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  • 1.
    Cederström, Björn
    KTH, Superseded Departments, Physics.
    A multi-prism lens for hard X-Rays2002Doctoral thesis, monograph (Other scientific)
    Abstract [en]

    This thesis describes a new type of refractive lens for hardx-rays. It is shown that a linear array of prisms, slightlyinclined with respect to the optical axis, will form a linefocus at a certain distance from the lens. Hence, the namemulti-prism lens. These lenses are free from sphericalaberration and are similar to planar parabolic compoundrefractive lenses in terms of performance. However, theydistinguish themselves in that only planar surfaces need to befabricated. A special feature is that the focal length can beeasily varied by adjusting the inclination angle.

    Theoretical calculations, based on geometrical and physicaloptics, are used to characterize the lenses. Aberrations arediscussed, as well as the sensitivity to fabricationimperfections, and insufficient flatness is identified as apotential problem. Ray-tracing is used to test theapproximations and assumptions used in the theory. Applicationsin x-ray microscopy and mammography are discussed.

    Lenses have been made of beryllium, silicon, epoxy anddiamond using different methods. Results from measurements ofsurface roughness and figure error show that the imperfectionsof the silicon and epoxy lenses should have a small impact,while the beryllium lenses should suffer from strongscattering. Experiments were performed at the EuropeanSynchrotron Radiation Facility and sub-µm focal linewidths, close to theoretical expectations, were measured forsilicon and epoxy lenses at 30 keV and 14 keV, respectively.Insertion gains up to 40 were reached. Two crossed lenses wereused to obtain focusing in two dimensions and a point focus.The smallest measured focal spot size was 1.0 µm by 5 .4µm, and an insertion gain exceeding 100 was achieved usingepoxy lenses.

    The diamond lenses suffered from voids in the materialformed in the chemical vapor deposition process, butnevertheless provided focal lines less than 2 µm in width,albeit at at relatively low insertion gain of 13. Due to theirexcellent thermal properties, these lenses are put forward ascandidates for optics at the next generationultra-high-intensity synchrotron beams and x-ray free electronlasers.

    Keywords:x-ray, optics, refractive, lens, mammography,synchrotron.

  • 2.
    Cederström, Björn
    KTH, Superseded Departments, Physics.
    Refractive Optics for Hard X-rays2001Licentiate thesis, monograph (Other scientific)
  • 3.
    Cederström, Björn
    et al.
    KTH, Superseded Departments, Physics.
    Cahn, R. N.
    Danielsson, Mats
    KTH, Superseded Departments, Physics.
    Lundqvist, M.
    Nygren, D. R.
    Focusing hard X-rays with old LPs2000In: Nature, ISSN 0028-0836, Vol. 404, no 6781, 951-951 p.Article in journal (Refereed)
  • 4.
    Cederström, Björn
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Fredenberg, Erik
    KTH, School of Engineering Sciences (SCI), Physics.
    The influence of anatomical noise on optimal beam quality in mammography2014In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 41, no 12, 121903- p.Article in journal (Refereed)
    Abstract [en]

    Purpose: Beam-quality optimization in digital mammography traditionally considers detection of a target obscured by quantum noise in a homogeneous background. This does not correspond well to the clinical imaging task because real mammographic images contain a complex superposition of anatomical structures, resulting in anatomical noise that may dominate over quantum noise. The purpose of this paper is to assess the influence on optimal beam quality in mammography when anatomical noise is taken into account. Methods: The detectability of microcalcifications and masses was quantified using a theoretical ideal-observer model that included quantum noise as well as anatomical noise and a simplified model of a photon-counting mammography system. The outcome was experimentally verified using two types of simulated tissue phantoms. Results: The theoretical model showed that the detectability of tumors and microcalcifications behaves differently with respect to beam quality and dose. The results for small microcalcifications were similar to what traditional optimization methods yield, which is to be expected because quantum noise dominates over anatomical noise at high spatial frequencies. For larger tumors, however, low-frequency anatomical noise was the limiting factor. Because anatomical structure noise has similar energy dependence as tumor contrast, the optimal x-ray energy was found to be higher and the useful energy region was wider than traditional methods suggest. A simplified scalar model was able to capture this behavior using a fitted noise mixing parameter. The phantom measurements confirmed these theoretical results. Conclusions: It was shown that since quantum noise constitutes only a small fraction of the noise, the dose could be reduced substantially without sacrificing tumor detectability. Furthermore, when anatomical noise is included, the tube voltage can be increased well beyond what is conventionally considered optimal and used clinically, without loss of image quality. However, no such conclusions can be drawn for the more complex mammographic imaging task as a whole. (C) 2014 American Association of Physicists in Medicine.

  • 5.
    Cederström, Björn
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Fredenberg, Erik
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Lundqvist, Mats
    Sectra Mamea AB, Solna, Sweden.
    Ericson, Tove
    Sectra Mamea AB, Solna, Sweden.
    Åslund, Magnus
    Sectra Mamea AB, Solna, Sweden.
    Observer-model optimization of X-ray system in photon-counting breast imaging2011In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, Vol. 648, no Supplement 1, S54-S57 p.Article in journal (Refereed)
    Abstract [en]

    An ideal-observer model is applied to optimize the design of an X-ray tube intended for use in a multi-slit scanning photon-counting mammography system. The design is such that the anode and the heel effect are reversed and the projected focal spot is smallest at the chest wall. Using linear systems theory, detectability and dose efficiency for a 0.1-mm disk are calculated for different focal spot sizes and anode angles. It is shown that the image acquisition time can be reduced by about 25% with spatial resolution and dose efficiency improved near the chest wall and worsened further away. The image quality is significantly more homogeneous than for the conventional anode orientation, both with respect to noise and detectability of a small object. With the tube rotated 90°, dose efficiency can be improved by 20% for a fixed image acquisition time. 

  • 6.
    Cederström, Björn
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Lundqvist, Mats
    KTH, School of Engineering Sciences (SCI), Physics.
    Ribbing, Carolina
    Uppsala Universitet.
    Multi-prism x-ray lens2002In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 81, no 8, 1399-1401 p.Article in journal (Refereed)
    Abstract [en]

    Refractive x-ray lenses with a triangular surface profile have been used to focus a synchrotron beam to sub-mum line width. These lenses are free from spherical aberration and work in analogy with one-dimensional focusing parabolic compound refractive lenses. However, the focal length can be easily varied by changing the gap between the two jaws. Silicon lenses were fabricated by wet anisotropic etching, and epoxy replicas were molded from the silicon masters. The lenses provided intensity gains up to a factor of 32 and the smallest focal line width was 0.87 mum. The simplified geometry and associated fabrication technique open possibilities for low-Z materials such as beryllium, which should greatly enhance the performance of refractive x-ray optics. (C) 2002 American Institute of Physics.

  • 7.
    Cederström, Björn
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Ribbing, C.
    Lundqvist, M.
    Generalized prism-array lenses for hard X-rays2005In: Journal of Synchrotron Radiation, ISSN 0909-0495, E-ISSN 1600-5775, Vol. 12, 340-344 p.Article in journal (Refereed)
    Abstract [en]

    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.

  • 8.
    Cederström, Björn
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Streubuehr, Ursula
    Comparison of photon-counting to storage phosphor plate mammography using contrast-detail phantom analysis2007In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 580, no 2, 1101-1104 p.Article in journal (Refereed)
    Abstract [en]

    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.

  • 9.
    Chen, Han
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Xu, Cheng
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Persson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Karlsson, Staffan
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    A photon-counting silicon-strip detector for digital mammography with an ultrafast 0.18-mu m CMOS ASIC2014In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 749, 1-6 p.Article in journal (Refereed)
    Abstract [en]

    We have evaluated a silicon-strip detector with a 0.18-mu m CMOS application specific integrated circuits (ASIC) containing 160 channels for use in photon-counting digital mammography. Measurements were performed at the Elettra light source using monochromatic X-ray beams with different energies and intensities. Energy resolution, Delta E/E-in, was measured to vary between 0.10 and 0.23 in the energy range of 15-40 keV. Pulse pileup has shown little effect on energy resolution.

  • 10.
    Chen, Han
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    On imaging with or without grid in digital mammography2014In: Proceedings of SPIE, the International Society for Optical Engineering, ISSN 0277-786X, Vol. 9033, 903346- p.Article in journal (Refereed)
    Abstract [en]

    The grids used in digital mammography to reduce scattered radiation from the breast are not perfect and lead to partial absorption of primary radiation at the same time as not all of the scattered radiation is absorbed. It has therefore lately been suggested to remove the grids and correct for effects of scattered radiation by post-processing the images. In this paper, we investigated the dose reduction that might be achieved if the gird were to be removed. Dose reduction is determined as a function of PMMA thickness by comparing the contrast-to-noise ratios (CNRs) of images acquired with and without grid at a constant exposure. We used a theoretical model validated with Monte Carlo simulations and phantom studies. To evaluate the CNR, we applied aluminum filters of two different sizes, 4x8 cm2 and 1x1 cm 2. When the large Al filter was used, the resulting CNR value for the grid-less images was overestimated as a result of a difference in amount of scattered radiation in the background region and of the region covered by the filter, a difference that could be eliminated by selecting a region of interest close to the edge of the filter. The optimal CNR when the PMMA thickness was above about 4 cm was obtained with a grid, whereas removing the grid leaded to a dose saving in thinner PMMAs. The results suggest not removing grids in breast cancer screening.

  • 11.
    Chen, Han
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Xu, Cheng
    KTH, School of Engineering Sciences (SCI), Physics.
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Physics.
    On image quality metrics and the usefulness of grids in digital mammography2015In: Journal of medical imaging (Bellingham, Wash.), ISSN 2329-4302, Vol. 2, no 1, 013501-013501 p.Article in journal (Refereed)
    Abstract [en]

    Antiscatter grids are used in digital mammography to reduce the scattered radiation from the breast and improve image contrast. They are, however, imperfect and lead to partial absorption of primary radiation, as well as failing to absorb all scattered radiation. Nevertheless, the general consensus has been that antiscatter grids improve image quality for the majority of breast types and sizes. There is, however, inconsistency in the literature, and recent results show that a substantial image quality improvement can be achieved even for thick breasts if the grid is disposed of. The purpose of this study was to investigate if differences in the considered imaging task and experimental setup could explain the different outcomes. We estimated the dose reduction that can be achieved if the grid were to be removed as a function of breast thickness with varying geometries and experimental conditions. Image quality was quantified by the signal-difference-to-noise ratio (SDNR) measured using an aluminum (Al) filter on blocks of poly(methyl methacrylate) (PMMA), and images were acquired with and without grid at a constant exposure. We also used a theoretical model validated with Monte Carlo simulations. Both theoretically and experimentally, the main finding was that when a large [Formula: see text] Al filter was used, the SDNR values for the gridless images were overestimated up to 25% compared to the values for the small [Formula: see text] filter, and gridless imaging was superior for any PMMA thickness. For the small Al filter, gridless imaging was only superior for PMMAs thinner than 4cm. This discrepancy can be explained by a different sensitivity to and sampling of the angular scatter spread function, depending on the size of the contrast object. The experimental differences were eliminated either by using a smaller region of interest close to the edge of the large filter or by applying a technique of scatter correction by subtracting the estimated scatter image. These results explain the different conclusions reported in the literature and show the importance of the selection of measurement methods. Since the interesting structures in mammography are below the 1-cm scale, we advocate the use of smaller contrast objects for assessment of antiscatter grid performance.

  • 12.
    Fredenberg, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Energy filtering with x-ray lenses: Optimization for photon-counting mammography2010In: Radiation Protection Dosimetry, ISSN 0144-8420, E-ISSN 1742-3406, Vol. 139, 339-342 p.Article in journal (Refereed)
    Abstract [en]

    Chromatic properties of the multi-prism and prism-array x-ray lenses (MPL and PAL) can potentially be utilized for efficient energy filtering and dose reduction in mammography. The line-shaped foci of the lenses are optimal for coupling to photon-counting silicon strip detectors in a scanning system. A theoretical model was developed and used to investigate the benefit of two lenses compared to an absorption-filtered reference system. The dose reduction of the MPL filter was 15% compared to the reference system at matching scan time, and the spatial resolution was higher. The dose of the PAL-filtered system was found to be 20% lower than for the reference system at equal scan time and resolution, and only 20% higher than for a monochromatic beam. An investigation of some practical issues remains, including the feasibility of brilliant-enough x-ray sources and manufacturing of a polymer PAL.

  • 13.
    Fredenberg, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Lundqvist, Mats
    Ribbing, Carolina
    Åslund, Magnus
    Diekmann, Felix
    Nishikawa, Robert
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Contrast-enhanced dual-energy subtraction imaging using electronic spectrum-splitting and multi-prism x-ray lenses2008In: Medical Imaging 2008 - Physics of Medical Imaging: PTS 1-3 / [ed] Hsieh, J; Samei, E, 2008, Vol. 6913, 91310-91310 p.Conference paper (Refereed)
    Abstract [en]

    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.

  • 14.
    Fredenberg, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Nillius, Peter
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Ribbing, Carolina
    Uppsala Univ..
    Karlsson, Staffan
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    A low-absorption x-ray energy filter for small-scale applications2009In: Optics Express, ISSN 1094-4087, Vol. 17, no 14, 11388-11398 p.Article in journal (Refereed)
    Abstract [en]

    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.

  • 15.
    Fredenberg, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Applied Physics, Biomedical and X-ray Physics.
    Ribbing, Carolina
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Prism-array lenses for energy filtering in medical X-ray imaging: Physics of Medical Imaging, Pts 1-32007In: Medical Imaging 2007: Physics of Medical Imaging, Pts 1-3 / [ed] Hsieh, J; Flynn, MJ, BELLINGHAM: SPIE-INT SOC OPTICAL ENGINEERING , 2007, Vol. 6510, U270-U281 p.Conference paper (Refereed)
    Abstract [en]

    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.

  • 16.
    Fredenberg, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Åslund, Magnus
    KTH, School of Engineering Sciences (SCI), Physics.
    Nillius, Peter
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    An efficient pre-object collimator based on an x-ray lens2009In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 36, no 2, 626-633 p.Article in journal (Refereed)
    Abstract [en]

    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.

  • 17.
    Fredenberg, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Åslund, Magnus
    Nillius, Peter
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Lundqvist, Mats
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Imaging with multi-prism x-ray lenses2008In: Medical Imaging 2008 - Physics of Medical Imaging: PTS 1-3 / [ed] Hsieh, J; Samei, E, 2008, Vol. 6913, 91308-91308 p.Conference paper (Refereed)
    Abstract [en]

    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).

  • 18.
    Fredenberg, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Åslund, Magnus
    Ribbing, Carolina
    Uppsala universitet.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    A Tunable Energy Filter for Medical X-Ray Imaging2008In: X-Ray Optics and Instrumentation, ISSN 1687-7632, Vol. 2008Article in journal (Refereed)
    Abstract [en]

    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.

  • 19.
    Fredenberg, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Hemmendorff, Magnus
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Åslund, Magnus
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Contrast-enhanced spectral mammography with a photon-counting detector2010In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 37, no 5, 2017-2029 p.Article in journal (Refereed)
    Abstract [en]

    Purpose: 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. In particular, the detectability of a contrast agent can be improved over a lumpy background. We have investigated a photon-counting spectral imaging system with two energy bins for contrast-enhanced mammography. System optimization and the potential benefit compared to conventional non-energy-resolved imaging was studied.

    Methods: A framework for system characterization was set up that included quantum and anatomical noise, and a theoretical model of the system was benchmarked to phantom measurements.

    Results: It was found that optimal combination of the energy-resolved images corresponded approximately to minimization of the anatomical noise, and an ideal-observer detectability index could be improved more than a factor of two compared to absorption imaging in the phantom study. In the clinical case, an improvement close to 80% was predicted for an average glandularity breast, and a factor of eight for dense breast tissue. Another 70% was found to be within reach for an optimized system.

    Conclusions: Contrast-enhanced spectral mammography is feasible and beneficial with the current system, and there is room for additional improvements.

  • 20.
    Fredenberg, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Lundqvist, Mats
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Åslund, Magnus
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Energy resolution of a photon-counting silicon strip detector2010In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, 156-162 p.Article in journal (Refereed)
    Abstract [en]

    A photon-counting silicon strip detector with two energy thresholds was investigated for spectral X-ray imaging in a mammography system. Preliminary studies already indicate clinical benefit of the detector, and the purpose of the present study is optimization with respect to energy resolution. Factors relevant for the energy response were measured, simulated, or gathered from previous studies, and used as input parameters to a cascaded detector model. Threshold scans over several X-ray spectra were used to calibrate threshold levels to energy, and to validate the model. The energy resolution of the detector assembly was assessed to range over ΔE/E=0.12–0.26 in the mammography region. Electronic noise dominated the peak broadening, followed by charge sharing between adjacent detector strips, and a channel-to-channel threshold spread. The energy resolution may be improved substantially if these effects are reduced to a minimum. Anti-coincidence logic mitigated double counting from charge sharing, but erased the energy resolution of all detected events, and optimization of the logic is desirable. Pile-up was found to be of minor importance at typical mammography rates.

  • 21.
    Fredenberg, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Lundqvist, Mats
    Åslund, Magnus
    Hemmendorff, Magnus
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    A photon-counting detector for dual-energy breast tomosynthesis2009In: Medical Imaging 2009: Physics of Medical Imaging, 2009, Vol. 7258, 72581- p.Conference paper (Refereed)
    Abstract [en]

    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.

  • 22.
    Fredenberg, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Svensson, B.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Lazzari, B.
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Optimization of mammography with respect to anatomical noise2011In: MEDICAL IMAGING 2011: PHYSICS OF MEDICAL IMAGING / [ed] Pelc, NJ; Samei, E; Nishikawa, RM, 2011, Vol. 7961Conference paper (Refereed)
    Abstract [en]

    Beam quality optimization in mammography traditionally considers detection of a target obscured by quantum noise on a homogenous background. It can be argued that this scheme does not correspond well to the clinical imaging task because real mammographic images contain a complex superposition of anatomical structures, resulting in anatomical noise that may dominate over quantum noise. Using a newly developed spectral mammography system, we measured the correlation and magnitude of the anatomical noise in a set of mammograms. The results from these measurements were used as input to an observer-model optimization that included quantum noise as well as anatomical noise. We found that, within this framework, the detectability of tumors and microcalcifications behaved very differently with respect to beam quality and dose. The results for small microcalcifications were similar to what traditional optimization methods would yield, which is to be expected since quantum noise dominates over anatomical noise at high spatial frequencies. For larger tumors, however, low-frequency anatomical noise was the limiting factor. Because anatomical structure has similar energy dependence as tumor contrast, optimal x-ray energy was significantly higher and the useful energy region wider than traditional methods suggest. Measurements on a tissue phantom confirmed these theoretical results. Furthermore, since quantum noise constitutes only a small fraction of the noise, the dose could be reduced substantially without sacrificing tumor detectability. Exposure settings used clinically are therefore not necessarily optimal for this imaging task. The impact of these findings on the mammographic imaging task as a whole is, however, at this stage unclear.

  • 23.
    Fredenberg, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Åslund, Magnus
    Sectra Mamea AB.
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Lundqvist, Mats
    Sectra Mamea AB.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Observer model optimization of a spectral mammography system2010In: MEDICAL IMAGING 2010: PHYSICS OF MEDICAL IMAGING / [ed] Samei E; Pelc NJ, 2010, Vol. 7622Conference paper (Refereed)
    Abstract [en]

    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.

  • 24. Lundqvist, M.
    et al.
    Cederström, Björn
    KTH, Superseded Departments, Physics.
    Chmill, V.
    Danielsson, Mats
    Hasegawa, B.
    Evaluation of a photon-counting X-ray imaging system2001In: IEEE Transactions on Nuclear Science, ISSN 0018-9499, E-ISSN 1558-1578, Vol. 48, no 4, 1530-1536 p.Article in journal (Refereed)
    Abstract [en]

    A digital imaging system has been developed for mammography using silicon strip detectors operated in a pulse-counting mode and configured in a scanned slit geometry. More than 90% of the photons are absorbed in the sensor. The scatter rejection is very efficient and the image formation is nearly optimal since each X ray is processed individually. The result is a very dose-efficient system. In this paper, we present measurements that verify that the performance of the readout electronics is sufficient to count photons at high rates with high quantum efficiency and a charge collection efficiency that does not limit the dose efficiency. The spatial resolution of the system was measured to provide a modulation transfer function of approximately 0.5 at a spatial frequency of 10 lp/mm. Images of a mammography phantom were recorded experimentally to test overall system performance.

  • 25. Lundqvist, M.
    et al.
    Cederström, Björn
    KTH, Superseded Departments, Physics.
    Chmill, V.
    Danielsson, Mats
    KTH, Superseded Departments, Physics.
    Nygren, D.
    Computer simulations and performance measurements on a silicon strip detector for edge-on imaging2000In: IEEE Transactions on Nuclear Science, ISSN 0018-9499, E-ISSN 1558-1578, Vol. 47, no 4, 1487-1492 p.Article in journal (Refereed)
    Abstract [en]

    Silicon strip detectors to be used edge-on for imaging in a scanned slit geometry have been simulated. A software program was developed which can simulate the motion of free charges in the bulk detector and calculate the signals they induce on the electrodes. The purpose was to quantify the impact of charge sharing on system detective quantum efficiency(DQE). The energy spectrum that was used in this study is typical for mammography. The detectors are working in single photon counting mode and the optimal threshold level to discriminate noise from useful signals has been calculated. The loss in detective quantum efficiency due to charge sharing was found to be around 5% for a 100 mu m pitched detector. Coincidence circuits can be included in the electronics to eliminate this problem. Furthermore, it is described how the relationship between charge collection efficiency and photon interaction position in the detector can be measured.

  • 26.
    Nillius, Peter
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Karlsson, Staffan
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Fredenberg, Erik
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Large-aperture focusing of high-energy x rays with a rolled polyimide film2011In: Optics Letters, ISSN 0146-9592, E-ISSN 1539-4794, Vol. 36, no 4, 555-557 p.Article in journal (Refereed)
    Abstract [en]

    We describe a point-focusing x-ray lens made of a rolled polyimide film with etched prisms. The resulting lens is a cylinder with a large number of prisms forming an internal conic structure. The method allows for the manufacturing of lenses with large apertures and short focal lengths, for energies up to at least 100 keV. In order to evaluate the concept, we have hand-rolled a few lenses and evaluated them at a synchrotron source. The measured performance of the prototype is promising, and deviations from the theoretical limits are quantitatively explained. (C) 2011 Optical Society of America

  • 27. Norell, B.
    et al.
    Fredenberg, Erik
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Leifland, K.
    Lundqvist, M.
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Lesion characterization using spectral mammography2012In: Medical Imaging 2012: Physics Of Medical Imaging, SPIE - International Society for Optical Engineering, 2012, Vol. 8313, 83130I- p.Conference paper (Refereed)
    Abstract [en]

    We present a novel method for characterizing mammographic findings using spectral imaging without the use of contrast agent. Within a statistical framework, suspicious findings are analyzed to determine if they are likely to be benign cystic lesions or malignant tissue. To evaluate the method, we have designed a phantom where combinations of different tissue types are realized by decomposition into the material bases aluminum and polyethylene. The results indicate that the lesion size limit for reliable characterization is below 10 mm diameter, when quantum noise is the only considered source of uncertainty. Furthermore, preliminary results using clinical images are encouraging, but allow no conclusions with significance.

  • 28. Ribbing, C.
    et al.
    Cederström, Björn
    KTH, Superseded Departments, Physics.
    Lundqvist, M.
    Microfabrication of saw-tooth refractive x-ray lenses in low-Z materials2003In: Journal of Micromechanics and Microengineering, ISSN 0960-1317, E-ISSN 1361-6439, Vol. 13, no 5, 714-720 p.Article in journal (Refereed)
    Abstract [en]

    Saw-tooth x-ray refractive lenses have been fabricated in silicon, epoxy and diamond. Silicon lenses were made by anisotropic wet etching of single crystalline silicon. Epoxy lenses were moulded from silicon masters. Diamond lenses were replicated by. chemical vapour deposition on silicon masters and subsequent sacrificial etching of silicon. Beryllium saw-tooth test structures were embossed using a diamond master. Silicon and epoxy lenses gave sub-micron focal lines and provided gains of up to 40 when tested in a synchrotron set-up. Focal lengths ranged from 0.33 to 0.61 m for x-ray energies between 14 and 30 keV.

  • 29. Ribbing, C.
    et al.
    Cederström, Björn
    KTH, Superseded Departments, Physics.
    Lundqvist, M.
    Microstructured diamond X-ray source and refractive lens2003In: Diamond and related materials, ISSN 0925-9635, Vol. 12, no 11-okt, 1793-1799 p.Article in journal (Refereed)
    Abstract [en]

    This paper treats microstructured CVD diamond in two X-ray applications, a miniature X-ray source and a refractive X-ray lens. The X-ray source consists of boron doped diamond membrane electrodes and an intermediate insulator. The cathode has a pyramidal shape, which is field-emitting and the anode is a metal film on a diamond membrane. Anode radiation emerges through both membrane electrodes. The source has not been vacuum sealed, therefore, all measurements so far have been made in a vacuum chamber. The refractive X-ray lens has saw-tooth geometry and a tunable focal length. It was made by microwave plasma assisted CVD of diamond onto anisotropically etched silicon masters. The lens has been used for one-dimensional focusing of a synchrotron beam to 1.9 mum line width.

  • 30. Shastri, S. D.
    et al.
    Almer, J.
    Ribbing, C.
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    High-energy X-ray optics with silicon saw-tooth refractive lenses2007In: Journal of Synchrotron Radiation, ISSN 0909-0495, E-ISSN 1600-5775, Vol. 14, 204-211 p.Article in journal (Refereed)
    Abstract [en]

    Silicon saw-tooth refractive lenses have been in successful use for vertical focusing and collimation of high-energy X-rays (50-100 keV) at the 1-ID undulator beamline of the Advanced Photon Source. In addition to presenting an effectively parabolic thickness profile, as required for aberration-free refractive optics, these devices allow high transmission and continuous tunability in photon energy and focal length. Furthermore, the use of a single-crystal material (i.e. Si) minimizes small-angle scattering background. The focusing performance of such saw-tooth lenses, used in conjunction with the 1-ID beamline's bent double-Laue monochromator, is presented for both short (similar to 1: 0.02) and long (similar to 1: 0.6) focal-length geometries, giving line-foci in the 2 mu m-25 mu m width range with 81 keV X-rays. In addition, a compound focusing scheme was tested whereby the radiation intercepted by a distant short-focal-length lens is increased by having it receive a collimated beam from a nearer ( upstream) lens. The collimation capabilities of Si saw-tooth lenses are also exploited to deliver enhanced throughput of a subsequently placed small-angular-acceptance high-energy-resolution post-monochromator in the 50-80 keV range. The successful use of such lenses in all these configurations establishes an important detail, that the pre-monochromator, despite being comprised of vertically reflecting bent Laue geometry crystals, can be brilliance-preserving to a very high degree.

  • 31.
    Svensson, Björn
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Biomedical and X-ray Physics. KTH, School of Biotechnology (BIO), Centres, Albanova VinnExcellence Center for Protein Technology, ProNova.
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Applied Physics, Biomedical and X-ray Physics. KTH, School of Biotechnology (BIO), Centres, Albanova VinnExcellence Center for Protein Technology, ProNova.
    Aslund, Magnus
    Lundqvist, Mats
    Teleman, Max
    Variable height multi-slit collimator and optimized image reconstruction in a photon-counting system for digital mammography2011In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 648, S216-S219 p.Article in journal (Refereed)
    Abstract [en]

    We have investigated a pre-breast collimator able to operate at variable heights for a photon-counting scanned multi-slit mammography system. A prototype system was built and used to evaluate how different collimator heights combined with optimized collimator slit widths affect dose efficiency and scan time. Moreover, new image construction software which takes into account the sub-pixel shift of each detector line was implemented and evaluated. With maintained scan time a collimator 79 mm above patient support improved dose efficiency for a 100 mu m disc by 12% and by 22% for a scan time extended by 10%. For image reconstruction and the same disc size a dose efficiency improvement of 32% +/- 9% was measured indicating a higher gain than expected. (c) 2010 Elsevier B.V. All rights reserved.

  • 32.
    Tibbelin, Sandra
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Nillius, Peter
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    HyperSPECT: a new system for pre-clinical imaging in vivo2009In: Medical Imaging 2009: Physics of Medical Imaging, SPIE - International Society for Optical Engineering, 2009, Vol. 7258, no 1Conference paper (Refereed)
    Abstract [en]

    Imaging in general is becoming increasingly important in the medical science. At the cell level it is possible to label and trace almost individual molecules in vivo to study biochemical reactions using microscopy. In vivo imaging of living organisms is today mainly accomplished by PET, SPECT and fMRI. The problem is that the spatial resolution for realistic image acquisition times is of the order 1-3 mm, which is a serious limitation. We propose a new imaging modality, based on the same principles as SPECT but with drastically improved efficiency and spatial resolution. This is achieved by incorporating a large number of x-ray lenses between the detectors and the object. In current SPECT a pin-hole geometry is standard, involving an unfortunate trade-off between efficiency and spatial resolution, our solution would change this. The agent for radiolabelling is assumed to be 125I, with an emission peak at 27 keV, since it is widely used and easy to handle. The large area, photon counting detectors will consist of a columnar CsI scintillator coupled to a CMOS integrated circuit for electronic read-out. Our simulations of the entire system and of the detector indicate that a resolution of 50 μm for the system is possible.

  • 33. Åslund, M.
    et al.
    Cederström, Björn
    KTH, Superseded Departments, Physics.
    Lundqvist, M.
    Danielsson, Mats
    KTH, Superseded Departments, Physics.
    Scatter rejection in scanned multi-slit digital mammography2004In: Proceedings of SPIE - The International Society for Optical Engineering, 2004, no 1, 478-487 p.Conference paper (Refereed)
    Abstract [en]

    Measurements and Monte Carlo simulations were used to investigate the scatter properties of a scanned multi-slit digital mammography system. Scatter to primary ratio (S/P) in the center of the image field was calculated for different thickness of breast equivalent material and different tube potentials. The simulated model also varied the angular acceptance, the number of slits and the distance between the slits of a dedicated scatter rejection device. In addition to the expected scatter from the breast equivalent material, scatter within the detector contributes to the S/P-ratio. The main part of the scatter is identified as coming from this process. Measured total 5/P-ratios below 3% are reported for breast range 3-8 cm. The scatter-DQE is used as figure-of-merit for comparison to other imaging geometries and scatter rejection schemes.

  • 34.
    Åslund, Magnus
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Physics.
    Lundqvist, M
    Danielsson, M
    Optimized AEC for scanning digital mammography based on local variation of scan velocity2005In: Medical Imaging 2005: Physics of Medical Imaging, Pts 1 and 2 / [ed] Flynn, MJ, BELLINGHAM: SPIE-INT SOC OPTICAL ENGINEERING , 2005, Vol. 5745, 468-477 p.Conference paper (Refereed)
    Abstract [en]

    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.

  • 35.
    Åslund, Magnus
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Physics.
    Lundqvist, Mats
    KTH, School of Engineering Sciences (SCI), Physics.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics.
    AEC for scanning digital mammography based on variation of scan velocity2005In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 32, no 11, 3367-3374 p.Article in journal (Refereed)
    Abstract [en]

    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.

  • 36.
    Åslund, Magnus
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Physics.
    Lundqvist, Mats
    KTH, School of Engineering Sciences (SCI), Physics.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics.
    Physical characterization of a scanning photon counting digital mammography system based on Si-strip detectors2007In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 34, no 6, 1918-1925 p.Article in journal (Refereed)
    Abstract [en]

    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.

  • 37.
    Åslund, Magnus
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Physics.
    Lundqvist, Mats
    KTH, School of Engineering Sciences (SCI), Physics.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics.
    Evaluation of an AEC system for scanning photon counting mammography based on variation of scan velocityIn: Medical physics (Lancaster), ISSN 0094-2405Article in journal (Other academic)
  • 38.
    Åslund, Magnus
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Physics.
    Lundqvist, Mats
    KTH, School of Engineering Sciences (SCI), Physics.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Scatter rejection in multislit digital mammography2006In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 33, no 4, 933-940 p.Article in journal (Refereed)
    Abstract [en]

    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.

  • 39.
    Åslund, Magnus
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Physics.
    Lundqvist, Mats
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics.
    Optimization of operating conditions in photon counting multi-slit mammography based on Si-strip detectors - art. no. 61420A2006In: Medical Imaging 2006: Physics of Medical Imaging, Pts 1-3 / [ed] Flynn, MJ; Hsieh, J, 2006, Vol. 6142, A1420-A1420 p.Conference paper (Refereed)
    Abstract [en]

    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.

  • 40.
    Åslund, Magnus
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Fredenberg, Erik
    KTH, School of Engineering Sciences (SCI), Physics.
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Physics.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics.
    Spectral shaping for photon counting digital mammography2007In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, Vol. 580, no 2, 1046-1049 p.Article in journal (Refereed)
    Abstract [en]

    The spectral shaping properties of conventional filters have been evaluated for a photon counting digital mammography system, and the result has been compared with the theoretical spectrum from a multi-prism X-ray lens (MPL). The absorption filters and the MPL were evaluated using a theoretical model of the system which has been verified experimentally. The spectral shaping performance is quantified with the spectral quantum efficiency (SQE), calculated as the polychromatic signal-difference-to-noise ratio (SDNR) squared over the optimal monochromatic SDNR squared at the same average glandular dose. The MPL increases the SQE by 25% compared to the investigated absorption filter when compared with a Tungsten anode. This translates into a potential dose reduction of 20% at maintained SDNR.

1 - 40 of 40
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