| 1. |
- Aslund, Magnus, et al.
(författare)
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Detectors for the future of X-ray imaging
- 2010
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Ingår i: Radiation Protection Dosimetry. - : Oxford University Press (OUP). - 0144-8420 .- 1742-3406. ; 139:1-3, s. 327-333
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Tidskriftsartikel (refereegranskat)abstract
- In recent decades, developments in detectors for X-ray imaging have improved dose efficiency. This has been accomplished with for example, structured scintillators such as columnar CsI, or with direct detectors where the X rays are converted to electric charge carriers in a semiconductor. Scattered radiation remains a major noise source, and fairly inefficient anti-scatter grids are still a gold standard. Hence, any future development should include improved scatter rejection. In recent years, photon-counting detectors have generated significant interest by several companies as well as academic research groups. This method eliminates electronic noise, which is an advantage in low-dose applications. Moreover, energy-sensitive photon-counting detectors allow for further improvements by optimising the signal-to-quantum-noise ratio, anatomical background subtraction or quantitative analysis of object constituents. This paper reviews state-of-the-art photon-counting detectors, scatter control and their application in diagnostic X-ray medical imaging. In particular, spectral imaging with photon-counting detectors, pitfalls such as charge sharing and high rates and various proposals for mitigation are discussed.
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| 2. |
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| 3. |
- Berggren, Karl, 1989-, et al.
(författare)
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Characterization of photon-counting multislit breast tomosynthesis
- 2018
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Ingår i: Medical Physics. - : John Wiley & Sons. - 2473-4209 .- 0094-2405.
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Tidskriftsartikel (refereegranskat)abstract
- Purpose: It has been shown that breast tomosynthesis may improve sensitivity and specificity compared to two-dimensional mammography, resulting in increased detection-rate of cancers or lowered call-back rates. The purpose of this study is to characterize a spectral photon-counting multislit breast tomosynthesis system that is able to do single-scan spectral imaging with multiple collimated x-ray beams. The system differs in many aspects compared to conventional tomosynthesis using energyintegrating flat-panel detectors. Methods: The investigated system was a prototype consisting of a dual-threshold photon-counting detector with 21 collimated line detectors scanning across the compressed breast. A review of the system is done in terms of detector, acquisition geometry, and reconstruction methods. Three reconstruction methods were used, simple back-projection, filtered back-projection and an iterative algebraic reconstruction technique. The image quality was evaluated by measuring the modulation transfer-function (MTF), normalized noise-power spectrum, detective quantum-efficiency (DQE), and artifact spread-function (ASF) on reconstructed spectral tomosynthesis images for a total-energy bin (defined by a low-energy threshold calibrated to remove electronic noise) and for a high-energy bin (with a threshold calibrated to split the spectrum in roughly equal parts). Acquisition was performed using a 29 kVp W/Al x-ray spectrum at a 0.24 mGy exposure. Results: The difference in MTF between the two energy bins was negligible, that is, there was no energy dependence on resolution. The MTF dropped to 50% at 1.5 lp/mm to 2.3 lp/mm in the scan direction and 2.4 lp/mm to 3.3 lp/mm in the slit direction, depending on the reconstruction method. The full width at half maximum of the ASF was found to range from 13.8 mm to 18.0 mm for the different reconstruction methods. The zero-frequency DQE of the system was found to be 0.72. The fraction of counts in the high-energy bin was measured to be 59% of the total detected spectrum. Scantimes ranged from 4 s to 16.5 s depending on voltage and current settings. Conclusions: The characterized system generates spectral tomosynthesis images with a dual-energy photon-counting detector. Measurements show a high DQE, enabling high image quality at a low dose, which is beneficial for low-dose applications such as screening. The single-scan spectral images open up for applications such as quantitative material decomposition and contrast-enhanced tomosynthesis.
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| 4. |
- Berggren, Karl, et al.
(författare)
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In vivo measurement of the effective atomic number of breast skin using spectral mammography
- 2018
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Ingår i: Physics in Medicine and Biology. - : Institute of Physics Publishing (IOPP). - 0031-9155 .- 1361-6560. ; 63:21
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Tidskriftsartikel (refereegranskat)abstract
- X-ray characteristics of body tissues are of crucial importance for developing and optimizing x-ray imaging techniques, in particular for dosimetry and spectral imaging applications. For breast imaging, the most important tissues are fibro-glandular, adipose and skin tissue. Some work has and is being done to better characterize these tissue types, in particular fibro-glandular and adipose tissue. In the case of breast skin, several recent studies have been published on the average skin thickness, but with regards to x-ray attenuation, the only published data, to the knowledge of the authors, is the elemental composition analysis of Hammerstein et al (1979 Radiology 130 485-91). This work presents an overview of breast skin thickness studies and a measurement of the effective atomic number (Z(eff)) of breast skin using spectral mammography. Z(eff), which together with the density forms the attenuation, is used to validate the work by Hammerstein et al, and the dependence of clinical parameters on Z(eff) is explored. Measurements were conducted on the skin edge of spectral mammograms using clinical data from a screening population (n = 709). The weighted average of breast skin thickness reported in studies between 1997 and 2013 was found to be 1.56 +/- 0.28 mm. Mean Z(eff) was found to be 7.365 (95% CI: 7.364,7.366) for normal breast skin and 7.441 (95% CI: 7.440,7.442) for the nipple and areola. Z(eff) of normal breast skin is in agreement with Hammerstein et al, despite the different methods and larger sample size used. A small but significant increase in Z(eff) was found with age, but the increase is too small to be relevant for most applications. We conclude that normal breast skin is well described by a 1.56 mm skin layer and the elemental composition presented by Hammerstein et al (1979 Radiology 130 485-91) and recommend using these characteristics when modelling breast skin.
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| 5. |
- Berggren, Karl, et al.
(författare)
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Physical characterization of photon-counting tomosynthesis
- 2015
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Konferensbidrag (refereegranskat)abstract
- Tomosynthesis is emerging as a next generation technology in mammography. Combined with photon-counting detectors with the ability for energy discrimination, a novel modality is enabled - spectral tomosynthesis. Further advantages of photon-counting detectors in the context of tomosynthesis include elimination of electronic noise, efficient scatter rejection (in some geometries) and no lag. Fourier-based linear-systems analysis is a well-established method for optimizing image quality in two-dimensional x-ray systems. The method has been successfully adapted to three-dimensional imaging, including tomosynthesis, but several areas need further investigation. This study focuses on two such areas: 1) Adaption of the methodology to photon-counting detectors, and 2) violation of the shift-invariance and stationarity assumptions in non-cylindrical geometries. We have developed a Fourier-based framework to study the image quality in a photon-counting tomosynthesis system, assuming locally linear, stationary, and shift-invariant system response. The framework includes a cascaded-systems model to propagate the modulation-transfer function (MTF) and noise-power spectrum (NPS) through the system. The model was validated by measurements of the MTF and NPS. High degrees of non-shift invariance and non-stationarity were observed, in particular for the depth resolution as the angle of incidence relative the reconstruction plane varied throughout the imaging volume. The largest effects on image quality in a given point in space were caused by interpolation from the inherent coordinate system of the x-rays to the coordinate system that was used for reconstruction. This study is part of our efforts to fully characterize the spectral tomosynthesis system, we intend to extend the model further to include the detective-quantum efficiency, observer modelling, and spectral effects.
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| 6. |
- Berggren, Karl, et al.
(författare)
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Rayleigh imaging in spectral mammography
- 2016
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Ingår i: MEDICAL IMAGING 2016. - : SPIE. - 9781510600188
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Konferensbidrag (refereegranskat)abstract
- Spectral imaging is the acquisition of multiple images of an object at different energy spectra. In mammography, dual-energy imaging (spectral imaging with two energy levels) has been investigated for several applications, in particular material decomposition, which allows for quantitative analysis of breast composition and quantitative contrast-enhanced imaging. Material decomposition with dual-energy imaging is based on the assumption that there are two dominant photon interaction effects that determine linear attenuation: the photoelectric effect and Compton scattering. This assumption limits the number of basis materials, i.e. the number of materials that are possible to differentiate between, to two. However, Rayleigh scattering may account for more than 10% of the linear attenuation in the mammography energy range. In this work, we show that a modified version of a scanning multi-slit spectral photon-counting mammography system is able to acquire three images at different spectra and can be used for triple-energy imaging. We further show that triple-energy imaging in combination with the efficient scatter rejection of the system enables measurement of Rayleigh scattering, which adds an additional energy dependency to the linear attenuation and enables material decomposition with three basis materials. Three available basis materials have the potential to improve virtually all applications of spectral imaging.
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| 7. |
- Berggren, Karl, 1989-
(författare)
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Spectral image quality and applications in breast tomosynthesis
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In the 1970s, it was determined that screening mammography is an efficient tool in fighting the increasing number of women dying from breast cancer, and many countries have established screening programs since then. Mammography systems have improved substantially over the years with one of the major advances being the transition from x-ray film to digital x-ray detectors. Following this development, the number of women dying from breast cancer has decreased, but there is still much room for improvement. One technology that is changing the breast imaging landscape is breast tomosynthesis; tomographic imaging with in-plane resolution similar to that of mammography, albeit limited height resolution. Breast tomosynthesis is commonly implemented with flat-panel detectors, but line detectors in a slit-scanning geometry can also be used. The latter configuration allows for more complex detector technologies, such as spectral photon-counting detectors that enable single-shot spectral imaging. The combination of spectral imaging and tomosynthesis opens up for a range of new applications, but the slit scanning geometry, which differs substantially from that of flat-panel tomosynthesis systems, and the factors affecting image quality have not been well understood. This thesis aims at filling this gap. Image quality and the parameters that influence image quality in spectral photon-counting slit-scanning breast tomosynthesis are characterized and analyzed using cascaded-systems modelling and linear image quality metrics. In addition, the thesis goes into characterizing the x-ray properties of breast tissue, an important input parameter for accurate material decomposition of in-vivo tissue. Material decomposition with spectral imaging opens up a range of applications, such as accurate measurement of volumetric breast density and spectral lesion characterization for decision support as part of mammography screening, and contrast-enhanced K-edge imaging for diagnostics. Tomosynthesis combined with material decomposition has the potential to improve these methods further by, for instance, separating lesions or regions of interest from surrounding fibro-glandular tissue in quantitative 3D maps of breast tissue.
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| 8. |
- Berggren, Karl, et al.
(författare)
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Technical Note: Comparison of first‐ and second‐generation photon‐counting slit‐scanning tomosynthesis systems
- 2018
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Ingår i: Medical physics (Lancaster). - : John Wiley & Sons. - 0094-2405. ; 45:2, s. 635-638
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Tidskriftsartikel (refereegranskat)abstract
- Purpose: Digital breast tomosynthesis (DBT) is an emerging tool for breast-cancer screening and diagnostics. The purpose of this study is to present a second-generation photon-counting slitscanning DBT system and compare it to the first-generation system in terms of geometry and image quality. The study presents the first image-quality measurements on the second-generation system. Method: The geometry of the new system is based on a combined rotational and linear motion, in contrast to a purely rotational scan motion in the first generation. In addition, the calibration routines have been updated. Image quality was measured in the center of the image field in terms of in-slice modulation transfer function (MTF), artifact spread function (ASF), and in-slice detective quantum efficiency (DQE). Images were acquired using a W/Al 29 kVp spectrum at 13 mAs with 2 mm Al additional filtration and reconstructed using simple back-projection. Result: The in-slice 50% MTF was improved in the chest-mammilla direction, going from 3.2 to 3.5 lp/mm, and the zero-frequency DQE increased from 0.71 to 0.77. The MTF and ASF were otherwise found to be on par for the two systems. The new system has reduced in-slice variation of the tomographic angle. Conclusions: The new geometry is less curved, which reduces in-slice tomographic-angle variation, and increases the maximum compression height, making the system accessible for a larger population. The improvements in MTF and DQE were attributed to the updated calibration procedures. We conclude that the second-generation system maintains the key features of the photon-counting system while maintaining or improving image quality and improving the maximum compression height.
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| 9. |
- Berglund, Johan, et al.
(författare)
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Energy weighting improves dose efficiency in clinical practice : implementation on a spectral photon-counting mammography system.
- 2014
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Ingår i: Journal of Medical Imaging. - 2329-4302 .- 2329-4310. ; 1:3
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Tidskriftsartikel (refereegranskat)abstract
- In x-ray imaging, contrast information content varies with photon energy. It is, therefore, possible to improve image quality by weighting photons according to energy. We have implemented and evaluated so-called energy weighting on a commercially available spectral photon-counting mammography system. The technique was evaluated using computer simulations, phantom experiments, and analysis of screening mammograms. The CNR benefit of energy weighting for a number of relevant target-background combinations measured by the three methods fell in the range of 2.2 to 5.2% when using optimal weight factors. This translates to a potential dose reduction at constant CNR in the range of 4.5 to 11%. We expect the choice of weight factor in practical implementations to be straightforward because (1) the CNR improvement was not very sensitive to weight, (2) the optimal weight was similar for all investigated target-background combinations, (3) aluminum/PMMA phantoms were found to represent clinically relevant tasks well, and (4) the optimal weight could be calculated directly from pixel values in phantom images. Reasonable agreement was found between the simulations and phantom measurements. Manual measurements on microcalcifications and automatic image analysis confirmed that the CNR improvement was detectable in energy-weighted screening mammograms.
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| 10. |
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