| 1. |
- Persson, Mats, et al.
(författare)
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Energy-resolved CT imaging with a photon-counting silicon-strip detector
- 2014
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Ingår i: Medical Imaging 2014. - : SPIE - International Society for Optical Engineering. - 9780819498267 ; , s. 90333L-
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Konferensbidrag (refereegranskat)abstract
- 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.
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| 2. |
- Persson, Mats, et al.
(författare)
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Energy-resolved CT imaging with a photon-counting silicon-strip detector
- 2014
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Ingår i: Physics in Medicine and Biology. - : IOP Publishing. - 0031-9155 .- 1361-6560. ; 59:22, s. 6709-6727
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 3. |
- Xu, Cheng, et al.
(författare)
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Evaluation of an ultra-fast photon-counting energy-resolved ASIC for spectral CT
- 2012
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Ingår i: Progress in Biomedical Optics and Imaging - Proceedings of SPIE. - : SPIE - International Society for Optical Engineering. - 9780819489623 ; , s. 83130Y-
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Konferensbidrag (refereegranskat)abstract
- We have developed an ultra-fast photon-counting energy-resolved application specific integrated circuit (ASIC) for spectral computed tomography (CT). A comprehensive characterization has been carried out to investigate the performance of the ASIC in terms of energy resolution under different photon flux rates and the count rate linearity in photon-counting mode. An energy resolution of 4.7 % has been achieved for 59.5 keV low flux photons. The count rate performance of the ASIC was measured with 120 kVp polychromatic x-rays. The results indicate that the count rate linearity can be kept for a flux rate up to 150 Mphotons s -1 mm -2 with retained energy information, and this value is increased to be 250 Mphotons s -1 mm -2 in photon-counting mode.
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| 4. |
- Yveborg, Moa, et al.
(författare)
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Eliminated risk of iodine contrast cancellation with multibin spectral CT
- 2013
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Ingår i: Physics in Medicine and Biology. - : Institute of Physics (IOP). - 0031-9155 .- 1361-6560. ; 58:14, s. N201-N209
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 5. |
- Yveborg, Moa, et al.
(författare)
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Optimal frequency-based weighting for spectral x-ray projection imaging
- 2015
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Ingår i: IEEE Transactions on Medical Imaging. - 0278-0062 .- 1558-254X. ; 34:3, s. 779-787
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 6. |
- Yveborg, Moa, et al.
(författare)
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Performance evaluation of a sub-millimeter spectrally resolved CT system on pediatric imaging tasks : a simulation
- 2011
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Ingår i: MEDICAL IMAGING 2011. - : SPIE.
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Konferensbidrag (refereegranskat)abstract
- We are developing a photon counting silicon strip detector with 0.4x0.5 mm(2) square detector elements for clinical CT applications. Except the somewhat limited detection efficiency at higher kVp's the largest discrepancies from ideal spectral behavior have been shown to be Compton interactions in the detector combined with electronic noise. Using the framework of cascaded systems analysis, we reconstruct the 3D MTF and NPS of a silicon strip detector using "optimal" projection based weighting, including the influence of scatter and charge sharing inside the detector. We compare the reconstructed noise and signal characteristics with a reconstructed 3D MTF and NPS of an ideal energy integrating detector by calculating the detectability index for several clinically relevant imaging task. This work demonstrates that although the detection efficiency of the silicon detector rapidly drops for the acceleration voltages encountered in clinical computed tomography practice and the high fraction of Compton interactions due to the low atomic number, silicon detectors can perform on par with ideal energy integrating detectors for routine imaging tasks contaning low frequency components. For imaging task containing high frequency components, silicon detectors can perform approximately 1.4 - 1.8 times better than a fully ideal energy integrating system with unity detection, no scatter or charge sharing inside the detector and 1x1 mm(2) square detector elements.
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| 7. |
- Yveborg, Moa, et al.
(författare)
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Performance evaluation of a sub-millimetre spectrally resolved CT system on high- and low-frequency imaging tasks : a simulation
- 2012
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Ingår i: Physics in Medicine and Biology. - : IOP Publishing. - 0031-9155 .- 1361-6560. ; 57:8, s. 2373-2391
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Tidskriftsartikel (refereegranskat)abstract
- We are developing a photon-counting silicon strip detector with 0.4 x 0.5 mm(2) detector elements for clinical CT applications. Except for the limited detection efficiency of approximately 0.8 for a spectrum of 80 kVp, the largest discrepancies from ideal spectral behaviour have been shown to be Compton interactions in the detector and electronic noise. Using the framework of cascaded system analysis, we reconstruct the 3D MTF and NPS of a silicon strip detector including the influence of scatter and charge sharing inside the detector. We compare the reconstructed noise and signal characteristics with a reconstructed 3D MTF and NPS of an ideal energy-integrating detector system with unity detection efficiency, no scatter or charge sharing inside the detector, unity presampling MTF and 1 x 1 mm(2) detector elements. The comparison is done by calculating the dose-normalized detectability index for some clinically relevant imaging tasks and spectra. This work demonstrates that although the detection efficiency of the silicon detector rapidly drops for the acceleration voltages encountered in clinical computed tomography practice, and despite the high fraction of Compton interactions due to the low atomic number, silicon detectors can perform on a par with ideal energy-integrating detectors for routine imaging tasks containing low-frequency components. For imaging tasks containing high-frequency components, the proposed silicon detector system can perform approximately 1.1-1.3 times better than a fully ideal energy-integrating system.
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| 8. |
- Yveborg, Moa, et al.
(författare)
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Photon-counting CT with silicon detectors : feasibility for pediatric imaging
- 2009
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Ingår i: Medical Imaging 2009. - : SPIE. - 9780819475091
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Konferensbidrag (refereegranskat)abstract
- X-ray detectors made of crystalline silicon have several advantages including low dark currents, fast charge collection and high energy resolution. For high-energy x-rays, however, silicon suffers from its low atomic number, which might result in low detection efficiency, as well as low energy and spatial resolution due to Compton scattering. We have used a monte-carlo model to investigate the feasibility of a detector for pediatric CT with 30 to 40 mm of silicon using x-ray spectra ranging from 80 to 140 kVp. A detection efficiency of 0.74 was found at 80 kVp, provided the noise threshold could be set low. Scattered photons were efficiently blocked by a thin metal shielding between the detector units, and Compton scattering in the detector could be well separated from photo absorption at 80 kVp. Hence, the detector is feasible at low acceleration voltages, which is also suitable for pediatric imaging. We conclude that silicon detectors may be an alternative to other designs for this special case.
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| 9. |
- Yveborg, Moa, 1982-
(författare)
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Quantification and Maximization of Performance Measures for Photon Counting Spectral Computed Tomography
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- During my time as a PhD student at the Physics of Medical Imaging group at KTH, I have taken part in the work of developing a photon counting spectrally resolved silicon detector for clinical computed tomography. This work has largely motivated the direction of my research, and is the main reason for my focus on certain issues. Early in the work, a need to quantify and optimize the performance of a spectrally resolved detector was identified. A large part of my work have thus consisted of reviewing conventional methods used for performance quantification and optimization in computed tomography, and identifying which are best suited for the characterization of a spectrally resolved system. In addition, my work has included comparisons of conventional systems with the detector we are developing. The collected result after a little more than four years of work are four publications and three conference papers.This compilation thesis consists of five introductory chapters and my four publications. The introductory chapters are not self-contained in the sense that the theory and results from all my published work are included. Rather, they are written with the purpose of being a context in which the papers should be read.The first two chapters treat the general purpose of the introductory chapters, and the theory of computed tomography including the distinction between conventional, non-spectral, computed tomography, and different practical implementations of spectral computed tomography. The second chapter consists of a review of the conventional methods developed for quantification and optimization of image quality in terms of detectability and signal-to-noise ratio, part of which are included in my published work. In addition, the theory on which the method of material basis decomposition is based on is presented, together with a condensed version of the results from my work on the comparison of two systems with fundamentally different practical solutions for material quantification. In the fourth chapter, previously unpublished measurements on the photon counting spectrally resolved detector we are developing are presented, and compared to Monte Carlo simulations. In the fifth and final chapter, a summary of the appended publications is included.
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| 10. |
- Yveborg, Moa, et al.
(författare)
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Task based weights for spectral computed tomography
- 2012
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Ingår i: Progress in Biomedical Optics and Imaging - Proceedings of SPIE. - : SPIE - International Society for Optical Engineering. - 9780819489623 ; , s. 831334-
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Konferensbidrag (refereegranskat)abstract
- In photon counting multi-bin CT, both projection based weighting and image based weighting (designed to maximize the signal-difference-to-noise ratio SDNR) in the final image, are pixel based and do not account for any spatial frequency dependency of signal and noise among the bins. The same weighting scheme will be used when imaging objects with a large fraction of high spatial frequencies and those with predominantly low spatial frequency content. Any effect on the detectability of a certain target due to correlation between detector elements and bins that might arise in pulse height discriminating systems will not be captured using such an approach. We show how to take the spatial frequency dependency of signal and noise transfer for each bin, and the spatial frequency composition of a target, into account when determining optimal task based weights for photon-counting multi-bin CT imaging using the 2D slice detectability index by applying cascaded system analysis to a spectrally resolved photon counting CT detector system with multiple bins.
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