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| 44. |
- 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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| 45. |
- 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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| 46. |
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| 47. |
- Persson, Mats, et al.
(författare)
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Quantification of ring artifact visibility in CT
- 2012
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Ingår i: Medical Imaging 2012. - : SPIE - International Society for Optical Engineering. - 9780819489623 ; , s. 83132J-
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Konferensbidrag (refereegranskat)abstract
- 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.
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| 48. |
- Persson, Mats, et al.
(författare)
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Resolution improvement in x-ray imaging with an energy-resolving detector
- 2017
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Ingår i: Medical Imaging 2017. - : SPIE - International Society for Optical Engineering. - 9781510607095
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Konferensbidrag (refereegranskat)abstract
- In x-ray imaging, improving spatial resolution is an important goal, but developing detectors with smaller pixels is technically challenging. We demonstrate a technique for improving the spatial resolution by utilizing the fact that linear attenuation coefficients of all substances within the human body can be expressed, to a good approximation, as a linear combination of two basis functions, or three if there is iodine contrast present in the image. When the x rays pass an interface parallel to the beam direction, the exponential attenuation law makes the linear attenuation coefficient measured by the detector a nonlinear combination of the linear attenuation coefficients on each side of the interface. This so-called nonlinear partial volume effect causes the spectral response to be dependent on the steepness of interfaces in the imaged volume. In this work, we show how this effect can be used to improve the spatial resolution in spectral projection x-ray imaging and quantify the achievable resolution improvement. We simulate x-ray transmission imaging of sharp and gradual changes in the projected path length of iodine contrast with an ideal energy-resolving photon-counting detector and demonstrate that the slope of the transition can be determined from the registered spectrum. We simulate piecewise-linear transitions and show that the algorithm is able to reproduce the transition profile on a subpixel scale. The FWHM resolution of the method is 5-30 % of the pixel width. The results show that an energy-resolving detector can be used to improve spatial resolution when imaging interfaces of highly attenuating objects.
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| 49. |
- Persson, Mats, 1987-
(författare)
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Spectral Computed Tomography with a Photon-Counting Silicon-Strip Detector
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 50. |
- Persson, Mats, 1987-, et al.
(författare)
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Sub-pixel information retrieval from spectral x-ray images
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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