| 1. |
- Cederström, Björn, et al.
(author)
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Comparison of photon-counting to storage phosphor plate mammography using contrast-detail phantom analysis
- 2007
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In: Nuclear Instruments and Methods in Physics Research Section A. - : Elsevier. - 0168-9002 .- 1872-9576. ; 580:2, s. 1101-1104
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Journal article (peer-reviewed)abstract
- Two digital mammography systems, one based on scanning photon counting silicon detectors and the other on storage phosphor plates (CR), have been compared in terms of image quality and dose. Sets of images of a contrast-detail phantom (CDMAM3.4) were acquired for each system and dose level. The images were evaluated in the disc diameter range 0.16-1 mm using a computer program (CDcom) and the results were fitted to a psychometric curve for each disc diameter. The contrast-detail curve was summarized into one single figure of merit, the image quality index, and the dose efficiency was calculated. The errors of the calculated parameters were assessed using statistical analysis. It was found that the scanning photon-counting system can achieve the same image quality as the storage phosphor plate (CR) system at 30-38% of the average glandular dose.
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| 2. |
- Cederström, Björn, et al.
(author)
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Generalized prism-array lenses for hard X-rays
- 2005
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In: Journal of Synchrotron Radiation. - : International Union of Crystallography (IUCr). - 0909-0495 .- 1600-5775. ; 12, s. 340-344
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Journal article (peer-reviewed)abstract
- A Fresnel-like X-ray lens can be constructed by a triangular array of identical prisms whose base corresponds to the 2 pi-shift length. Each column of prisms is progressively shifted from the optical axis by an arbitrary fraction of the prism height. Similarly to the multi-prism lens, quasi-parabolic profiles are formed by a superposition of straight-line segments. The resulting projected lens profile is approximately linear with a Fresnel-lens pattern superimposed on it to provide the focusing. This geometry exhibits a significantly larger effective aperture than conventional parabolic refractive lenses. Prototype lenses were fabricated by deep reactive ion etching of silicon. These one-dimensionally focusing lenses were tested at a synchrotron beamline and provided focal line-widths down to 1.4 mu m FWHM and an intensity gain of 39 at a photon energy of 13.4 keV. Fabrication imperfections gave rise to unwanted interference effects resulting in several intensity maxima in the focal plane. The presented design allows the focal length to be shortened without decreasing the feature size of the lens. Furthermore, this feature size does not limit the resolution as for real Fresnel optics.
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| 3. |
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| 4. |
- Fredenberg, Erik, PhD, 1979-, et al.
(author)
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A low-absorption x-ray energy filter for small-scale applications
- 2009
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In: Optics Express. - : The Optical Society. - 1094-4087. ; 17:14, s. 11388-11398
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Journal article (peer-reviewed)abstract
- We present an experimental and theoretical evaluation of an x-ray energy filter based on the chromatic properties of a prism-array lens (PAL). It is intended for small-scale applications such as medical imaging. The PAL approximates a Fresnel lens and allows for high efficiency compared to filters based on ordinary refractive lenses, however at the cost of a lower energy resolution. Geometrical optics was found to provide a good approximation for the performance of a flawless lens, but a field-propagation model was used for quantitative predictions. The model predicted a 0.29 ΔE/E energy resolution and an intensity gain of 6.5 for a silicon PAL at 23.5 keV. Measurements with an x-ray tube showed good agreement with the model in energy resolution and peak energy, but a blurred focal line contributed to a 29% gain reduction. We believe the blurring to be caused mainly by lens imperfections, in particular at the periphery of the lens.
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| 5. |
- Fredenberg, Erik, PhD, 1979-, et al.
(author)
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A photon-counting detector for dual-energy breast tomosynthesis
- 2009
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In: Medical Imaging 2009. - : SPIE. ; , s. 72581-
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Conference paper (peer-reviewed)abstract
- We present the first evaluation of a recently developed silicon-strip detector for photon-counting dual-energy breast tomosynthesis. The detector is well suited for tomosynthesis with high dose efficiency and intrinsic scatter rejection. A method was developed for measuring the spatial resolution of a system based on the detector in terms of the three-dimensional modulation transfer function (MTF). The measurements agreed well with theoretical expectations, and it was seen that depth resolution was won at the cost of a slightly decreased lateral resolution. This may be a justifiable trade-off as clinical images acquired with the system indicate improved conspicuity of breast lesions. The photon-counting detector enables dual-energy subtraction imaging with electronic spectrumsplitting. This improved the detectability of iodine in phantom measurements, and the detector was found to be stable over typical clinical acquisition times. A model of the energy resolution showed that further improvements are witn reach by optimization of the detector.
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| 6. |
- Fredenberg, Erik, PhD, 1979-, et al.
(author)
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A Tunable Energy Filter for Medical X-Ray Imaging
- 2008
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In: X-Ray Optics and Instrumentation. - : Hindawi. - 1687-7632 .- 1687-7640. ; 2008
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Journal article (peer-reviewed)abstract
- A multiprism lens (MPL) is a refractive X-ray lens, and its chromatic properties can be employed in an energy filtering setup to obtain a narrow tunable X-ray spectrum. We present the first evaluation of such a filter for medical X-ray imaging. The experimental setup yields a 6.6 gain of flux at 20 keV, and we demonstrate tunability by altering the energy spectrum to center also around 17 and 23 keV. All measurements are found to agree well with ray-tracing and a proposed geometrical model. Compared to a model mammography system with absorption filtering, the experimental MPL filter reduces dose 13–25% for 3–7 cm breasts if the spectrum is centered around the optimal energy. Additionally, the resolution is improved 2.5 times for a 5 cm breast. The scan time is increased 3 times but can be reduced with a slightly decreased energy filtering and resolution.
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| 7. |
- Fredenberg, Erik, 1979-, et al.
(author)
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An efficient pre-object collimator based on an x-ray lens
- 2009
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In: Medical physics (Lancaster). - : Wiley. - 0094-2405. ; 36:2, s. 626-633
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Journal article (peer-reviewed)abstract
- A multiprism lens (MPL) is a refractive x-ray lens with one-dimensional focusing properties. If used as a pre-object collimator in a scanning system for medical x-ray imaging, it reduces the divergence of the radiation and improves on photon economy compared to a slit collimator. Potential advantages include shorter acquisition times, a reduced tube loading, or improved resolution. We present the first images acquired with a MPL in a prototype for a scanning mammography system. The lens showed a gain of flux of 1.32 compared to a slit collimator at equal resolution, or a gain in resolution of 1.31–1.44 at equal flux. We expect the gain of flux in a clinical setup with an optimized MPL and a custom-made absorption filter to reach 1.67, or 1.45–1.54 gain in resolution.
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| 8. |
- Fredenberg, Erik, PhD, 1979-, et al.
(author)
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Contrast-enhanced dual-energy subtraction imaging using electronic spectrum-splitting and multi-prism x-ray lenses
- 2008
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In: Medical Imaging 2008 - Physics of Medical Imaging. - San Diego, CA, USA : SPIE. - 9780819470973 ; , s. 91310-91310
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Conference paper (peer-reviewed)abstract
- Dual-energy subtraction imaging (DES) is a method to improve the detectability of contrast agents over a lumpy background. Two images, acquired at x-ray energies above and below an absorption edge of the agent material, are logarithmically subtracted, resulting in suppression of the signal from the tissue background and a relative enhancement of the signal from the agent. Although promising, DES is still not widely used in clinical practice. One reason may be the need for two distinctly separated x-ray spectra that are still close to the absorption edge, realized through dual exposures which may introduce motion unsharpness. In this study, electronic spectrum-splitting with a silicon-strip detector is theoretically and experimentally investigated for a mammography model with iodinated contrast agent. Comparisons are made to absorption imaging and a near-ideal detector using a signal-to-noise ratio that includes both statistical and structural noise. Similar to previous studies, heavy absorption filtration was needed to narrow the spectra at the expense of a large reduction in x-ray flux. Therefore, potential improvements using a chromatic multi-prism x-ray lens (MPL) for filtering were evaluated theoretically. The MPL offers a narrow tunable spectrum, and we show that the image quality can be improved compared to conventional filtering methods.
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| 9. |
- Fredenberg, Erik, PhD, 1979-, et al.
(author)
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Imaging with multi-prism x-ray lenses
- 2008
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In: Medical Imaging 2008 - Physics of Medical Imaging. - : SPIE. - 9780819470973 ; , s. 91308-91308
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Conference paper (peer-reviewed)abstract
- The multi-prism lens (MPL) is a refractive x-ray lens consisting of two rows of prisms facing each other at an angle. Rays entering the lens at the periphery will encounter a larger number of prisms than will central ones, hence experiencing a greater refraction. The focusing effect of the MPL can be used to gather radiation from a large aperture onto a smaller detector, and accordingly to make better use of the available x-ray flux in medical x-ray imaging. Potential advantages of a better photon economy include shorter acquisition times, a reduced tube loading, or an improved resolution. Since the focusing effect is one-dimensional it matches the design of scanning systems. In this study we present the first images acquired with an MPL instead of the pre-breast slit collimator in a scanning mammography system. According to the measurements, the MPL is able to increase the flux 32% at equal resolution compared to the slit collimator, or to improve the resolution 2.4 mm(-1) at equal flux. If used with a custom-made absorption filter in a clinical set-up, the gain of flux of the MPL is expected to be at least 45%, and the corresponding improvement in resolution to be 3 mm(-1).
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| 10. |
- Fredenberg, Erik, PhD, 1979-, et al.
(author)
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Prism-array lenses for energy filtering in medical X-ray imaging
- 2007
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In: Medical Imaging 2007: Physics of Medical Imaging. - BELLINGHAM : SPIE-INT SOC OPTICAL ENGINEERING. - 9780819466280 ; , s. U270-U281
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Conference paper (peer-reviewed)abstract
- Conventional energy filters for x-ray imaging are based on absorbing materials which attenuate low energy photons, sometimes combined with an absorption edge, thus also discriminating towards 'photons of higher energies. These filters are fairly inefficient, in particular for photons of higher energies, and other methods for achieving a narrower bandwidth have been proposed. Such methods include various types of monochromators, based on for instance mosaic crystals or refractive multi-prism x-ray lenses (MPL's). Prism-array lenses (PAL's) are similar to MPL's, but are shorter, have larger apertures, and higher transmission. A PAL consists of a number of small prisms arranged in columns perpendicular to the optical axis. The column height decreases along the optical axis so that the projection of lens material is approximately linear with a Resnel phase-plate pattern superimposed on it. The focusing effect is one dimensional, and the lens is chromatic. Hence, unwanted energies can be blocked by placing a slit in the image plane of a desired energy. We present the first experimental and theoretical results on an energy filter based on a silicon PAL. The study includes an evaluation of the spectral shaping properties of the filter as well as a quantification of the achievable increase in dose efficiency compared to standard methods. Previously, PAL's have been investigated with synchrotron radiation, but in this study a medical imaging setup, based on a regular x-ray tube, is considered.
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