST PHYSICS, 2016
Keywords
fluence rate, count rate requirements, count rate problem, photon counting CT
National Category
Medical Equipment Engineering
Identifiers
urn:nbn:se:kth:diva-243843 (URN)10.1118/1.4954008 (DOI)000379171900044 ()27370155 (PubMedID)2-s2.0-84976381566 (Scopus ID)
Note

QC 20190207

Available from: 2019-02-07 Created: 2019-02-07 Last updated: 2019-02-07Bibliographically approved
Bornefalk, H., Persson, M. & Danielsson, M. (2015). Allowable forward model misspecification for accurate basis decomposition in a silicon detector based spectral CT. IEEE Transactions on Medical Imaging, 34(3), 788-795
Open this publication in new window or tab >>Allowable forward model misspecification for accurate basis decomposition in a silicon detector based spectral CT
2015 (English)In: IEEE Transactions on Medical Imaging, ISSN 0278-0062, E-ISSN 1558-254X, Vol. 34, no 3, p. 788-795Article in journal (Refereed) Published
Abstract [en]

Material basis decomposition in the sinogram domain requires accurate knowledge of the forward model in spectral computed tomography (CT). Misspecifications over a certain limit will result in biased estimates and make quantum limited (where statistical noise dominates) quantitative CT difficult. We present a method whereby users can determine the degree of allowed misspecification error in a spectral CT forward model and still have quantification errors that are limited by the inherent statistical uncertainty. For a particular silicon detector based spectral CT system, we conclude that threshold determination is the most critical factor and that the bin edges need to be known to within 0.15 keV in order to be able to perform quantum limited material basis decomposition. The method as such is general to all multibin systems.

Place, publisher, year, edition, pages
IEEE Press, 2015
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-163858 (URN)10.1109/TMI.2014.2361680 (DOI)000350870700011 ()2-s2.0-84923886681 (Scopus ID)
Note

QC 20150420

Available from: 2015-04-13 Created: 2015-04-13 Last updated: 2017-12-04Bibliographically approved
Liu, X., Chen, H., Bornefalk, H., Danielsson, M., Karlsson, S., Persson, M., . . . Huber, B. (2015). Energy Calibration of a Silicon-Strip Detector for Photon-Counting Spectral CT by Direct Usage of the X-ray Tube Spectrum. IEEE Transactions on Nuclear Science, 62(1), 68-75
Open this publication in new window or tab >>Energy Calibration of a Silicon-Strip Detector for Photon-Counting Spectral CT by Direct Usage of the X-ray Tube Spectrum
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2015 (English)In: IEEE Transactions on Nuclear Science, ISSN 0018-9499, E-ISSN 1558-1578, Vol. 62, no 1, p. 68-75Article in journal (Refereed) Published
Abstract [en]

The variation among energy thresholds in a multibin detector for photon-counting spectral CT can lead to ring artefacts in the reconstructed images. Calibration of the energy thresholds can be used to achieve homogeneous threshold settings or to develop compensation methods to reduce the artefacts. We have developed an energy-calibrationmethod for the different comparator thresholds employed in a photon-counting silicon-strip detector. In our case, this corresponds to specifying the linear relation between the threshold positions in units of mV and the actual deposited photon energies in units of keV. This relation is determined by gain and offset values that differ for different detector channels due to variations in the manufacturing process. Typically, the calibration is accomplished by correlating the peak positions of obtained pulse-height spectra to known photon energies, e. g. with the aid of mono-energetic x rays from synchrotron radiation, radioactive isotopes or fluorescence materials. Instead of mono-energetic x rays, the calibrationmethod presented in this papermakes use of a broad x-ray spectrum provided by commercial x-ray tubes. Gain and offset as the calibration parameters are obtained by a regression analysis that adjusts a simulated spectrum of deposited energies to ameasured pulse-height spectrum. Besides the basic photon interactions such as Rayleigh scattering, Compton scattering and photo-electric absorption, the simulation takes into account the effect of pulse pileup, charge sharing and the electronic noise of the detector channels. We verify the method for different detector channels with the aid of a table-top setup, where we find the uncertainty of the keV-value of a calibrated threshold to be between 0.1 and 0.2 keV.

Keywords
Calibration, computed tomography, Monte-Carlo simulation, photon-counting, silicon-strip detector, spectral CT
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-161964 (URN)10.1109/TNS.2014.2373641 (DOI)000349672700008 ()2-s2.0-84923304829 (Scopus ID)
Note

QC 20150407

Available from: 2015-04-07 Created: 2015-03-20 Last updated: 2017-12-04Bibliographically approved
Grönberg, F., Persson, M. & Bornefalk, H. (2015). Image reconstruction based on energy-resolved image data from a photon-counting multi-bin detector. us 9870628B2.
Open this publication in new window or tab >>Image reconstruction based on energy-resolved image data from a photon-counting multi-bin detector
2015 (English)Patent (Other (popular science, discussion, etc.))
Abstract [en]

There is provided a method of image reconstruction based on energy-resolved image data from a photon-counting multi-bin detector or an intermediate storage. The method comprises processing (S1) the energy-resolved image data by performing at least two separate basis decompositions using different number of basis functions for modeling linear attenuation, wherein a first basis decomposition is performed using a first smaller set of basis functions to obtain at least one first basis image representation, and wherein a second basis decomposition is performed using a second larger set of basis functions to obtain at least one second basis image representation. The method also comprises reconstructing a first image based on said at least one first basis image representation obtained from the first basis decomposition, and combining the first image with information representative of said at least one second basis image representation.

National Category
Medical Image Processing
Identifiers
urn:nbn:se:kth:diva-260501 (URN)
Patent
US 9870628B2 (2018-01-16)
Note

QC 20191014

Available from: 2019-09-30 Created: 2019-09-30 Last updated: 2019-10-14Bibliographically approved
Liu, X., Chen, H., Bornefalk, H., Danielsson, M., Karlsson, S., Persson, M., . . . Huber, B. (2015). Modelling the channel-wise count response of a photon-counting spectral CT detector to a broad x-ray spectrum. In: Medical Imaging 2015: Physics of Medical Imaging: . Paper presented at Conference on Medical Imaging - Physics of Medical Imaging, FEB 22-25, 2015, Orlando, FL. , 9412, Article ID 941215.
Open this publication in new window or tab >>Modelling the channel-wise count response of a photon-counting spectral CT detector to a broad x-ray spectrum
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2015 (English)In: Medical Imaging 2015: Physics of Medical Imaging, 2015, Vol. 9412, article id 941215Conference paper, Published paper (Refereed)
Abstract [en]

Variations among detector channels in CT very sensitively lead to ring artefacts in the reconstructed images. For material decomposition in the projection domain, the variations can result in intolerable biases in the material line integral estimates. A typical way to overcome these effects is to apply calibration methods that try to unify spectral responses from different detector channels to an ideal response from a detector model. However, the calibration procedure can be rather complex and require excessive calibration measurements for a multitude of combinations of x-ray shapes, tissue combinations and thicknesses. In this paper, we propose a channel-wise model for a multibin photon-counting detector for spectral CT. Predictions of this channel-wise model match well with their physical performances, which can thus be used to eliminate ring artefacts in CT images and achieve projection-basis material decomposition. In an experimental validation, image data show significant improvement with respect to ring artefacts compared to images calibrated with flat-fielding data. Projection-based material decomposition gives basis material images showing good separation among individual materials and good quantification of iodine and gadolinium contrast agents. The work indicates that the channel-wise model can be used for quantitative CT with this detector.

National Category
Radiology, Nuclear Medicine and Medical Imaging
Identifiers
urn:nbn:se:kth:diva-170713 (URN)10.1117/12.2081776 (DOI)000355581700038 ()2-s2.0-84943327631 (Scopus ID)978-1-62841-502-5 (ISBN)
Conference
Conference on Medical Imaging - Physics of Medical Imaging, FEB 22-25, 2015, Orlando, FL
Note

QC 20150706

Available from: 2015-07-06 Created: 2015-07-03 Last updated: 2015-07-06Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0002-5092-8822

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