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  • 1.
    Angelopoulos, A.
    et al.
    -.
    Apostolakis, A.
    -.
    Aslanides, E.
    -.
    Backenstoss, G.
    -.
    Bargassa, P.
    -.
    Behnke, O.
    -.
    Benelli, A.
    -.
    Bertin, V.
    -.
    Blanc, F.
    -.
    Bloch, P.
    -.
    Danielsson, Mats
    KTH, Tidigare Institutioner (före 2005), Fysik.
    K 0–K̄0 mass and decay-width differences: CPLEAR evaluation1999Ingår i: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 471, nr 2, s. 332-338Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The CPT-violation parameters Re(δ) and Im(δ) determined recently by CPLEAR are used to evaluate the K0 mass and decay-width differences, as given by the difference between the diagonal elements of the neutral-kaon mixing matrix (M−iΓ/2). The results – GeV and GeV – are consistent with CPT invariance. The CPT invariance is also shown to hold within a few times 10−3–10−4 for many of the amplitudes describing neutral-kaon decays to different final states.

  • 2.
    Angelopoulos, Angelos
    et al.
    -.
    Apostolakis, A.
    -.
    Aslanides, E.
    -.
    Backenstoss, Gerhard
    -.
    Bargassa, P.
    -.
    Bee, C P.
    -.
    Behnke, O.
    -.
    Benelli, A.
    -.
    Bertin, V.
    -.
    Blanc, F.
    -.
    Danielsson, Mats
    KTH, Tidigare Institutioner (före 2005), Fysik.
    Physics at CPLEAR2003Ingår i: Physics Reports, Vol. 374, nr 3, s. 165-270Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    LEAR offered unique opportunities to study the symmetries which exist between matter and antimatter. At variance with other approaches at this facility, CPLEAR was an experiment devoted to the study of CP, T and CPT symmetries in the neutral-kaon system. A variety of measurements allowed us to determine with high precision the parameters which describe the time evolution of the neutral kaons and their antiparticles, including decay amplitudes, and the related symmetry properties. Limits concerning quantum-mechanical predictions (EPR, coherence of the wave function) or the equivalence principle of general relativity have been obtained. An account of the main features of the experiment and its performances is given here, together with the results achieved.

  • 3.
    Angelopoulos, Angelos
    et al.
    -.
    Aslanides, E.
    -.
    Backenstoss, G.
    -.
    Bargassa, P.
    -.
    Behnke, O.
    -.
    Benelli, A.
    -.
    Bertin, V.
    -.
    Blanc, F.
    -.
    Bloch, P.
    -.
    Carlson, P.
    -.
    Danielsson, Mats
    KTH, Tidigare Institutioner (före 2005), Fysik.
    K0⇋ K̄0 transitions monitored by strong interactions: a new determination of the K L–K S mass difference2001Ingår i: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 503, nr 1, s. 49-57Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The CPLEAR set-up (modified) has been used to determine the KL–KS mass difference by a method where neutral-kaon strangeness oscillations are monitored through kaon strong interactions, rather than semileptonic decays, thus requiring no assumptions on CPT invariance for the decay amplitudes. The result, Δm=(0.5343±0.0063stat±0.0025syst)×1010ℏ/s, provides a valuable input for CPT tests.

  • 4.
    Angelopoulos, Angelos
    et al.
    -.
    Locher, M P
    -.
    Markushin, V E
    -.
    Danielsson, Mats
    KTH, Tidigare Institutioner (före 2005), Fysik.
    Dispersion relation analysis of the neutral kaon regeneration amplitude in carbon1999Ingår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 10, nr 1, s. 19-25Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We apply a forward dispersion relation to the regeneration amplitude for kaon scattering on 12" style="position: relative;" tabindex="0" id="MathJax-Element-1-Frame" class="MathJax">12C using all available data. The CPLEAR data at low energies allow the determination of the net contribution from the subthreshold region which turns out to be much smaller than earlier evaluations, solving a long standing puzzle.

  • 5. Apostolakis, A
    et al.
    Aslanides, E.
    -.
    Backenstoss, G.
    -.
    Bargassa, P.
    -.
    Behnke, O.
    -.
    Benelli, A.
    -.
    Bertin, V.
    -.
    Blanc, F.
    -.
    Bloch, P.
    -.
    Carlson, P.
    KTH, Skolan för teknikvetenskap (SCI).
    Danielsson, Mats
    KTH, Tidigare Institutioner (före 2005), Fysik.
    A determination of the CP violation parameter η+- from the decay of strangeness-tagged neutral kaons1999Ingår i: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 458, nr 4, s. 545-552Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    LEAR offered unique opportunities to study the symmetries which exist between matter and antimatter. At variance with other approaches at this facility, CPLEAR was an experiment devoted to the study of CP, T and CPT symmetries in the neutral-kaon system. A variety of measurements allowed us to determine with high precision the parameters which describe the time evolution of the neutral kaons and their antiparticles, including decay amplitudes, and the related symmetry properties. Limits concerning quantum-mechanical predictions (EPR, coherence of the wave function) or the equivalence principle of general relativity have been obtained. An account of the main features of the experiment and its performances is given here, together with the results achieved.

  • 6.
    Apostolakis, A.
    et al.
    -.
    Aslanides, E.
    -.
    Backenstoss, G.
    -.
    Bargassa, P.
    -.
    Behnke, O.
    -.
    Benelli, A.
    -.
    Bertin, V.
    -.
    Blanc, F.
    -.
    Bloch, P.
    -.
    Carlson, P.
    Danielsson, Mats
    KTH, Tidigare Institutioner (före 2005), Fysik.
    Measurement of the energy dependence of the form factor f+ in K 0 e3 decay2000Ingår i: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 473, nr 1, s. 186-192Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Neutral-kaon decays to πeν were analysed to determine the q2 dependence of the K0e3 electroweak form factor f+. Based on 365612 events, this form factor was found to have a linear dependence on q2 with a slope λ+=0.0245±0.0012stat±0.0022syst.

  • 7. Aslund, Magnus
    et al.
    Fredenberg, Erik
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Telman, M.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Detectors for the future of X-ray imaging2010Ingår i: Radiation Protection Dosimetry, ISSN 0144-8420, E-ISSN 1742-3406, Vol. 139, nr 1-3, s. 327-333Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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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  • 8. Bakowski Holtryd, Mietek
    et al.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Xu, Cheng
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    X-ray sensor, method for constructing an x-ray sensor and an x-ray imaging system comprising such an x-ray sensor2020Patent (Övrig (populärvetenskap, debatt, mm))
    Abstract [en]

    An X-ray sensor (1) having an active detector region including a plurality of detector diodes (2) arranged on a surface region (3) of the X-ray sensor (1), a junction termination (4) surrounding the surface area (3) including the plurality of detector diodes (2), the junction termination (4) including a guard (5) arranged closest to the end of the surface region (3), a field stop (6) arranged outside the guard (2) and a number N of field limiting rings, FLRs (7) arranged between the guard (5) and the field stop (6), wherein each of the FLRs (7) are placed at positions selected so that distances between different FLRs (7) and between the guard and the first FLR lie within an effective area, the effective area being bounded by the lines α=(10+1.3×(n−1)) μm and β=(5+1.05×(n−1)) μm.

  • 9. Bakowski Holtryd, Mietek
    et al.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Xu, Cheng
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    X-ray sensor, x-ray detector system and x-ray imaging system2018Patent (Övrig (populärvetenskap, debatt, mm))
    Abstract [en]

    Disclosed is an x-ray sensor having an active detector region including a plurality of detector diodes at a first side of the sensor, and with placement of the junction termination at a second opposite side of the sensor. Normally, this implies that the junction termination is moved from the top side where the active detector area is located to the bottom side of the sensor, allowing for full utilization of the active detector area at the top side with detector diodes to the very edge of the sensor.  

  • 10. Bakowski Holtryd, Mietek
    et al.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Xu, Cheng
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    X-ray sensor, x-ray detector system and x-ray imaging system2019Patent (Övrig (populärvetenskap, debatt, mm))
    Abstract [en]

    There is provided an x-ray sensor (21) comprising an active detector region including a plurality of detector diodes (22) at a first side of the sensor, and a common junction termination (23) at a second opposite side of the sensor. Normally, this implies that the junction termination (23) is moved from the top side where the active detector area is located to the bottom side of the sensor, allowing for full utilization of the active detector area at the top side with detector diodes to the very edge of the sensor.  

  • 11.
    Behling, Rolf
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikelfysik, astrofysik och medicinsk bildbehandling.
    Hulme-Smith, Christopher
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Processer.
    Tolias, Panagiotis
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Elektroteknik, Rymd- och plasmafysik.
    Poludniowski, Gavin
    Medical Radiation Physics and Nuclear Medicine, Karolinska University Hospital, Framstegsgatan 21, Stockholm, 17176, Sweden, Framstegsgatan 21.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikelfysik, astrofysik och medicinsk bildbehandling.
    Microparticle Hybrid Target Simulation for keV X-ray Sources2024Ingår i: Instruments, E-ISSN 2410-390X, Vol. 8, nr 2, artikel-id 32Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The spatiotemporal resolution of diagnostic X-ray images obtained with rotating-anode X-ray tubes has remained limited as the development of rigid, high-performance target materials has slowed down. However, novel imaging techniques using finer detector pixels and orthovolt cancer therapy employing narrow X-ray focal spots demand improved output from brilliant keV X-ray sources. Since its advent in 1929, rotating-anode technology has become the greatest bottleneck to improvement. To overcome this limitation, the current authors have devised a novel X-ray generation technology based on tungsten microparticle targets. The current study investigated a hybrid solution of a stream of fast tungsten microparticles and a rotating anode to both harvest the benefits of the improved performance of the new solution and to reuse known technology. The rotating anode captures energy that may pass a partially opaque microparticle stream and thereby contributes to X-ray generation. With reference to fast-rotating anodes and a highly appreciated small focal spot of a standardized size of 0.3 for an 8° target angle (physical: 0.45 mm × 4.67 mm), we calculated a potential output gain of at least 85% for non-melting microparticles and of 124% if melting is envisioned. Microparticle charging can be remediated by electron backscattering and electron field emission. The adoption of such a solution enables substantially improved image resolution.

  • 12.
    Berggren, Karl
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik. Philips Healthcare, Sweden.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Fredenberg, Erik
    Philips Healthcare, Sweden.
    Rayleigh imaging in spectral mammography2016Ingår i: MEDICAL IMAGING 2016: PHYSICS OF MEDICAL IMAGING, 2016, artikel-id 97830AKonferensbidrag (Refereegranskat)
    Abstract [en]

    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.

  • 13.
    Berggren, Karl
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik. Philips Healthcare, S-17141 Solna, Sweden.
    Lundqvist, Mats
    Cederstrom, Bjorn
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Fredenberg, Erik
    Physical characterization of photon-counting tomosynthesis2015Konferensbidrag (Refereegranskat)
    Abstract [en]

    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.

  • 14.
    Bertilson, M.
    et al.
    Eclipse Optics, Vasagatan 52, Stockholm, Sweden, Vasagatan 52.
    von Hofsten, O.
    Eclipse Optics, Vasagatan 52, Stockholm, Sweden, Vasagatan 52.
    Maltz, J. S.
    GE HealthCare, Waukesha, WI, United States of America.
    Taphorn, K.
    Munich Institute of Biomedical Engineering, Technical University of Munich, D-85748, Garching, Germany; Chair of Biomedical Physics, Department of Physics, TUM School of Natural Sciences, Technical University of Munich, D-85748 Garching, Germany; Research Group Biomedical Imaging Physics, Department of Physics, TUM School of Natural Sciences, Technical University of Munich, D-85748 Garching, Germany.
    Herzen, J.
    Munich Institute of Biomedical Engineering, Technical University of Munich, D-85748, Garching, Germany; Chair of Biomedical Physics, Department of Physics, TUM School of Natural Sciences, Technical University of Munich, D-85748 Garching, Germany; Research Group Biomedical Imaging Physics, Department of Physics, TUM School of Natural Sciences, Technical University of Munich, D-85748 Garching, Germany.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Analyzer-free hard x-ray interferometry2024Ingår i: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 69, nr 4, artikel-id 045011Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Objective. To enable practical interferometry-based phase contrast CT using standard incoherent x-ray sources, we propose an imaging system where the analyzer grating is replaced by a high-resolution detector. Since there is no need to perform multiple exposures (with the analyzer grating at different positions) at each scan angle, this scheme is compatible with continuous-rotation CT apparatus, and has the potential to reduce patient radiation dose and patient motion artifacts. Approach. Grating-based x-ray interferometry is a well-studied technique for imaging soft tissues and highly scattering objects embedded in such tissues. In addition to the traditional x-ray absorption-based image, this technique allows reconstruction of the object phase and small-angle scattering information. When using conventional incoherent, polychromatic, hard x-ray tubes as sources, three gratings are usually employed. To sufficiently resolve the pattern generated in these interferometers with contemporary x-ray detectors, an analyzer grating is used, and consequently multiple images need to be acquired for each view angle. This adds complexity to the imaging system, slows image acquisition and thus increases sensitivity to patient motion, and is not dose efficient. By simulating image formation based on wave propagation, and proposing a novel phase retrieval algorithm based on a virtual grating, we assess the potential of a analyzer-grating-free system to overcome these limitations. Main results. We demonstrate that the removal of the analyzer-grating can produce equal image contrast-to-noise ratio at reduced dose (by a factor of 5), without prolonging scan duration. Significance. By demonstrating that an analyzer-free CT system, in conjuction with an efficient phase retrieval algorithm, can overcome the prohibitive dose and workflow penalties associated grating-stepping, an alternative path towards realizing clinical inteferometric CT appears possible.

  • 15.
    Bornefalk, Hans
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Photon-counting spectral computed tomography using silicon strip detectors: a feasibility study2010Ingår i: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 55, nr 7, s. 1999-2022Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We show how the spectral imaging framework should be modified to account for a high fraction of Compton interactions in low Z detector materials such as silicon. Using this framework, where deposited energies differ from actual photon energies, we compare the performance of a silicon strip detector, including the influence of scatter inside the detector and charge sharing but disregarding signal pileup, with an ideal energy integrating detector. We show that although the detection efficiency for silicon rapidly drops for the acceleration voltages encountered in clinical computed tomography practice, silicon detectors could perform on a par with ideal energy integrating detectors for routine imaging tasks. The use of spectrally sensitive detectors opens up the possibility for decomposition techniques such as k-edge imaging, and we show that the proposed modification of the spectral imaging framework is beneficial for such imaging tasks.

  • 16.
    Bornefalk, Hans
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Svensson, Christer
    Image quality in photon counting-mode detector systems2013Patent (Övrig (populärvetenskap, debatt, mm))
    Abstract [en]

    The current invention applies to photon counting silicon x-ray detectors with energy discriminating capabilities and applications in x-ray imaging systems. The overall image quality produced by such a system is improved by the presented novel methods for optimally using the energy information in Compton events and making selective use of counts induced from charges collected in neighboring pixels. The pile-up problem during high-flux imaging regimes is reduced by a novel method for signal reset, which improves the count efficiency by reducing the risk of losing event due to signal pile-up in the read out electronics chain.

  • 17.
    Bornefalk, Hans
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Grönberg, Fredrik
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Enhanced spectral x-ray imaging2021Patent (Övrig (populärvetenskap, debatt, mm))
    Abstract [en]

    An x-ray imaging apparatus includes an x-ray source and detector with multiple detector elements. The source and detector are on a support that rotates around a subject, enabling projections at different view angles. The apparatus operates the x-ray source in switched kVp mode for alternately applying different voltages, including lower and higher voltages, during rotation to enable lower-energy and higher-energy exposures over the projections, providing for lower-energy projections and higher-energy projections. The x-ray detector is a photon-counting multi-bin detector allocating photon counts to multiple energy bins, and the apparatus selects counts from at least a subset of the bins to provide corresponding photon count information for both lower- and higher-energy projections. The apparatus performs material basis decomposition for some of the lower-energy projections and higher-energy projections and/or for some combinations of at least one lower-energy projection and at least one higher-energy projection, based on the corresponding photon count information.

  • 18. Bornefalk, Hans
    et al.
    Lewin, John M.
    Danielsson, Mats
    Lundqvist, Mats
    Single-shot dual-energy subtraction mammography with electronic spectrum splitting: Feasibility2006Ingår i: European Journal of Radiology, ISSN 0720-048X, E-ISSN 1872-7727, Vol. 60, nr 2, s. 275-278Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present a single-shot dual-energy subtraction mammography technique using an energy sensitive photon counting detector. An electronic threshold near the middle of the X-ray spectrum discriminates between high- and low-energy photons, and allows the simultaneous acquisition of high- and low-energy images which can be combined to suppress anatomical clutter. By setting the electronic threshold close to 33.2 keV (the k-edge of iodine) the system is optimized for dual-energy contrast-enhanced imaging of breast tumors. This method eliminates the need for separate exposures which might otherwise lead to motion artifacts. The method is illustrated in phantom images.

  • 19.
    Bornefalk, Hans
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Persson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Allowable forward model misspecification for accurate basis decomposition in a silicon detector based spectral CT2015Ingår i: IEEE Transactions on Medical Imaging, ISSN 0278-0062, E-ISSN 1558-254X, Vol. 34, nr 3, s. 788-795Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Material basis decomposition in the sinogram domain requires accurate knowledge of the forward model in spectral computed tomography (CT). Misspecifications over a certain limit will result in biased estimates and make quantum limited (where statistical noise dominates) quantitative CT difficult. We present a method whereby users can determine the degree of allowed misspecification error in a spectral CT forward model and still have quantification errors that are limited by the inherent statistical uncertainty. For a particular silicon detector based spectral CT system, we conclude that threshold determination is the most critical factor and that the bin edges need to be known to within 0.15 keV in order to be able to perform quantum limited material basis decomposition. The method as such is general to all multibin systems.

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  • 20.
    Bornefalk, Hans
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Persson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Necessary forward model specification accuracy for basis material decomposition in spectral CT2014Ingår i: Medical Imaging 2014: Physics of Medical Imaging, SPIE - International Society for Optical Engineering, 2014, s. 90332I-Konferensbidrag (Refereegranskat)
    Abstract [en]

    Material basis decomposition in the sinogram domain requires accurate knowledge of the forward model in spectral CT. Misspecifications over a certain limit will result in biased estimates and make quantum limited quantitative CT difficult. We present a method whereby users can determine the degree of allowed misspecification error in a spectral CT forward model, and still have quantification errors that are quantum limited.

  • 21.
    Bornefalk, Hans
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Persson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Xu, Cheng
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Karlsson, Staffan
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Svensson, Christer
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Effect of Temperature Variation on the Energy Response of a Photon Counting Silicon CT Detector2013Ingår i: IEEE Transactions on Nuclear Science, ISSN 0018-9499, E-ISSN 1558-1578, Vol. 60, nr 2, s. 1442-1449Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The effect of temperature variation on pulse height determination accuracy is determined for a photon counting multibin silicon detector developed for spectral CT. Theoretical predictions of the temperature coefficient of the gain and offset are similar to values derived from synchrotron radiation measurements in a temperature controlled environment. By means of statistical modeling, we conclude that temperature changes affect all channels equally and with separate effects on gain and threshold offset. The combined effect of a 1 degrees C temperature increase is to decrease the detected energy by 0.1 keV for events depositing 30 keV. For the electronic noise, no statistically significant temperature effect was discernible in the data set, although theory predicts a weak dependence. The method is applicable to all x-ray detectors operating in pulse mode.

  • 22.
    Bornefalk, Hans
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Xu, Cheng
    Svensson, Christer
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Design considerations to overcome cross talk in a photon counting silicon strip detector for computed tomography2010Ingår i: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 621, nr 1-3, s. 371-378Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This article presents a Monte Carlo simulation of the detector energy response in the presence of pileup in a segmented silicon microstrip detector designed for high flux spectral computed tomography with sub-millimeter pixel size. Currents induced on the collection electrode of a pixel segment are explicitly modeled and signals emanating from events in neighboring pixels are superimposed together with electronic noise before the entire pulse train is processed by a model of the readout electronics to obtain the detector energy response function. The article shows how the lower threshold and the time constant of the electronic filters need to be set in order to minimize the detrimental influence of cross talk from neighboring pixel segments, an issue that is aggravated by the sub-millimeter pixel size and the proposed segmented detector design. (C) 2010 Elsevier B.V. All rights reserved.

  • 23.
    Bornefalk, Hans
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Xu, Cheng
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Svensson, Christer
    Division of Electronic Devices, Linköping University.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Simulation study of an energy sensitive photon counting silicon strip detector for computed tomography: identifying strengths and weaknesses and developing work-arounds2010Ingår i: MEDICAL IMAGING 2010: PHYSICS OF MEDICAL IMAGING / [ed] Samei E; Pelc NJ, 2010, Vol. 7622Konferensbidrag (Refereegranskat)
    Abstract [en]

    We model the effect of signal pile-up on the energy resolution of a photon counting silicon detector designed for high flux spectral CT with sub-millimeter pixel size. Various design parameters, such as bias voltage, lower threshold level for discarding of electronic noise and the entire electronic read out chain are modeled and realistic parameter settings are determined. We explicitly model the currents induced on the collection electrodes of a pixel and superimpose signals emanating from events in neighboring pixels, either due to charge sharing or signals induced during charge collection. Electronic noise is added to the pulse train before feeding it through a model of the read out electronics where the pulse height spectrum is saved to yield the detector energy response function. The main result of this study is that a lower threshold of 5 keV and a rather long time constant of the shaping filter (tau(0) = 30 ns) are needed to discard induced pulses from events in neighboring pixels. These induction currents occur even if no charge is being deposited in the analyzed pixel from the event in the neighboring pixel. There is also only a limited gain in energy resolution by increasing the bias voltage to 1000 V from 600 V. We show that with these settings the resulting energy resolution, as measured by the FWHM/E of the photo peak, is 5% at 70 keV.

  • 24. Brahme, A.
    et al.
    Danielsson, Mats
    KTH, Tidigare Institutioner (före 2005), Fysik.
    Iacobaeus, C.
    Ostling, J.
    Peskov, V.
    Wallmark, M.
    Evaluation of a GEM and CAT-based detector for radiation therapy beam monitoring2000Ingår i: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 454, nr 1, s. 136-141Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We are developing a radiation therapy beam monitor for the Karolinska Institute. This monitor will consist of two consecutive detectors confined in one gas chamber: a keV-photon detector, which will allow diagnostic quality visualization of the patient, and a MeV-photon detector, that will measure the absolute intensity of the therapy beam and its position with respect to the patient. Both detectors are based on highly radiation resistant gas and solid photon to electron converters, combined with GEMs and a CAT as amplification structures. We have performed systematic studies of the high-rate characteristics of the GEM and the CAT, as well as tested the electron transfer through these electron multipliers and various types of converters. The tests show that the GEM and the CAT satisfy all requirements for the beam monitoring system. As a result of these studies we successfully developed and tested a full section of the beam monitor equipped with a MeV-photon converter placed between the GEM and the CAT.

  • 25.
    Brunskog, Rickard
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik. MedTechLabs, BioClinicum, Karolinska University Hospital, Solna, Sweden, 17164.
    Persson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik. MedTechLabs, BioClinicum, Karolinska University Hospital, Solna, Sweden, 17164.
    Jin, Zihui
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik. MedTechLabs, BioClinicum, Karolinska University Hospital, Solna, Sweden, 17164.
    Experimental Evaluation of a Micron-Resolution CT Detector2024Ingår i: Medical Imaging 2024: Physics of Medical Imaging, SPIE-Intl Soc Optical Eng , 2024, Vol. 12925, artikel-id 129250BKonferensbidrag (Refereegranskat)
    Abstract [en]

    Purpose: Current photon-counting detectors are limited to a pixel size of 0.3 mm-1 mm, as decreasing the pixel size further generally introduces degraded dose efficiency and energy resolution from excessive charge sharing. In this work, we present experimental measurements of the first photon-counting detector prototype designed to leverage the charge sharing to estimate the photon interaction position, where simulations indicate a theoretical resolution of around 1 µm using a similar geometry. The goal of the measurements is to validate our Monte-Carlo simulation for further development. Approach: DAC sweeps are performed with an X-ray beam at specified locations on the sensor front, with the beam at 20 keV and 35 keV, as well as with different sensor biases with the beam at 35 keV. The experimental data are then compared to a Monte Carlo simulation combined with a charge transport model. In this first prototype wire bonds are used, and as such only a few channels are connected. Results: The experimental data agree generally well with the simulated data with the beam close to the electrodes, with the simulated data diverging from the experiments with the beam further away from the electrodes. The induced charge cloud signal exhibits a fairly linear dependency on the beam position, indicating that any estimation techniques will yield more precise position when the photon interacts further away from the electrodes, rather than closer. Conclusions: With the experimental data and the simulations agreeing generally well, together with the same software previously indicating a resolution of around 1 µm, we expect an ultra-high-resolution detector to be in reach, and are encouraged to continue development.

  • 26.
    Brunskog, Rickard
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik. Karolinska University Hospital, MedTechLabs, BioClinicum, Solna, Sweden.
    Persson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik. Karolinska University Hospital, MedTechLabs, BioClinicum, Solna, Sweden.
    Jin, Zihui
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik. Karolinska University Hospital, MedTechLabs, BioClinicum, Solna, Sweden.
    First experimental evaluation of a high-resolution deep silicon photon-counting sensor2024Ingår i: Journal of Medical Imaging, ISSN 2329-4302, E-ISSN 2329-4310, Vol. 11, nr 1, artikel-id 013503Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Purpose: Current photon-counting computed tomography detectors are limited to a pixel size of around 0.3 to 0.5 mm due to excessive charge sharing degrading the dose efficiency and energy resolution as the pixels become smaller. In this work, we present measurements of a prototype photon-counting detector that leverages the charge sharing to reach a theoretical sub-pixel resolution in the order of 1 μm. The goal of the study is to validate our Monte-Carlo simulation using measurements, enabling further development. Approach: We measure the channel response at the MAX IV Lab, in the DanMAX beamline, with a 35 keV photon beam, and compare the measurements with a 2D Monte Carlo simulation combined with a charge transport model. Only a few channels on the prototype are connected to keep the number of wire bonds low. Results: The measurements agree generally well with the simulations with the beam close to the electrodes but diverge as the beam is moved further away. The induced charge cloud signals also seem to increase linearly as the beam is moved away from the electrodes. Conclusions: The agreement between measurements and simulations indicates that the Monte-Carlo simulation can accurately model the channel response of the detector with the photon interactions close to the electrodes, which indicates that the unconnected electrodes introduce unwanted effects that need to be further explored. With the same Monte-Carlo simulation previously indicating a resolution of around 1 μm with similar geometry, the results are promising that an ultra-high resolution detector is not far in the future.

  • 27.
    Bujila, Robert
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik. Karolinska Univ Hosp, Med Radiat Phys & Nucl Med, Stockholm, Sweden.
    Kull, Love
    Sunderby Hosp, Dept Radiat Phys, Luleå, Sweden..
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Andersson, Jonas
    Umeå Univ, Dept Radiat Sci, Umeå, Sweden..
    Applying three different methods of measuring CTDIfree air to the extended CTDI formalism for wide-beam scanners (IEC 60601-2-44): A comparative study2018Ingår i: Journal of Applied Clinical Medical Physics, E-ISSN 1526-9914, Vol. 19, nr 4, s. 281-289Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Purpose: The weighted CT dose index (CTDIw) has been extended for a nominal total collimation width (nT) greater than 40 mm and relies on measurements of CTDfree air. The purpose of this work was to compare three methods of measuring CTDIfree air and subsequent calculations of CTDIw to investigate their clinical appropriateness. Methods: The CTDIfree air, for multiple nTs up to 160 mm, was calculated from (1) high-resolution air kerma profiles from a step-and-shoot translation of a liquid ionization chamber (LIC) (considered to be a dosimetric reference), (2) pencil ionization chamber (PIC) measurements at multiple contiguous positions, and (3) air kerma profiles obtained through the continuous translation of a solid-state detector. The resulting CTDIfree air was used to calculate the CTDIw, per the extended formalism, and compared. Results: The LIC indicated that a 40 mm nT should not be excluded from the extension of the CTDIw formalism. The solid-state detector differed by as much as 8% compared to the LIC. The PIC was the most straightforward method and gave equivalent results to the LIC. Conclusions: The CTDIw calculated with the latest CTDI formalism will differ most for 160 mm nTs (e.g., whole-organ perfusion or coronary CT angiography) compared to the previous CTDI formalism. Inaccuracies in the measurement of CTDIfree air will subsequently manifest themselves as erroneous calculations of the CTDIw, for nTs greater than 40 mm, with the latest CTDI formalism. The PIC was found to be the most clinically feasible method and was validated against the LIC.

  • 28.
    Cahn, R N
    et al.
    -.
    Cederström, B.
    -.
    Danielsson, Mats
    KTH, Tidigare Institutioner (före 2005), Fysik.
    Hall, A.
    -.
    Lundqvist, M.
    -.
    Nygren, D.
    -.
    Detective quantum efficiency dependence on x-ray energy weighting in mammography1999Ingår i: Medical physics, Vol. 26, nr 12, s. 2680-2683Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    An evaluation of the dependence of detective quantum efficiency (DQE) on the incident energy spectrum has been made for mammography. The DQE dependence on the energy spectrum has been evaluated for energy-integrating detectors, photon-counting detectors, and detectors that measure the energy of each photon. To isolate the effect of the x-ray energy spectrum the detector has been assumed to be ideal, i.e., all noise sources are assumed to be zero except for quantum fluctuations. The result shows that the improvement in DQE, if the energy-integrating detector is compared to a single-photon counting detector, is of the order of 10%. Comparing the energy-integrating detector and the detector measuring the energy for each photon the improvement is around 30% using a molybdenumanodespectrum typical in mammography. It is shown that the optimal weight factors to combine the data in the case the energy is measured are very well approximated if the weight factors are proportional to E−3." style="position: relative;" tabindex="0" id="MathJax-Element-1-Frame" class="MathJax">E−3. Another conclusion is that in calculating the DQE, a detector should be compared to one that uses ideal energy weighting for each photon since this provides the best signal-to-noise ratio. This has generally been neglected in the literature.

  • 29. Cederström, B.
    et al.
    Fredenberg, E.
    Berggren, Karl
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik. Mammography Solutions, Philips, Sweden.
    Erhard, K.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Wallis, M.
    Lesion characterization in spectral photon-counting tomosynthesis2017Ingår i: Medical Imaging 2017: Physics of Medical Imaging, SPIE - International Society for Optical Engineering, 2017, Vol. 10132, artikel-id 1013205Konferensbidrag (Refereegranskat)
    Abstract [en]

    It has previously been shown that 2D spectral mammography can be used to discriminate between (likely benign) cystic and (potentially malignant) solid lesions in order to reduce unnecessary recalls in mammography. One limitation of the technique is, however, that the composition of overlapping tissue needs to be interpolated from a region surrounding the lesion. The purpose of this investigation was to demonstrate that lesion characterization can be done with spectral tomosynthesis, and to investigate whether the 3D information available in tomosynthesis can reduce the uncertainty from the interpolation of surrounding tissue. A phantom experiment was designed to simulate a cyst and a tumor, where the tumor was overlaid with a structure that made it mimic a cyst. In 2D, the two targets appeared similar in composition, whereas spectral tomosynthesis revealed the exact compositional difference. However, the loss of discrimination signal due to spread from the plane of interest was of the same strength as the reduction of anatomical noise. Results from a preliminary investigation on clinical tomosynthesis images of solid lesions yielded results that were consistent with the phantom experiments, but were still to some extent inconclusive. We conclude that lesion characterization is feasible in spectral tomosynthesis, but more data, as well as refinement of the calibration and discrimination algorithms, are needed to draw final conclusions about the benefit compared to 2D.

  • 30.
    Cederström, Björn
    et al.
    KTH, Tidigare Institutioner (före 2005), Fysik.
    Cahn, R. N.
    Danielsson, Mats
    KTH, Tidigare Institutioner (före 2005), Fysik.
    Lundqvist, M.
    Nygren, D. R.
    Focusing hard X-rays with old LPs2000Ingår i: Nature, ISSN 0028-0836, E-ISSN 1476-4687, Vol. 404, nr 6781, s. 951-951Artikel i tidskrift (Refereegranskat)
  • 31.
    Chen, Han
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Cederström, Björn
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Xu, Cheng
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Persson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Karlsson, Staffan
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    A photon-counting silicon-strip detector for digital mammography with an ultrafast 0.18-mu m CMOS ASIC2014Ingår i: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 749, s. 1-6Artikel i tidskrift (Refereegranskat)
    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.

  • 32.
    Chen, Han
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Cederström, Björn
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    On imaging with or without grid in digital mammography2014Ingår i: Proceedings of SPIE, the International Society for Optical Engineering, ISSN 0277-786X, E-ISSN 1996-756X, Vol. 9033, s. 903346-Artikel i tidskrift (Refereegranskat)
    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.

  • 33.
    Chen, Han
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Xu, Cheng
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Size-dependent scanning parameters (kVp and mAs) for photon-counting spectral CT system in pediatric imaging: simulation study2016Ingår i: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 61, nr 11Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We are developing a photon-counting spectral CT detector with small pixel size of 0.40.5 mm2, o ering a potentialadvantage for better visualization of small structures in pediatric patients. The purpose of this study is to determinethe patient size dependent scanning parameters (kVp and mAs) for pediatric CT in two imaging cases: adipose imagingand iodinated blood imaging.Cylindrical soft-tissue phantoms of diameters between 10-25 cm were used to mimic patients of di erent ages from 0-15 y. For adipose imaging, a 5-mm-diameter adipose sphere was assumed as an imaging target, while an iodinated bloodsphere of 1 mm in diameter was assumed in the case of iodinated imaging. By applying the geometry of a commercial CTscanner (GE LightSpeed VCT), simulations were carried out to calculate the detectability index,d02, with tube potentialsvarying from 40 to 140 kVp. The optimal kVp for each phantom in each imaging case was determined such that the dose-normalized detectability index,d02=dose, is maximized. With the assumption that image quality in pediatric imagingis required the same as in typical adult imaging, the value of mAs at optimal kVp for each phantom was selected toachieve a reference detectability index that was obtained by scanning an adult phantom (30 cm in diameter) in a typicaladult CT procedure (120 kVp and 200 mAs) using a modeled energy-integrating system.For adipose imaging, the optimal kVps are 50, 60, 80, and 120 kVp, respectively, for phantoms of 10, 15, 20, and25-cm in diameter. The corresponding mAs values required to achieve the reference detectability index are only 9%,23%, 24%, and 54% of the mAs that is used for adult patients at 120 kVp, for 10, 15, 20, and 25-cm-diameter phantoms,respectively. In the case of iodinated imaging, a tube potential of 60 kVp was found optimal for all phantoms investigated,and the mAs values required to achieve the reference detectability index are 2%, 9%, 37%, and 109% of the adult mAs.The results also indicate that with the use of respective optimal kVps, the photon-counting spectral system o ers up to30% higherd02=dose than the modeled energy-integrating system for adipose imaging, and 70% for iodinated imaging.

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  • 34.
    Chen, Han
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Xu, Cheng
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Cederström, Björn
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    On image quality metrics and the usefulness of grids in digital mammography2015Ingår i: Journal of medical imaging (Bellingham, Wash.), ISSN 2329-4302, Vol. 2, nr 1, s. 013501-013501Artikel i tidskrift (Refereegranskat)
    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.

  • 35.
    Chen, Han
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Xu, Cheng
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Persson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Optimization Of Beam Quality For Photon-Counting Spectral Computed Tomography In Head Imaging: Simulation Study2015Ingår i: Journal of Medical Imaging, ISSN 2329-4302, E-ISSN 2329-4310, Vol. 2, nr 4, s. 043504-1-043504-16, artikel-id 043504Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Head computed tomography (CT) plays an important role in the comprehensive evaluation of acutestroke. Photon-counting spectral detectors, as promising candidates for use in the next generation of x-ray CTsystems, allow for assigning more weight to low-energy x-rays that generally contain more contrast information.Most importantly, the spectral information can be utilized to decompose the original set of energy-selectiveimages into several basis function images that are inherently free of beam-hardening artifacts, a potential ad-vantage for further improving the diagnosis accuracy. We are developing a photon-counting spectral detector forCT applications. The purpose of this work is to determine the optimal beam quality for material decomposition intwo head imaging cases: nonenhanced imaging and K-edge imaging. A cylindrical brain tissue of 16-cm diam-eter, coated by a 6-mm-thick bone layer and 2-mm-thick skin layer, was used as a head phantom. The imagingtarget was a 5-mm-thick blood vessel centered in the head phantom. In K-edge imaging, two contrast agents,iodine and gadolinium, with the same concentration (5mg∕mL) were studied. Three parameters that affect beamquality were evaluated: kVp settings (50 to 130 kVp), filter materials (Z¼13to 83), and filter thicknesses [0 to 2half-value layer (HVL)]. The image qualities resulting from the varying x-ray beams were compared in terms oftwo figures of merit (FOMs): squared signal-difference-to-noise ratio normalized by brain dose (SDNR2∕BD) andthat normalized by skin dose (SDNR2∕SD). For nonenhanced imaging, the results show that the use of the 120-kVp spectrum filtered by 2 HVL copper (Z¼29) provides the best performance in both FOMs. When iodine isused in K-edge imaging, the optimal filter is 2 HVL iodine (Z¼53) and the optimal kVps are 60 kVp in terms ofSDNR2∕BD and 75 kVp in terms of SDNR2∕SD. A tradeoff of 65 kVp was proposed to lower the potential riskof skin injuries if a relatively long exposure time is necessarily performed in the iodinated imaging. In the case ofgadolinium imaging, both SD and BD can be minimized at 120 kVp filtered with 2 HVL thulium (Z¼69). Theresults also indicate that with the same concentration and their respective optimal spectrum, the values ofSDNR2∕BD and SDNR2∕SD in gadolinium imaging are, respectively, around 3 and 10 times larger thanthose in iodine imaging. However, since gadolinium is used in much lower concentrations than iodine in theclinic, iodine may be a preferable candidate for K-edge imaging.

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  • 36.
    da Silva, Joakim
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Grönberg, Fredrik
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik. Prismatic Sensors AB, Stockholm, Sweden.
    Cederström, Björn
    Persson, Mats
    Sjölin, Martin
    Alagic, Zlatan
    Bujila, Robert
    KTH, Skolan för teknikvetenskap (SCI), Fysik. Karolinska University Hospital, Medical Radiation Physics and Nuclear Medicine, Stockholm, Sweden.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik. Prismatic Sensors AB, Stockholm, Sweden.
    Resolution characterization of a silicon-based, photon-counting computed tomography prototype capable of patient scanning2019Ingår i: Journal of Medical Imaging, ISSN 2329-4302, E-ISSN 2329-4310, Vol. 6, nr 4, artikel-id 043502Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Photon-counting detectors are expected to bring a range of improvements to patient imaging with x-ray computed tomography (CT). One is higher spatial resolution. We demonstrate the resolution obtained using a commercial CT scanner where the original energy-integrating detector has been replaced by a single-slice, silicon-based, photon-counting detector. This prototype constitutes the first full-field-of-view silicon-based CT scanner capable of patient scanning. First, the pixel response function and focal spot profile are measured and, combining the two, the system modulation transfer function is calculated. Second, the prototype is used to scan a resolution phantom and a skull phantom. The resolution images are compared to images from a state-of-the-art CT scanner. The comparison shows that for the prototype 19 lp∕cm are detectable with the same clarity as 14 lp∕cm on the reference scanner at equal dose and reconstruction grid, with more line pairs visible with increasing dose and decreasing image pixel size. The high spatial resolution remains evident in the anatomy of the skull phantom and is comparable to that of other photon-counting CT prototypes present in the literature. We conclude that the deep silicon-based detector used in our study could provide improved spatial resolution in patient imaging without increasing the x-ray dose.

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  • 37.
    Dahlman, Nils
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Fredenberg, Erik
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Aslund, Magnus
    Lundqvist, Mats
    Diekmann, Felix
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Evaluation of photon-counting spectral breast tomosynthesis2011Ingår i: Medical Imaging 2011: Physics of Medical Imaging / [ed] Pelc, NJ; Samei, E; Nishikawa, RM, 2011, Vol. 7961, artikel-id 796114Konferensbidrag (Refereegranskat)
    Abstract [en]

    We have designed a mammography system that for the first time combines photon-counting spectral imaging with tomosynthesis. The present study is a comprehensive physical evaluation of the system; tomosynthesis, spectral imaging, and the combination of both are compared using an ideal-observer model that takes anatomical noise into account. Predictions of signal and noise transfer through the system are verified by contrast measurements on a tissue phantom and 3D measurements of MTF and NPS. Clinical images acquired with the system are discussed in view of the model predictions.

  • 38.
    Daniel, Maxime
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik. MedTechLabs, BioClinicum, Karolinska University Hospital, Solna, Sweden.
    Yanoff, Brian D.
    GE Research Center, Niskayuna, NY 12309, USA.
    Persson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik. MedTechLabs, BioClinicum, Karolinska University Hospital, Solna, Sweden.
    Charge collection efficiency of CdTe detectors: Impact of charge collection time and polarisation2023Ingår i: Medical Imaging 2023: Physics of Medical Imaging, SPIE-Intl Soc Optical Eng , 2023, artikel-id 124630HKonferensbidrag (Refereegranskat)
    Abstract [en]

    Cadmium telluride (CdTe) is one of the materials used in photon-counting detectors for x-ray computed tomography. One challenge with this material is that it is susceptible to polarisation due to holes being trapped in impurities in the material. This can potentially lead to the buildup of bulk charge in the semiconductor, causing decreased charge collection efficiency and degraded energy resolution. In this work, we develop a simulation model of CdTe detectors with polarisation and use it to study the effect of polarisation on the measured energy spectrum for different charge collection times. To this end, we use a theoretical model of charge buildup to find the critical charge in the detector's bulk above which the detector can be considered completely polarised. We then simulate a 320-by-270-by-1600 μm CdTe detector used in CT clinical imaging, for varying degrees of polarisation (ratio between the actual charge and the critical charge) and charge collection time. Our results show that the measured spectrum gets heavily distorted for large degrees of polarisation or for short charge collection time. We also put these results in context by discussing how they relate to the critical fluence rate and the time of flight of the charge carriers. These results can lead to improved simulation models of CdTe detectors and a better understanding the factors affecting their imaging performance.

  • 39.
    Danielsson, Mats
    KTH, Tidigare Institutioner (före 2005), Fysik.
    A collimator arrangement for varying the exposed surface of an x-ray detector in an x-ray imaging system includes an arrangement of two relatively displaceable parts having overlapping slots producing an adjustable slot width2000Patent (Övrig (populärvetenskap, debatt, mm))
    Abstract [en]

    In an x-ray imaging system, a variable exposure of a semiconductor, gas or sensitive film x-ray detector (43) to x-rays from a source is provided by a collimator (42). The collimator is arranged as two parts (42a, 42b) that are displaceable relative to each other, each having several identical slots in different planes that overlap. Relative displacement by a motor/actuator produces a variable width slot (45) that is adjustable between fully closed and open positions.

  • 40.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Collimator element2001Patent (Övrig (populärvetenskap, debatt, mm))
    Abstract [en]

    A beam collimator arrangement for scanned-slot x-ray imaging having one or several collimators in an x-ray apparatus is disclosed. The beam collimator arrangement includes an x-ray source; an x-ray image receiver positioned to receive x-rays from the x-ray source; a compressor or means for compressing a female breast to be examined where the compressor is positionable between the x-ray source and the x-ray image receiver; and the beam collimator is positioned between the x-ray source and the compressor. The beam collimator arrangement is arranged on a carrying structure to displace the beam collimator arrangement between a first position when no x-ray exposure is conducted and a second position before x-ray exposure is initiated.

  • 41.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Detecting entire signal from any photon converting in detector in X-ray detecting arrangement for X-ray imaging in radiography2004Patent (Övrig (populärvetenskap, debatt, mm))
    Abstract [sv]

    Föreliggande uppfinning avser en metod och anordning för spåming av väsentligen den totala signalen från en foton som omvandlas i en detektor i en röntgenspåmings-anordning (200, 300, 400), vilket resulterar i laddningsdelning, nämnda anordning innefattande ett antal sensorer (220, 320,420) anordnade på ett avstånd emellan, varvid nämnda spårade fotoner indirekt skapar en mängd av fria laddningar proportionell mot fotonenergin. Metoden innefattar stegen att anordna ett blockeringsmedel (230, 330, 430) mellan nämnda sensorer anordnade på ett avstånd emellan för att stoppa röntgenstrålar som kommer att infalla nära kanterna hos nämnda sensorer och kommer att ge upphov till laddningsdelning mellan näraliggande sensorer. Anordningen innefattar nämnda blockeringsmedel.

  • 42.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Device and method related to X-ray imaging2007Patent (Övrig (populärvetenskap, debatt, mm))
    Abstract [en]

    The present invention relates to an arrangement for detecting X-ray radiations comprising a carrying member on one face arranged with detectors consisting of a plurality of sensors arranged on a substrate. The detectors are arranged substantially edge to edge at lease in one row on at least one side of said carrying member.  

  • 43.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Device and method relating to x-ray imaging2013Patent (Övrig (populärvetenskap, debatt, mm))
  • 44.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Enabling estimation of an initial point of interaction of an x-ray photon in a photon-counting x-ray detector2021Patent (Övrig (populärvetenskap, debatt, mm))
    Abstract [en]

    The disclosed method enables estimating an initial point of interaction of an x-ray photon in a photon-counting x-ray detector, based on a number of x-ray detector sub-modules or wafers, each including detector elements. The x-ray detector sub-modules are oriented in edge-on geometry with the edge directed towards the x-ray source, assuming the x-rays enter through the edge. Each detector sub-module or wafer has a thickness with two opposite sides of different potentials to enable charge drift towards the side, where the detector elements, also referred to as pixels, are arranged. Basically, the method includes: determining an estimate of charge diffusion originating from a Compton interaction or an interaction through photoeffect related to the x-ray photon in a detector sub-module or wafer of the x-ray detector; and estimating the initial point of interaction along the thickness of the detector sub-module based on the determined estimate of charge diffusion.

  • 45.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    MAMMOGRAPHY SHIFTS TOWARD SPECTRAL IMAGING-Photon counting is an intuitive way to detect x-rays, which by nature are digital and have a color spectrum2009Ingår i: Diagnostic Imaging, Vol. 25, nr 7Artikel i tidskrift (Refereegranskat)
  • 46.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Method and apparatus for detecting X-rays2005Patent (Övrig (populärvetenskap, debatt, mm))
    Abstract [en]

    The present invention relates to a method of detecting X-rays for obtaining improved radiographic images including a step of orienting a semiconductor radiation detector whose height is greater than its thickness. The orienting step comprises a selection of an acute angle between a direction of incident radiation and a side of said detector having said height such that said incident radiation mainly hit the side of said radiation detector. The hit area excludes at least a section between at least one edge of said detector and at least an active sensor area and that substantially all of the energy of the radiation is dissipated within the detector.

  • 47.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Method and apparatus for simplified alignment in x-ray imaging1999Patent (Övrig (populärvetenskap, debatt, mm))
    Abstract [en]

    The invention relates to a method and an apparatus for simplified alignment in scanned slot x-ray imaging. The apparatus has a first collimator (102) and a second collimator (104) arranged in a first distance (a) and a second distance (b), respectively, from a radiation source (100). Each collimator is provided with a slot (102a, 104a) and a detector (106) is located under the second collimator slot, said slot of said second collimator being wider than the said slot of said first collimator and said detector under the second slot is wider than the first collimator slot and the second collimator slot. The slot (104a) of said second collimator has a width (y') not less than a safety margin and the product of the width (x) of the slot (102a) of said first collimator (102) and said second distance (b) divided with the said first distance (a) for allowing a misalignment with respect to a central symmetry line (105) of said slots (102a, 104a).

  • 48.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    Method and arrangement relating to x-ray detection2003Patent (Övrig (populärvetenskap, debatt, mm))
    Abstract [en]

    A method an arrangement in an x-ray imaging apparatus (10), includes at least one x-ray source (11), a collimator (42, 52, 62a, 62b, 72) and a detcetor (43, 73); the arrangement is arranged for providing a variable exposure of the dector (43, 73) to x-ray radiation from the x-ray source (10) through slots (45, 55, 65a, 65b, 75) on the collimator (42, 52, 62a, 62b, 72). The arrangement comprises at least one of the collimator (42, 52, 62a, 62b, 72) or registering means (43, 73), which are arranged moveable relative each other to vary number of x-rays registered by the dector.

  • 49.
    Danielsson, Mats
    KTH, Tidigare Institutioner (före 2005), Fysik.
    Metod och anordning att genom etsning anordna ett blockerande medel för att förhindra laddningsdelning mellan näraliggande röntgensensorer: [Method and arrangement relating to x-ray imaging]2000Patent (Övrig (populärvetenskap, debatt, mm))
    Abstract [en]

    A detector (200) comprises a number of spaced-apart sensor strips (220) arranged on a substrate, while a blocking arrangement is in the form of strips (230) covering the spaces between the sensor strips. The blocking strips also cover parts of the sensor strips and are made of a material such as wolfram, lead etc. which can be arranged as strips directly on the sensors. AN Independent claim is included for an arrangement for detecting signals from converted photons.

  • 50.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk bildfysik.
    MO‐D‐210A‐01: Photon Counting Detectors for Mammography2009Ingår i: Medical physics (Lancaster), ISSN 0094-2405, Vol. 36, nr 6, s. 2699-2699Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Mammography is currently one of the most common x‐ray imaging examinations. More than 100 million women worldwide are screened every year and early detection of breast cancer through mammography has proven to be a key to significantly reduced mortality. The requirement on spatial resolution as well as contrast resolution is very high in order to detect and diagnose the cancer. Moreover, because of the large number of women going through this procedure and the fact that more than 99 % are healthy, it also becomes very important to minimize the radiation dose. Photon counting may be one way to meet the demands and mammography is the first modality in x‐ray imaging to implement photon counting detectors. FDA approval is still pending but they are currently in routine clinical use in more than 15 countries. The photon counting enables a discrimination of all electronic noise and a more optimum use of the information in each x‐ray. The absence of electronic noise is particularly important in low dose applications, in for example tomosynthesis a number of exposures from different angles are required and since the dose in each projection is just a fraction of the total dose for a mammogram the sensitivity to electronic noise will increase. Using the spectral information for each x‐ray it is in principle possible to deduce the elemental composition of an object in the breast. This could for example be used to enhance microcalcifications relative to soft tissue and differentiate water from fat in cysts. Recently contrast mammography has attracted significant attention. In this application Iodine is used as a contrast media to visualize the vascular structure. As in breast MRI the cancer stand out because of the leaky vessels resulting from its angiogenesis. A photon counting detector gives a unique opportunity to image the Iodine through spectral imaging by adjusting one of the thresholds to its K‐edge. Challenges for photon counting in mammography are high rates of x‐rays, both to generate the required flux at the source and to handle the rates at the detector without pile‐up. Even more difficult to handle are the charge sharing between detector pixels which, if not corrected for, will compromise the energy information. The current status of photon counting detectors in mammography will be described together with strategies to overcome the pit‐falls. Also future possibilities with spectral imaging in mammography will be investigated and examples from ongoing clinical trials will be given. Learning Objectives: 1. Status of photon counting detectors in mammography 2. Pit‐falls and opportunities with photon counting detectors for mammography 3. Future applications based on spectral detectors for mammography.

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