| 11. |
- Fredenberg, Erik, PhD, 1979-, et al.
(författare)
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Observer model optimization of a spectral mammography system
- 2010
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Ingår i: Medical Imaging 2010. - : SPIE. - 9780819480231
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Konferensbidrag (refereegranskat)abstract
- Spectral imaging is a method in medical x-ray imaging to extract information about the object constituents by the material-specific energy dependence of x-ray attenuation. Contrast-enhanced spectral imaging has been thoroughly investigated, but unenhanced imaging may be more useful because it comes as a bonus to the conventional non-energy-resolved absorption image at screening; there is no additional radiation dose and no need for contrast medium. We have used a previously developed theoretical framework and system model that include quantum and anatomical noise to characterize the performance of a photon-counting spectral mammography system with two energy bins for unenhanced imaging. The theoretical framework was validated with synthesized images. Optimal combination of the energy-resolved images for detecting large unenhanced tumors corresponded closely, but not exactly, to minimization of the anatomical noise, which is commonly referred to as energy subtraction. In that case, an ideal-observer detectability index could be improved close to 50% compared to absorption imaging. Optimization with respect to the signal-to-quantum-noise ratio, commonly referred to as energy weighting, deteriorated detectability. For small microcalcifications or tumors on uniform backgrounds, however, energy subtraction was suboptimal whereas energy weighting provided a minute improvement. The performance was largely independent of beam quality, detector energy resolution, and bin count fraction. It is clear that inclusion of anatomical noise and imaging task in spectral optimization may yield completely different results than an analysis based solely on quantum noise.
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| 12. |
- Fredenberg, Erik, PhD, 1979-, et al.
(författare)
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Phase contrast imaging apparatus
- 2016
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Patent (populärvet., debatt m.m.)abstract
- An x-ray imaging system includes an x-ray source, an x-ray detector including a plurality of detector strips arranged in a first direction of the x-ray detector. Each detector strip includes a plurality of detector pixels arranged in a second direction of the x-ray detector. A phase grating and a plurality of analyzer gratings including grating slits are disposed between the x-ray source and detectors. The x-ray source and the x-ray detector are adapted to perform a scanning movement in relation to an object in the first direction, in order to scan the object. Each of the plurality of analyzer gratings (162) is arranged in association with a respective detector strip with the grating slits arranged in the second direction. The grating slits of the analyzer gratings of the detector strips are offset relative to each other in the second direction.
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| 13. |
- Åslund, Magnus, et al.
(författare)
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AEC for scanning digital mammography based on variation of scan velocity
- 2005
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Ingår i: Medical physics (Lancaster). - : Wiley. - 0094-2405. ; 32:11, s. 3367-3374
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Tidskriftsartikel (refereegranskat)abstract
- A theoretical evaluation of nonuniform x-ray field distributions in mammography was conducted. An automatic exposure control (AEC) is proposed for a scanning full field digital mammography system. It uses information from the leading part of the detector to vary the scan velocity dynamically, thus creating a nonuniform x-ray field in the scan direction. Nonuniform radiation fields were also created by numerically optimizing the scan velocity profile to each breast's transmission distribution, with constraints on velocity and acceleration. The goal of the proposed AEC is to produce constant pixel signal-to-noise ratio throughout the image. The target pixel SNR for each image could be set based on the breast thickness, breast composition, and the beam quality as to achieve the same contrast-to-noise ratio between images for structures of interest. The results are quantified in terms of reduction in entrance surface air kerma (ESAK) and scan time relative to a uniform x-ray field. The theoretical evaluation was performed on a set of 266 mammograms. The performance of the different methods to create nonuniform fields decreased with increased detector width, from 18% to 11% in terms of ESAK reduction and from 30% to 25% in terms of scan time reduction for the proposed AEC and detector widths from 10 to 60 mm. Some correlation was found between compressed breast thickness and the projected breast area onto the image field. This translated into an increase of the ESAK and decrease of the scan time reduction with breast thickness. Ideally a nonuniform field in two dimensions could reduce the entrance dose by 39% on average, whereas a field nonuniform in only the scanning dimension ideally yields a 20% reduction. A benefit with the proposed AEC is that the risk of underexposing the densest region of the breast can be virtually eliminated.
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| 14. |
- Åslund, Magnus
(författare)
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Digital Mammography with a Photon Counting Detector in a Scanned Multislit Geometry
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Mammography screening aims to reduce the number of breast cancer deaths by early detection of the disease, which is one of the leading causes of deaths for middle aged women in the western world. The risk from the x-ray radiation in mammography is relatively low but still a factor in the benefit-risk ratio of screening. The characterization and optimization of a digital mammography system is presented in this thesis. The investigated system is shown to be highly dose efficient by employing a photon counting detector in a scanning multislit geometry. A novel automatic exposure control (AEC) is proposed and validated in clinical practise. The AEC uses the leading detector edge to measure the transmission of the breast. The exposure is modulated by altering the scan velocity during the scan. A W-Al anode-filter combination is proposed. The characterization of the photon counting detector is performed using the detective quantum efficiency. The effect of the photon counting detector and the multislit geometry on the measurement method is studied in detail. It is shown that the detector has a zero-frequency DQE of over 70\% and that it is quantum limited even at very low exposures. Efficient rejection of image-degrading secondary radiation is fundamental for a dose efficient system. The efficiency of the scatter rejection techniques currently used are quantified and compared to the multislit geometry. A system performance metric with its foundation in statistical decision theory is discussed. It is argued that a photon counting multislit system can operate at approximately half the dose compared to several other digital mammography techniques.
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| 15. |
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| 16. |
- Åslund, Magnus
(författare)
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Method and arrangement relating to x-ray imaging
- 2006
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Patent (populärvet., debatt m.m.)abstract
- The present invention relates to a method and arrangement for controlling exposure in an e-ray apparatus, for depicting an object. The apparatus comprises an x-ray source and a displaceable detector being arranged to move with a controllable speed across an image exposure area. The method comprises the step of: acquiring a signal relating to photons incident on at least a part of the detector, comparing said acquired signal with a target value, and controlling the speed of detector, displacement with respect to the result of the comparison.
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| 17. |
- Åslund, Magnus, et al.
(författare)
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Optimization of operating conditions in photon counting multi-slit mammography based on Si-strip detectors - art. no. 61420A
- 2006
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Ingår i: Medical Imaging 2006: Physics of Medical Imaging, Pts 1-3. - : SPIE. - 9780819461858 ; , s. A1420-A1420
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Konferensbidrag (refereegranskat)abstract
- Measurements and simulations of the signal-difference-to-noise ratio (SDNR) and average glandular dose (AGD) have been performed on a photon counting full-field digital mammography system to determine the optimal operating conditions. Several beam qualities were experimentally evaluated by using different combinations of tube voltage, added filters and thickness of BR12 with a tungsten target x-ray tube. The SDNR and AGD were also calculated theoretically for an extended number of operating conditions and a more accurate breast model. As figure of merit for each operating condition, a spectral quantum efficiency (SQE) was calculated as the polychromatic SDNR squared over the optimal monochromatic SDNR squared at the same AGD. The theoretical model agreed within 4% relative the measured SDNR throughout the evaluated breast thickness (30-70 mm) and tube voltage range (26-38 kV). The optimization was performed with a constant SDNR-rate as compared to using a fixed filter thickness. The optimal combinations of tube voltage-filter material were: 32 kV-Ag, 34 kV-Cd, 36 kV-Sn for a breast thickness of 30, 50 and 70 mm respectively. These K-edge filter materials increased the SQE by less than 4% compared to the optimal Al filtration.
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| 18. |
- Åslund, Magnus, et al.
(författare)
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Optimized AEC for scanning digital mammography based on local variation of scan velocity
- 2005
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Ingår i: Medical Imaging 2005: Physics of Medical Imaging, Pts 1 and 2. - BELLINGHAM : SPIE-INT SOC OPTICAL ENGINEERING. - 0819457191 ; , s. 468-477
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Konferensbidrag (refereegranskat)abstract
- In mammography, there is an optimal photon energy and current time product that produce the required image quality at the minimal dose. The task of an automatic exposure control (AEC), in full field digital mammography (FFDM) is to minimize the dose by using optimized exposure settings. Each point in a mammogram has different radiological thickness. A conventional AEC samples the thickness in some regions to set the current time product and possibly also the beam quality. We define an ideal AEC as one that optimizes the beam quality and exposure in each point to produce a constant contrast-to-noise ratio (CNR) of structures of interest throughout the image. This paper presents the results from a theoretical evaluation of an AEC proposed for a scanning photon-counting FFDM system. The geometry enables the AEC to use information from the leading detector line to adjust the scan velocity during the scan. Thus, the irradiation can be better optimized in the scanning-direction as compared to a conventional AEC. The scan time is further reduced by increased velocity over sections that contain no tissue. The results are quantified in terms of reduction of entrance dose and scan time. The presented AEC is compared to an ideal AEC, a conventional AEC and is also benchmarked against an ideal regulator. The effect of the detector width is evaluated. Compared to a conventional AEC, both evaluated on a set of 266 mammograms, the ideal AEC would reduce the entrance dose by 39% on average while the proposed AEC for scanning systems reduces the entrance dose by 10-20% and scan-time by 25-32% on average, depending on detector width.
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| 19. |
- Åslund, Magnus, et al.
(författare)
-
Physical characterization of a scanning photon counting digital mammography system based on Si-strip detectors
- 2007
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Ingår i: Medical physics (Lancaster). - : Wiley. - 0094-2405. ; 34:6, s. 1918-1925
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Tidskriftsartikel (refereegranskat)abstract
- The physical performance of a scanning multislit full field digital mammography system was determined using basic image quality parameters. The system employs a direct detection detector comprised of linear silicon strip sensors in an edge-on geometry connected to photon counting electronics. The pixel size is 50 mu m and the field of view 24 x 26 cm(2). The performance was quantified using the presampled modulation transfer function, the normalized noise power spectrum and the detective quantum efficiency (DQE). Compared to conventional DQE methods, the scanning geometry with its intrinsic scatter rejection poses additional requirements on the measurement setup, which are investigated in this work. The DQE of the photon counting system was found to be independent of the dose level to the detector in the 7.6-206 mu Gy range. The peak DQE was 72% and 73% in the scan and slit direction, respectively, measured with a 28 kV W-0.5 mm Al anodefilter combination with an added 2 mm Al filtration.
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| 20. |
- Åslund, Magnus, et al.
(författare)
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Scatter rejection in multislit digital mammography
- 2006
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Ingår i: Medical physics (Lancaster). - : Wiley. - 0094-2405. ; 33:4, s. 933-940
-
Tidskriftsartikel (refereegranskat)abstract
- The scatter to primary ratio (SPR) was measured on a scanning multislit full-field digital mammography system for different thickness of breast equivalent material and different tube voltages. Scatter within the detector was measured separately and was found to be the major source of scatter in the assembly. Measured total SPRs below 6% are reported for breast range 3-7 cm. The performance of the multislit assembly is compared to other imaging geometries with different scatter rejection schemes by using the scatter detective quantum efficiency.
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