Ändra sökning
Avgränsa sökresultatet
1 - 31 av 31
RefereraExporteraLänk till träfflistan
Permanent länk
Referera
Referensformat
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Annat format
Fler format
Språk
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Annat språk
Fler språk
Utmatningsformat
  • html
  • text
  • asciidoc
  • rtf
Träffar per sida
  • 5
  • 10
  • 20
  • 50
  • 100
  • 250
Sortering
  • Standard (Relevans)
  • Författare A-Ö
  • Författare Ö-A
  • Titel A-Ö
  • Titel Ö-A
  • Publikationstyp A-Ö
  • Publikationstyp Ö-A
  • Äldst först
  • Nyast först
  • Skapad (Äldst först)
  • Skapad (Nyast först)
  • Senast uppdaterad (Äldst först)
  • Senast uppdaterad (Nyast först)
  • Disputationsdatum (tidigaste först)
  • Disputationsdatum (senaste först)
  • Standard (Relevans)
  • Författare A-Ö
  • Författare Ö-A
  • Titel A-Ö
  • Titel Ö-A
  • Publikationstyp A-Ö
  • Publikationstyp Ö-A
  • Äldst först
  • Nyast först
  • Skapad (Äldst först)
  • Skapad (Nyast först)
  • Senast uppdaterad (Äldst först)
  • Senast uppdaterad (Nyast först)
  • Disputationsdatum (tidigaste först)
  • Disputationsdatum (senaste först)
Markera
Maxantalet träffar du kan exportera från sökgränssnittet är 250. Vid större uttag använd dig av utsökningar.
  • 1.
    Bornefalk, Hans
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Persson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Theoretical Comparison of the Iodine Quantification Accuracy of Two Spectral CT Technologies2014Ingår i: IEEE Transactions on Medical Imaging, ISSN 0278-0062, E-ISSN 1558-254X, Vol. 33, nr 2, s. 556-565Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We compare the theoretical limits of iodine quantification for the photon counting multibin and dual energy technologies. Dual energy systems by necessity have to make prior assumptions in order to quantify iodine. We explicitly allow the multibin system to make the same assumptions and also allow them to be wrong. We isolate the effect of technology from imperfections and implementation issues by assuming both technologies to be ideal, i.e., without scattered radiation, unity detection efficiency and perfect energy response functions, and by applying the Cramer-Rao lower bound methodology to assess the quantification accuracy. When priors are wrong the maximum likelihood estimates will be biased and the mean square error of the quantification error is a more appropriate figure of merit. The evaluation assumes identical X-ray spectra for both methodologies and for that reason a sensitivity analysis is performed with regard to the assumed X-ray spectrum. We show that when iodine is quantified over regions of interest larger than 6 cm, multibin systems benefit by independent estimation of three basis functions. For smaller regions of interest multibin systems can increase quantification accuracy by making the same prior assumptions as dual energy systems.

  • 2.
    Bornefalk, Hans
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Persson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Allowable forward model misspecification for accurate basis decomposition in a silicon detector based spectral CT2015Ingår i: IEEE Transactions on Medical Imaging, ISSN 0278-0062, E-ISSN 1558-254X, Vol. 34, nr 3, s. 788-795Artikel i tidskrift (Refereegranskat)
    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.

  • 3.
    Bornefalk, Hans
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Persson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Necessary forward model specification accuracy for basis material decomposition in spectral CT2014Ingår i: Medical Imaging 2014: Physics of Medical Imaging, SPIE - International Society for Optical Engineering, 2014, s. 90332I-Konferensbidrag (Refereegranskat)
    Abstract [en]

    Material basis decomposition in the sinogram domain requires accurate knowledge of the forward model in spectral CT. Misspecifications over a certain limit will result in biased estimates and make quantum limited 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 quantum limited.

  • 4.
    Bornefalk, Hans
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk avbildning.
    Persson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk avbildning.
    Xu, Cheng
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk avbildning.
    Karlsson, Staffan
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk avbildning.
    Svensson, Christer
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk avbildning.
    Effect of Temperature Variation on the Energy Response of a Photon Counting Silicon CT Detector2013Ingår i: IEEE Transactions on Nuclear Science, ISSN 0018-9499, E-ISSN 1558-1578, Vol. 60, nr 2, s. 1442-1449Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The effect of temperature variation on pulse height determination accuracy is determined for a photon counting multibin silicon detector developed for spectral CT. Theoretical predictions of the temperature coefficient of the gain and offset are similar to values derived from synchrotron radiation measurements in a temperature controlled environment. By means of statistical modeling, we conclude that temperature changes affect all channels equally and with separate effects on gain and threshold offset. The combined effect of a 1 degrees C temperature increase is to decrease the detected energy by 0.1 keV for events depositing 30 keV. For the electronic noise, no statistically significant temperature effect was discernible in the data set, although theory predicts a weak dependence. The method is applicable to all x-ray detectors operating in pulse mode.

  • 5.
    Chen, Han
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Cederström, Björn
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Xu, Cheng
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Persson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Karlsson, Staffan
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    A photon-counting silicon-strip detector for digital mammography with an ultrafast 0.18-mu m CMOS ASIC2014Ingår i: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 749, s. 1-6Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We have evaluated a silicon-strip detector with a 0.18-mu m CMOS application specific integrated circuits (ASIC) containing 160 channels for use in photon-counting digital mammography. Measurements were performed at the Elettra light source using monochromatic X-ray beams with different energies and intensities. Energy resolution, Delta E/E-in, was measured to vary between 0.10 and 0.23 in the energy range of 15-40 keV. Pulse pileup has shown little effect on energy resolution.

  • 6.
    Chen, Han
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Xu, Cheng
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Persson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Optimization Of Beam Quality For Photon-Counting Spectral Computed Tomography In Head Imaging: Simulation Study2015Ingår i: Journal of Medical Imaging, ISSN 2329-4302, E-ISSN 2329-4310, Vol. 2, nr 4, s. 043504-1-043504-16, artikel-id 043504Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Head computed tomography (CT) plays an important role in the comprehensive evaluation of acutestroke. Photon-counting spectral detectors, as promising candidates for use in the next generation of x-ray CTsystems, allow for assigning more weight to low-energy x-rays that generally contain more contrast information.Most importantly, the spectral information can be utilized to decompose the original set of energy-selectiveimages into several basis function images that are inherently free of beam-hardening artifacts, a potential ad-vantage for further improving the diagnosis accuracy. We are developing a photon-counting spectral detector forCT applications. The purpose of this work is to determine the optimal beam quality for material decomposition intwo head imaging cases: nonenhanced imaging and K-edge imaging. A cylindrical brain tissue of 16-cm diam-eter, coated by a 6-mm-thick bone layer and 2-mm-thick skin layer, was used as a head phantom. The imagingtarget was a 5-mm-thick blood vessel centered in the head phantom. In K-edge imaging, two contrast agents,iodine and gadolinium, with the same concentration (5mg∕mL) were studied. Three parameters that affect beamquality were evaluated: kVp settings (50 to 130 kVp), filter materials (Z¼13to 83), and filter thicknesses [0 to 2half-value layer (HVL)]. The image qualities resulting from the varying x-ray beams were compared in terms oftwo figures of merit (FOMs): squared signal-difference-to-noise ratio normalized by brain dose (SDNR2∕BD) andthat normalized by skin dose (SDNR2∕SD). For nonenhanced imaging, the results show that the use of the 120-kVp spectrum filtered by 2 HVL copper (Z¼29) provides the best performance in both FOMs. When iodine isused in K-edge imaging, the optimal filter is 2 HVL iodine (Z¼53) and the optimal kVps are 60 kVp in terms ofSDNR2∕BD and 75 kVp in terms of SDNR2∕SD. A tradeoff of 65 kVp was proposed to lower the potential riskof skin injuries if a relatively long exposure time is necessarily performed in the iodinated imaging. In the case ofgadolinium imaging, both SD and BD can be minimized at 120 kVp filtered with 2 HVL thulium (Z¼69). Theresults also indicate that with the same concentration and their respective optimal spectrum, the values ofSDNR2∕BD and SDNR2∕SD in gadolinium imaging are, respectively, around 3 and 10 times larger thanthose in iodine imaging. However, since gadolinium is used in much lower concentrations than iodine in theclinic, iodine may be a preferable candidate for K-edge imaging.

  • 7.
    Grönberg, Fredrik
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Persson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Bornefalk, Hans
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Image reconstruction based on energy-resolved image data from a photon-counting multi-bin detector2015Patent (Övrig (populärvetenskap, debatt, mm))
    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.

  • 8.
    Grönberg, Fredrik
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Persson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Bornefalk, Hans
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Third material separation in spectral CT with basis decomposition2015Konferensbidrag (Övrigt vetenskapligt)
  • 9.
    Liu, Xuejin
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Bornefalk, Hans
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Chen, Han
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Karlsson, Staffan
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Persson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Xu, Cheng
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Huber, Ben
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    A Silicon-Strip Detector for Photon-Counting Spectral CT: Energy Resolution From 40 keV to 120 keV2014Ingår i: IEEE Transactions on Nuclear Science, ISSN 0018-9499, E-ISSN 1558-1578, Vol. 61, nr 3, s. 1099-1105Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We are developing a segmented silicon-strip detector for spectral computed tomography. The detector operates in photon-counting mode and allows pulse-height discrimination with 8 adjustable energy bins. In this work, we determine the energy resolution of a detector module using monoenergetic x-rays from 40 keV to 120 keV, provided at the European Synchrotron Radiation Facility, Grenoble. For each incident x-ray energy, pulse height spectra at different input photon fluxes are obtained. We investigate changes of the energy resolution due to charge sharing between pixels and pulse pileup. The different incident energies are used to channel-wise calibrate the pulse-height response in terms of signal gain and offset and to probe the homogeneity of the detector module. The detector shows a linear pulse-height response in the energy range from 40 keV to 120 keV. The gain variation among the channels is below 4%, whereas the variation of the offsets is on the order of 1 keV. We find an absolute energy resolution (sigma(E)) that degrades from 1.5 keV to 1.9 keV with increasing x-ray energy from 40 keV to 100 keV. With increasing input count rate, sigma(E) degrades by approximately 4 . 10(-3) keV Mcps(-1) mm(2), which is, within error bars, the same for the different energies. The effect of charge sharing on the width of the response peak is found to be negligible.

  • 10.
    Liu, Xuejin
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Bornefalk, Hans
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Chen, Han
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Karlsson, Staffan
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Persson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Xu, Cheng
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Huber, Ben
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Characterization of a silicon strip detector for photon-counting spectral CT using monoenergetic photons from 40 keV to 120 keV2014Ingår i: Medical Imaging 2014: Physics of Medical Imaging, SPIE - International Society for Optical Engineering, 2014, s. 90333O-Konferensbidrag (Refereegranskat)
    Abstract [en]

    Background: We are developing a segmented silicon strip detector that operates in photon-counting mode and allows pulse-height discrimination with 8 adjustable energy bins. In this work, we determine the energy resolution of the detector using monoenergetic x-ray radiation from 40 keV to 120 keV. We further investigate the effects of pulse pileup and charge sharing between detector channels that may lead to a decreased energy resolution. Methods: For each incident monochromatic x-ray energy, we obtain count spectra at different photon fluxes. These spectra corresponds to the pulse-height response of the detector and allow the determination of energy resolution and charge-sharing probability. The energy resolution, however, is influenced by signal pileup and charge sharing. Both effects are quantified using Monte Carlo simulations of the detector that aim to reproduce the conditions during the measurements. Results: The absolute energy resolution is found to increase from 1.7 to 2.1 keV for increasing energies 40 keV to 120 keV at the lowest measured photon flux. The effect of charge sharing is found to increase the absolute energy resolution by a factor of 1.025 at maximum. This increase is considered as negligibly small. The pileup of pulses leads to a deterioration rate of the energy resolution of 4 · 10-3 keV Mcps-1 mm2, corresponding to an increase of 0.04keV per 10 Mcps increase of the detected count rate.

  • 11.
    Liu, Xuejin
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Chen, Han
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Bornefalk, Hans
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Karlsson, Staffan
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Persson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Xu, Cheng
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Huber, Ben
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Energy Calibration of a Silicon-Strip Detector for Photon-Counting Spectral CT by Direct Usage of the X-ray Tube Spectrum2015Ingår i: IEEE Transactions on Nuclear Science, ISSN 0018-9499, E-ISSN 1558-1578, Vol. 62, nr 1, s. 68-75Artikel i tidskrift (Refereegranskat)
    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.

  • 12.
    Liu, Xuejin
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Chen, Han
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Bornefalk, Hans
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Karlsson, Staffan
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Persson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Xu, Cheng
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Huber, Ben
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Modelling the channel-wise count response of a photon-counting spectral CT detector to a broad x-ray spectrum2015Ingår i: Medical Imaging 2015: Physics of Medical Imaging, 2015, Vol. 9412, artikel-id 941215Konferensbidrag (Refereegranskat)
    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.

  • 13.
    Liu, Xuejin
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Persson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Bornefalk, Hans
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Karlsson, Staffan
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Xu, Cheng
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Huber, Ben
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Spectral response model for a multibin photon-counting spectral computed tomography detector and its applications2015Ingår i: Journal of Medical Imaging, ISSN 2329-4302, E-ISSN 2329-4310, Vol. 2, nr 3, artikel-id 033502Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Variations among detector channels in computed tomography can lead to ring artifacts in the reconstructed images and biased estimates in projection-based material decomposition. Typically, the ring artifacts are corrected by compensation methods based on flat fielding, where transmission measurements are required for a number of material-thickness combinations. Phantoms used in these methods can be rather complex and require an extensive number of transmission measurements. Moreover, material decomposition needs knowledge of the individual response of each detector channel to account for the detector inhomogeneities. For this purpose, we have developed a spectral response model that binwise predicts the response of a multibin photon-counting detector individually for each detector channel. The spectral response model is performed in two steps. The first step employs a forward model to predict the expected numbers of photon counts, taking into account parameters such as the incident x-ray spectrum, absorption efficiency, and energy response of the detector. The second step utilizes a limited number of transmission measurements with a set of flat slabs of two absorber materials to fine-tune the model predictions, resulting in a good correspondence with the physical measurements. To verify the response model, we apply the model in two cases. First, the model is used in combination with a compensation method which requires an extensive number of transmission measurements to determine the necessary parameters. Our spectral response model successfully replaces these measurements by simulations, saving a significant amount of measurement time. Second, the spectral response model is used as the basis of the maximum likelihood approach for projection-based material decomposition. The reconstructed basis images show a good separation between the calcium-like material and the contrast agents, iodine and gadolinium. The contrast agent concentrations are reconstructed with more than 94% accuracy.

  • 14.
    Liu, Xuejin
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Persson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Bornefalk, Hans
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Karlsson, Staffan
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Xu, Cheng
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Huber, Ben
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Spectral response model for a multibin photon-counting spectral computed tomography detector and its applications (vol 2, 033502, 2015)2016Ingår i: Journal of Medical Imaging, ISSN 2329-4302, E-ISSN 2329-4310, Vol. 3, nr 4, artikel-id 049801Artikel i tidskrift (Refereegranskat)
  • 15.
    Persson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Spectral Computed Tomography with a Photon-Counting Silicon-Strip Detector2016Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    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.  

  • 16.
    Persson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Spectral x-ray imaging2014Patent (Övrig (populärvetenskap, debatt, mm))
  • 17.
    Persson, Mats
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Adler, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Matematik (Inst.), Matematik (Avd.).
    Spectral CT reconstruction with anti-correlated noise model and joint prior2017Konferensbidrag (Övrigt vetenskapligt)
    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.

  • 18.
    Persson, Mats
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk avbildning.
    Bornefalk, Hans
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk avbildning.
    A Framework for Evaluating Threshold Variation Compensation Methods in Photon Counting Spectral CT2012Ingår i: IEEE Transactions on Medical Imaging, ISSN 0278-0062, E-ISSN 1558-254X, Vol. 31, nr 10, s. 1861-1874Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    One of the challenges in the development of photon counting spectral computed tomography (CT) detectors is that the location of the energy thresholds tends to vary among detector elements. If not compensated for, this threshold variation leads to ring artifacts in the reconstructed images. In this paper, a framework is presented for the systematic comparison of different methods of compensating for inhomogeneities among detector elements in photon counting CT with multiple energy bins. Furthermore, we propose the use of an affine minimum mean square error estimator, calibrated against transmission measurements on different combinations of two materials, for inhomogeneity compensation. Using the framework developed here, this method is compared to two other compensation schemes, flatfielding using an air scan and signal-to-thickness calibration using a step wedge calibrator, in a simulation study. The results show that for all but the lowest studied level of threshold spread, the proposed method is superior to signal-to-thickness calibration, which in turn is superior to flatfielding. We also demonstrate that the effects of threshold variation can be countered to a large extent by substructuring each detector element into depth segments.

  • 19.
    Persson, Mats
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Bujila, Robert
    Unit of X-ray Physics, Section of Imaging Physics Solna, Department of Medical Physics,Karolinska University Hospital.
    Nowik, Patrik
    Unit of X-ray Physics, Section of Imaging Physics Solna, Department of Medical Physics,Karolinska University Hospital.
    Andersson, Henrik
    Unit of X-ray Physics, Section of Imaging Physics Solna, Department of Medical Physics, Karolinska University Hospital.
    Kull, Love
    Medical Radiation Physics, Sunderby Hospital.
    Andersson, Jonas
    Department of Radiation Sciences, Radiation Physics, Umeå University.
    Bornefalk, Hans
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Upper limits of the photon fluence rate on CT detectors: case study on a commercial scannerIngår i: Artikel i tidskrift (Övrigt vetenskapligt)
    Abstract [en]

    Purpose: The highest photon fluence rate that a CT detector must be able to measure is animportant parameter. We calculate the maximum transmitted fluence rate in a commercial CT scanner as a function of patient size for standard head, chest and abdomen protocols.Method: We 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 currentapplied with the tube current modulation (TCM) system. By rescaling this tube current usingpublished measurements on the tube current modulation of a GE scanner we could estimate the tube current that these protocols would have resulted in for other patient sizes. An ECG gatedchest protocol was also simulated. Using measured dose rate profiles along the bowtie filters, wesimulated imaging of anonymized patient images with a range of sizes on a GE VCT scanner andcalculated the maximum transmitted fluence rate. In addition, the 99th and the 95th percentilesof 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 protocolswith centered patients are found for head and chest images of small patients, with a maximumof 7.1 · 107 mm−2 s−1 for head and 9.6 · 107 mm−2 s−1 for chest. Miscentering the head by 50 mm downwards increases the maximum transmitted fluence rate to 3.9 · 108 mm−2 s−1 . The ECG gatedchest protocol gives fluence rates up to 2.3 · 108 − 2.4 · 108 mm−2 s−1 depending on miscentering.Conclusion: The fluence rate on a CT detector reaches 1 · 108 − 4 · 108 mm−2 s−1 in standardimaging protocols, with the highest rates occurring for ECG gated chest and miscentered headscans. These results will be useful to developers of CT detectors, in particular photon countingdetectors.

  • 20.
    Persson, Mats
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik. Royal Inst Technol, Dept Phys, SE-10691 Stockholm, Sweden..
    Bujila, Robert
    Karolinska Univ Hosp, Unit Xray Phys, Sect Imaging Phys Solna, Dept Med Phys, SE-17176 Stockholm, Sweden..
    Nowik, Patrik
    Karolinska Univ Hosp, Unit Xray Phys, Sect Imaging Phys Solna, Dept Med Phys, SE-17176 Stockholm, Sweden..
    Andersson, Henrik
    Karolinska Univ Hosp, Unit Xray Phys, Sect Imaging Phys Solna, Dept Med Phys, SE-17176 Stockholm, Sweden..
    Kull, Love
    Sunderby Hosp, Med Radiat Phys, SE-97180 Lulea, Sweden..
    Andersson, Jonas
    Umea Univ, Radiat Phys, Dept Radiat Sci, SE-90185 Umea, Sweden..
    Bornefalk, Hans
    KTH, Skolan för teknikvetenskap (SCI), Fysik. Royal Inst Technol, Dept Phys, SE-10691 Stockholm, Sweden..
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik. Royal Inst Technol, Dept Phys, SE-10691 Stockholm, Sweden..
    Upper limits of the photon fluence rate on CT detectors: Case study on a commercial scanner2016Ingår i: Medical physics (Lancaster), ISSN 0094-2405, Vol. 43, nr 7, s. 4398-4411Artikel i tidskrift (Refereegranskat)
    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.

  • 21.
    Persson, Mats
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Grönberg, Fredrik
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Bias-variance tradeoff in anticorrelated noise reduction for spectral CT2017Ingår i: Medical physics (Lancaster), ISSN 0094-2405, Vol. 44, nr 9, s. E242-E254Artikel i tidskrift (Refereegranskat)
    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.

  • 22.
    Persson, Mats
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Grönberg, Fredrik
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Spatial-frequency-domain study of anticorrelated noise reduction in spectral CT2016Ingår i: CT-Meeting 2016, Proceedings, 2016, s. 283-286Konferensbidrag (Refereegranskat)
  • 23.
    Persson, Mats
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik. Stanford Univ, Dept Bioengn, Stanford, CA 94305 USA..
    Holmin, Staffan
    Karolinska Inst, Dept Clin Neurosci, Stockholm, Sweden.;Karolinska Univ Hosp, Dept Neuroradiol, Stockholm, Sweden..
    Karlsson, Staffan
    Bornefalk, Hans
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Subpixel x-ray imaging with an energy-resolving detector2018Ingår i: Journal of Medical Imaging, ISSN 2329-4302, E-ISSN 2329-4310, Vol. 5, nr 1, artikel-id 013507Artikel i tidskrift (Refereegranskat)
    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.

  • 24.
    Persson, Mats
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Holmin, Staffan
    Department of Clinical Neuroscience, Karolinska Institutet, SE-17177 Stockholm, Sweden and Department of Neuroradiology, Karolinska University Hospital Solna, SE-17176 Stockholm, Sweden.
    Karlsson, Staffan
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Sub-pixel information retrieval from spectral x-ray imagesManuskript (preprint) (Övrigt vetenskapligt)
    Abstract [en]

    The detector pixel size can be a severe limitation in projection x-ray imaging of fine details inthe human body, but developing higher resolution detectors is technically challenging. Wedemonstrate a novel method of using spectral x-ray measurements, from an energy-resolvingdetector or from multiple acquisitions with different beam quality, to obtain information aboutthe spatial distribution of the linear attenuation coefficient on a length scale smaller than onepixel. The method builds on the fact that the linear attenuation coefficient of all materials inthe human body can be expressed as linear combinations of a small number of basis functions.However, an interface parallel to the x-ray beam has a unique spectral responose which makesit distinguishable from homogeneous materials.To demonstrate the method experimentally, a 120 mm polyethylene phantom with a 6 mmiodine-filled hole in its centre was imaged in a projection geometry using a photon-countingsilicon-strip detector with eight energy bins. X-ray transmission measurements of differentthicknesses of polyethylene and iodine were used to calibrate a forward model describing thedetector response for different objects in the beam. Using the proposed method, an imagespecific to the spectral response of an iodine-polyethylene interface was generated. Theresults show that the borders of the iodine insert are highlighted in the resulting image, ingood agreement with simulations.Our study demonstrates that spectral x-ray measurements can be used to distinguish betweensharp and gradual transitions in an x-ray image. The method may potentially be used forimproving visualization of blood vessel boundaries in stroke care.

  • 25.
    Persson, Mats
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Huber, Ben
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Karlsson, Staffan
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Liu, Xuejin
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Chen, Han
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Xu, Cheng
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Yveborg, Moa
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Bornefalk, Hans
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Energy-resolved CT imaging with a photon-counting silicon-strip detector2014Ingår i: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 59, nr 22, s. 6709-6727Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Photon-counting detectors are promising candidates for use in the next generation of x-ray computed tomography (CT) scanners. Among the foreseen benefits are higher spatial resolution, better trade-off between noise and dose and energy discriminating capabilities. Silicon is an attractive detector material because of its low cost, mature manufacturing process and high hole mobility. However, it is sometimes overlooked for CT applications because of its low absorption efficiency and high fraction of Compton scatter. The purpose of this work is to demonstrate that silicon is a feasible material for CT detectors by showing energy-resolved CT images acquired with an 80 kVp x-ray tube spectrum using a photon-counting silicon-strip detector with eight energy thresholds developed in our group. We use a single detector module, consisting of a linear array of 50 0.5 x 0.4 mm detector elements, to image a phantom in a table-top lab setup. The phantom consists of a plastic cylinder with circular inserts containing water, fat and aqueous solutions of calcium, iodine and gadolinium, in different concentrations. By using basis material decomposition we obtain water, calcium, iodine and gadolinium basis images and demonstrate that these basis images can be used to separate the different materials in the inserts. We also show results showing that the detector has potential for quantitative measurements of substance concentrations.

  • 26.
    Persson, Mats
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Huber, Ben
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Karlsson, Staffan
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Liu, Xuejin
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Chen, Han
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Xu, Cheng
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Yveborg, Moa
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Bornefalk, Hans
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Energy-resolved CT imaging with a photon-counting silicon-strip detector2014Ingår i: Medical Imaging 2014: Physics of Medical Imaging, SPIE - International Society for Optical Engineering, 2014, s. 90333L-Konferensbidrag (Refereegranskat)
    Abstract [en]

    Photon-counting detectors are promising candidates for use in the next generation of x-ray CT scanners. Among the foreseen benefits are higher spatial resolution, better trade-off between noise and dose, and energy discriminating capabilities. Silicon is an attractive detector material because of its low cost, mature manufacturing process and high hole mobility. However, it is sometimes claimed to be unsuitable for use in computed tomography because of its low absorption efficiency and high fraction of Compton scatter. The purpose of this work is to demonstrate that high-quality energy-resolved CT images can nonetheless be acquired with clinically realistic exposure parameters using a photon-counting silicon-strip detector with eight energy thresholds developed in our group. We use a single detector module, consisting of a linear array of 50 0.5 × 0.4 mm detector elements, to image a phantom in a table-top lab setup. The phantom consists of a plastic cylinder with circular inserts containing water, fat and aqueous solutions of calcium, iodine and gadolinium, in different concentrations. We use basis material decomposition to obtain water, calcium, iodine and gadolinium basis images and demonstrate that these basis images can be used to separate the different materials in the inserts. We also show results showing that the detector has potential for quantitative measurements of substance concentrations.

  • 27.
    Persson, Mats
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Meyer, Bettina
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Bornefalk, Hans
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Quantification of ring artifact visibility in CT2012Ingår i: Medical Imaging 2012: Physics Of Medical Imaging, SPIE - International Society for Optical Engineering, 2012, Vol. 8313, s. 83132J-Konferensbidrag (Refereegranskat)
    Abstract [en]

    Ring artifacts appear in computed tomography images if there are too large inhomogeneities between different detector elements. The question of how large inhomogeneities are acceptable is gaining in importance due to the development of energy discriminating photon counting CT, where detector homogeneity is an important design parameter. We propose using the systematic-to-statistical error quotient q, defined as the variance of the expected log-normalized count number between detector elements (dels) divided by the variance of log-normalized count numbers measured with the same del, as a metric of ring artifact visibility. With a simple observer study using simulated images, it is shown that rings are visible in the reconstructed image if q exceeds a threshold which lies close to 1.2·10 -3 for 1500 detector elements and 2000 projection angles. It is also shown by visual inspection of simulated images that the threshold value is, to a good approximation, inversely proportional to the number of angle measurements and independent of the number of detector elements. The results suggest that a simple oberver study, together with these scaling relationships, is sufficient for establishing sinogram homogeneity requirements for a particular reconstruction method.

  • 28.
    Xu, Cheng
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Chen, Han
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Persson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Karlsson, Staffan
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Svensson, Christer
    Bornefalk, Hans
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Energy resolution of a segmented silicon strip detector for photon-counting spectral CT2013Ingår i: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 715, s. 11-17Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We investigated the energy resolution of a segmented silicon strip detector for photon-counting spectral computed tomography (CT). The detector response to different monochromatic photon energies and various photon fluxes was characterized at the Elettra synchrotron. An RMS energy resolution of 1.50 keV has been demonstrated for 22 keV photons at zero flux, and it deteriorated as a function of input count rate at a rate of 5.13 eV mm2 /Mcps. The charge sharing effect has been evaluated. The results show that around 11.1% of the interacting photons experience charge sharing for 22 keV photons and 15.3% for 30 keV.

  • 29.
    Xu, Cheng
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk avbildning.
    Persson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk avbildning.
    Chen, Han
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk avbildning.
    Karlsson, Staffan
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk avbildning.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk avbildning.
    Svensson, Christer
    Bornefalk, Hans
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk avbildning.
    Evaluation of a Second-Generation Ultra-Fast Energy-Resolved ASIC for Photon-Counting Spectral CT2013Ingår i: IEEE Transactions on Nuclear Science, ISSN 0018-9499, E-ISSN 1558-1578, Vol. 60, nr 1, s. 437-445Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A second-generation ultra-fast energy-resolved application specific integrated circuit (ASIC) has been developed for photon-counting spectral computed tomography (CT). The energy resolution, threshold dispersion and gain of the ASIC were characterized with synchrotron radiation at Diamond Light Source. The standard deviation of threshold offsets at zero keV is 0.89 keV. An RMS energy resolution of 1.09 keV has been demonstrated for 15 keV photon energy at a count rate of 40 kcps, and it deteriorates at a rate of 0.29 keV/Mcps with the increase of output cout rate. The count rate performance of the ASIC has also been evaluated with 120 kV polychromatic x-rays produced by a tungsten anode tube and the results are presented.

  • 30.
    Yveborg, Moa
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Hans, Bornefalk
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Mats, Persson
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Optimal frequency-based weighting for spectral x-ray projection imaging2015Ingår i: IEEE Transactions on Medical Imaging, ISSN 0278-0062, E-ISSN 1558-254X, Vol. 34, nr 3, s. 779-787Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The purpose of this work is to derive a weighting scheme that maximizes the frequency-dependent ideal observer signal-difference-to-noise ratio, commonly referred to as detectability index or Hotelling-SDNR, for spectral X-ray projection imaging. Starting from basic statistical decision theory, optimal frequency-dependent weights are derived for a multiple-bin system and the Hotelling-SDNR calculated. A 28% increase in detectability index is found for high frequency objects when applying optimal frequency-dependent weights instead of pixel-based weights to a simplified model of a silicon detector, decreasing towards 0% for low frequency objects. Simulation results indicate a potentially large increase in detectability for high-frequency object imaging using silicon detectors, thus meriting further evaluations on a real system.

  • 31.
    Yveborg, Moa
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk avbildning.
    Persson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk avbildning.
    Crafoord, Joakim
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk avbildning.
    Bornefalk, Hans
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk avbildning.
    Eliminated risk of iodine contrast cancellation with multibin spectral CT2013Ingår i: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 58, nr 14, s. N201-N209Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This note compares the extent of contrast cancellation induced by iodinated contrast agents in energy integrating and photon counting multibin CT images. The contrast between a hypodense target and soft tissue is modeled for the two systems for a range of iodine concentrations and tube voltages. In energy integrating systems, we show that the contrast vanishes for low concentrations of iodine whereas the same effect is not seen in multibin systems. We conclude that it is the ability of multibin systems to apply weighting schemes post-acquisition that allows the operator to eliminate the risk of contrast cancellation between iodinated targets and the background.

1 - 31 av 31
RefereraExporteraLänk till träfflistan
Permanent länk
Referera
Referensformat
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Annat format
Fler format
Språk
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Annat språk
Fler språk
Utmatningsformat
  • html
  • text
  • asciidoc
  • rtf