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  • 51.
    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.

  • 52.
    Fredenberg, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Stayman, J. Webster
    Siewerdsen, Jeffrey H.
    Åslund, Magnus
    Ideal-observer detectability in photon-counting differential phase-contrast imaging using a linear-systems approach2012In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 39, no 9, p. 5317-5335Article in journal (Refereed)
    Abstract [en]

    Purpose: To provide a cascaded-systems framework based on the noise-power spectrum (NPS), modulation transfer function (MTF), and noise-equivalent number of quanta (NEQ) for quantitative evaluation of differential phase-contrast imaging (Talbot interferometry) in relation to conventional absorption contrast under equal-dose, equal-geometry, and, to some extent, equal-photon-economy constraints. The focus is a geometry for photon-counting mammography. Methods: Phase-contrast imaging is a promising technology that may emerge as an alternative or adjunct to conventional absorption contrast. In particular, phase contrast may increase the signal-difference-to-noise ratio compared to absorption contrast because the difference in phase shift between soft-tissue structures is often substantially larger than the absorption difference. We have developed a comprehensive cascaded-systems framework to investigate Talbot interferometry, which is a technique for differential phase-contrast imaging. Analytical expressions for the MTF and NPS were derived to calculate the NEQ and a task-specific ideal-observer detectability index under assumptions of linearity and shift invariance. Talbot interferometry was compared to absorption contrast at equal dose, and using either a plane wave or a spherical wave in a conceivable mammography geometry. The impact of source size and spectrum bandwidth was included in the framework, and the trade-off with photon economy was investigated in some detail. Wave-propagation simulations were used to verify the analytical expressions and to generate example images. Results: Talbot interferometry inherently detects the differential of the phase, which led to a maximum in NEQ at high spatial frequencies, whereas the absorption-contrast NEQ decreased monotonically with frequency. Further, phase contrast detects differences in density rather than atomic number, and the optimal imaging energy was found to be a factor of 1.7 higher than for absorption contrast. Talbot interferometry with a plane wave increased detectability for 0.1-mm tumor and glandular structures by a factor of 3-4 at equal dose, whereas absorption contrast was the preferred method for structures larger than similar to 0.5 mm. Microcalcifications are small, but differ from soft tissue in atomic number more than density, which is favored by absorption contrast, and Talbot interferometry was barely beneficial at all within the resolution limit of the system. Further. Talbot interferometry favored detection of "sharp" as opposed to "smooth" structures, and discrimination tasks by about 50% compared to detection tasks. The technique was relatively insensitive to spectrum bandwidth, whereas the projected source size was more important. If equal photon economy was added as a restriction, phase-contrast efficiency was reduced so that the benefit for detection tasks almost vanished compared to absorption contrast, but discrimination tasks were still improved close to a factor of 2 at the resolution limit. Conclusions: Cascaded-systems analysis enables comprehensive and intuitive evaluation of phase-contrast efficiency in relation to absorption contrast under requirements of equal dose, equal geometry, and equal photon economy. The benefit of Talbot interferometry was highly dependent on task, in particular detection versus discrimination tasks, and target size, shape, and material. Requiring equal photon economy weakened the benefit of Talbot interferometry in mammography.

  • 53. Fredenberg, Erik
    et al.
    Erhard, Klaus
    Berggren, Karl
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging. Philips Healthcare, S-17141 Solna, Sweden.
    Dance, David R.
    Young, Kenneth C.
    Cederstrom, Bjorn
    Johansson, Henrik
    Lundqvist, Mats
    Moa, Elin
    Homan, Hanno
    Willsher, Paula
    Kilburn-Toppin, Fleur
    Wallis, Matthew
    X-ray attenuation of adipose breast tissue: In-vitro and in-vivo measurements using spectral imaging2015In: MEDICAL IMAGING 2015: PHYSICS OF MEDICAL IMAGING, 2015, Vol. 9412, article id 94121UConference paper (Refereed)
    Abstract [en]

    The development of new x-ray imaging techniques often requires prior knowledge of tissue attenuation, but the sources of such information are sparse. We have measured the attenuation of adipose breast tissue using spectral imaging, in vitro and in vivo. For the in-vitro measurement, fixed samples of adipose breast tissue were imaged on a spectral mammography system, and the energy-dependent x-ray attenuation was measured in terms of equivalent thicknesses of aluminum and poly-methyl methacrylate (PMMA). For the in-vivo measurement, a similar procedure was applied on a number of spectral screening mammograms. The results of the two measurements agreed well and were consistent with published attenuation data and with measurements on tissue-equivalent material.

  • 54.
    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, p. 2017-2029Article 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.

  • 55. Fredenberg, Erik
    et al.
    Kilburn-Toppin, Fleur
    Willsher, Paula
    Moa, Elin
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Dance, David R.
    Young, Kenneth C.
    Wallis, Matthew G.
    Measurement of breast-tissue x-ray attenuation by spectral mammography: solid lesions2016In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 61, no 7, p. 2595-2612Article in journal (Refereed)
    Abstract [en]

    Knowledge of x-ray attenuation is essential for developing and evaluating x-ray imaging technologies. For instance, techniques to distinguish between cysts and solid tumours at mammography screening would be highly desirable to reduce recalls, but the development requires knowledge of the x-ray attenuation for cysts and tumours. We have previously measured the attenuation of cyst fluid using photon-counting spectral mammography. Data on x-ray attenuation for solid breast lesions are available in the literature, but cover a relatively wide range, likely caused by natural spread between samples, random measurement errors, and different experimental conditions. In this study, we have adapted a previously developed spectral method to measure the linear attenuation of solid breast lesions. A total of 56 malignant and 5 benign lesions were included in the study. The samples were placed in a holder that allowed for thickness measurement. Spectral (energy-resolved) images of the samples were acquired and the image signal was mapped to equivalent thicknesses of two known reference materials, which can be used to derive the x-ray attenuation as a function of energy. The spread in equivalent material thicknesses was relatively large between samples, which is likely to be caused mainly by natural variation and only to a minor extent by random measurement errors and sample inhomogeneity. No significant difference in attenuation was found between benign and malignant solid lesions. The separation between cyst-fluid and tumour attenuation was, however, significant, which suggests it may be possible to distinguish cystic from solid breast lesions, and the results lay the groundwork for a clinical trial. In addition, the study adds a relatively large sample set to the published data and may contribute to a reduction in the overall uncertainty in the literature.

  • 56.
    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, E-ISSN 1872-9576, p. 156-162Article 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.

  • 57.
    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, p. 72581-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.

  • 58.
    Fredenberg, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Roessl, E.
    Koehler, T.
    van Stevendaal, U.
    Schulze-Wenck, I.
    Wieberneit, N.
    Stampanoni, M.
    Wang, Z.
    Kubik-Huch, R. A.
    Hauser, N.
    Lundqvist, M.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Aslund, M.
    Photon-Counting Spectral Phase-Contrast Mammography2012In: Medical Imaging 2012: Physics Of Medical Imaging / [ed] Pelc, NJ; Nishikawa, RM; Whiting, BR, SPIE - International Society for Optical Engineering, 2012, p. 83130F-Conference paper (Refereed)
    Abstract [en]

    Phase-contrast imaging is an emerging technology that may increase the signal-difference-to-noise ratio in medical imaging. One of the most promising phase-contrast techniques is Talbot interferometry, which, combined with energy-sensitive photon-counting detectors, enables spectral differential phase-contrast mammography. We have evaluated a realistic system based on this technique by cascaded-systems analysis and with a task-dependent ideal-observer detectability index as a figure-of-merit. Beam-propagation simulations were used for validation and illustration of the analytical framework. Differential phase contrast improved detectability compared to absorption contrast, in particular for fine tumor structures. This result was supported by images of human mastectomy samples that were acquired with a conventional detector. The optimal incident energy was higher in differential phase contrast than in absorption contrast when disregarding the setup design energy. Further, optimal weighting of the transmitted spectrum was found to have a weaker energy dependence than for absorption contrast. Taking the design energy into account yielded a superimposed maximum on both detectability as a function of incident energy, and on optimal weighting. Spectral material decomposition was not facilitated by phase contrast, but phase information may be used instead of spectral information.

  • 59.
    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.

  • 60.
    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.

  • 61.
    Grönberg, Fredrik
    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.
    Sjölin, Martin
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Count statistics of nonparalyzable photon-counting detectors with nonzero pulse length2018In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 45, no 8, p. 3800-3811Article in journal (Refereed)
    Abstract [en]

    PurposePhoton-counting detectors are expected to be the next big step in the development of medical computed tomography (CT). Accurate modeling of the behavior of photon-counting detectors in both low and high count rate regimes is important for accurate image reconstruction and detector performance evaluations. The commonly used ideal nonparalyzable (delta pulse) model is built on crude assumptions that make it unsuitable for predicting the behavior of photon-counting detectors at high count rates. The aim of this work is to present an analytical count statistics model that better describes the behavior of photon-counting detectors with nonzero pulse length. MethodsAn analytical statistical count distribution model for nonparalyzable detectors with nonzero pulse length is derived using tools from statistical analysis. To validate the model, a nonparalyzable photon-counting detector is simulated using Monte Carlo methods and compared against. Image performance metrics are computed using the Fisher information metric and a comparison between the proposed model, approximations of the proposed model, and those made by the ideal nonparalyzable model is presented and analyzed. ResultsIt is shown that the presented model agrees well with the results from the Monte Carlo simulation and is stable for varying x-ray beam qualities. It is also shown that a simple Gaussian approximation of the distribution can be used to accurately model the behavior and performance of nonparalyzable detectors with nonzero pulse length. Furthermore, the comparison of performance metrics show that the proposed model predicts a very different behavior than the ideal nonparalyzable detector model, suggesting that the proposed model can fill an important gap in the understanding of pileup effects. ConclusionsAn analytical model for the count statistics of a nonparalyzable photon-counting detector with nonzero pulse length is presented. The model agrees well with results obtained from Monte Carlo simulations and can be used to improve, speed up and simplify modeling of photon-counting detectors.

  • 62.
    Grönberg, Fredrik
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Persson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Bornefalk, Hans
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Image reconstruction based on energy-resolved image data from a photon-counting multi-bin detector2015Patent (Other (popular science, discussion, etc.))
    Abstract [en]

    There is provided a method of image reconstruction based on energy-resolved image data from a photon-counting multi-bin detector or an intermediate storage. The method comprises processing (S1) the energy-resolved image data by performing at least two separate basis decompositions using different number of basis functions for modeling linear attenuation, wherein a first basis decomposition is performed using a first smaller set of basis functions to obtain at least one first basis image representation, and wherein a second basis decomposition is performed using a second larger set of basis functions to obtain at least one second basis image representation. The method also comprises reconstructing a first image based on said at least one first basis image representation obtained from the first basis decomposition, and combining the first image with information representative of said at least one second basis image representation.

  • 63.
    Grönberg, Fredrik
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Persson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Bornefalk, Hans
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Third material separation in spectral CT with basis decomposition2015Conference paper (Other academic)
  • 64.
    Grönberg, Fredrik
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Sjölin, Martin
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Count statistics and pileup correction for nonparalyzable photon counting detectors with finite pulse length2018In: Medical Imaging 2018: Physics Of Medical Imaging / [ed] Lo, JY Schmidt, TG Chen, GH, SPIE - International Society for Optical Engineering, 2018, article id UNSP 105730ZConference paper (Refereed)
    Abstract [en]

    Photon counting detectors are expected to be the next big step in the development of medical computed tomography. Accurate modeling of the behavior of photon counting detectors in the high count rate regime is therefore important for detector performance evaluations and the development of accurate image reconstruction methods. The commonly used ideal nonparalyzable detector model is based on the assumption that photon interactions are converted to pulses with zero extent in time, which is too simplistic to accurately predict the behavior of photon counting detectors in both low and high count rate regimes. In this work we develop a statistical count model for a nonparalyzable detector with finite pulse length and use it to derive the asymptotic mean and variance of the output count distribution using tools from renewal theory. We use the statistical moments of the distribution to construct an estimator of the true number of counts for pileup correction. We con firm the accuracy of the model and evaluate the pileup correction using Monte Carlo simulations. The results show that image quality is preserved for surprisingly high count rates.

  • 65. Gustavsson, Mikael
    et al.
    Ul Amin, Farooq
    Björklid, Anders
    Ehliar, Andreas
    Xu, Cheng
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Svensson, Christer
    A High-Rate Energy-Resolving Photon-Counting ASIC for Spectral Computed Tomography2012In: IEEE Transactions on Nuclear Science, ISSN 0018-9499, E-ISSN 1558-1578, Vol. 59, no 1, p. 30-39Article in journal (Refereed)
    Abstract [en]

    We describe a high-rate energy-resolving photon-counting ASIC aimed for spectral computed tomography. The chip has 160 channels and 8 energy bins per channel. It demonstrates a noise level of ENC= electrons at 5 pF input load at a power consumption of <5mW/channel. Maximum count rate is 17 Mcps at a peak time of 40 ns, made possible through a new filter reset scheme, and maximum read-out frame rate is 37 kframe/s.

  • 66. Hjärn, Torbjörn
    et al.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Hemmendorff, Magnus
    Method and arrangement relating to x-ray imaging2005Patent (Other (popular science, discussion, etc.))
    Abstract [en]

    novel X-ray apparatus is provided for three-dimensional imaging and in particular for tomosynthesis examination, which includes an X-ray source having a focal spot, a collimator including a plurality of slits, a detector assembly including a plurality of line detectors corresponding to respective ones of the plurality of slits and an exposure volume arranged between the collimator and the detector assembly. The X-ray source, the collimator and the detector assembly are arranged in series, so that each line detector is aligned with the corresponding collimator slit and the focal spot, and is simultaneously displaceable by a scan motion relative to the exposure volume. The scan motion is primarily a rotation around a rotation axis arranged such that the detector assembly is situated essentially between the rotation axis and the X-ray source.; Combined two and three-dimensional examination are also permitted according to the disclosed methods and apparatus.

  • 67.
    Hsieh, Scott S.
    et al.
    Univ Calif Los Angeles, Dept Radiol Sci, Los Angeles, CA 90024 USA..
    Sjölin, Martin
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Digital count summing vs analog charge summing for photon counting detectors: A performance simulation study2018In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 45, no 9, p. 4085-4093Article in journal (Refereed)
    Abstract [en]

    PurposeCharge sharing is a significant problem for CdTe-based photon counting detectors (PCDs) and can cause high-energy photons to be misclassified as one or more low-energy events. Charge sharing is especially problematic in PCDs for CT because the high flux necessitates small pixels, which increase the magnitude of charge sharing. Analog charge summing (ACS) is a powerful solution to reduce spectral distortion arising from charge sharing but may be difficult to implement. We investigate correction of the signal after digitization by the comparator (digital count summing), which is only able to correct a subset of charge sharing events but may have implementation advantages. We compare and quantify the relative performance of digital and ACS in simulations. MethodsTransport of photons in CdTe was modeled using Monte Carlo simulations. Energy deposited in the CdTe substrate was converted to electrical charges of a predetermined shape, and all charges within the detector pixel are assumed to be perfectly collected. In ACS, the maximum charge received over any 2x2 block of pixels was grouped together prior to digitization. In digital count summing (DCS), the charge was digitized in each pixel, and subsequently, adjacent pixels that detected events grouped their charge to record a single, higher energy event. All simulations were performed at the limit of low flux (no pileup). The default tube voltage was 120kVp, object thickness was 20cm of water, pixel pitch was 250m, and charge cloud modeled as a Gaussian with sigma=40m. Variation of these parameters was examined in a sensitivity analysis. ResultsDetectors that used no correction, DCS, and ACS misclassified 51%, 39%, and 15% of incident photons, respectively. For iodine basis material imaging, DCS exhibited 100% greater dose efficiency compared to uncorrected, and ACS exhibited an additional 111% greater dose efficiency compared to digital charge summing. For a nonspectral task, the dose efficiency improvement as estimated by improvement of zero-frequency detective quantum efficiency, DQE(0) was 10% for DCS compared to uncorrected and 10% for ACS compared to DCS. A sensitivity analysis showed that DCS generally achieved half the benefit of ACS over a range of conditions, although the benefit was markedly less if the charge cloud was instead modeled as a small sphere. ConclusionsSumming of counts after digitization may be a simpler alternative to summing of charge prior to digitization due to the relative complexity of analog circuit design. Over most conditions studied, it provides roughly half the benefit of ACS and may offer certain implementation advantages.

  • 68.
    Kördel, Mikael
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    A Physical X-Ray Scintillator Detector Model for CBCT Imaging Applications2014Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Download full text (pdf)
    fulltext
  • 69.
    Liu, Xuejin
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Bornefalk, Hans
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Chen, Han
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Danielsson, 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.
    Persson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Xu, Cheng
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Huber, Ben
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    A Silicon-Strip Detector for Photon-Counting Spectral CT: Energy Resolution From 40 keV to 120 keV2014In: IEEE Transactions on Nuclear Science, ISSN 0018-9499, E-ISSN 1558-1578, Vol. 61, no 3, p. 1099-1105Article in journal (Refereed)
    Abstract [en]

    We are developing a segmented silicon-strip detector for spectral computed tomography. The detector operates in photon-counting mode and allows pulse-height discrimination with 8 adjustable energy bins. In this work, we determine the energy resolution of a detector module using monoenergetic x-rays from 40 keV to 120 keV, provided at the European Synchrotron Radiation Facility, Grenoble. For each incident x-ray energy, pulse height spectra at different input photon fluxes are obtained. We investigate changes of the energy resolution due to charge sharing between pixels and pulse pileup. The different incident energies are used to channel-wise calibrate the pulse-height response in terms of signal gain and offset and to probe the homogeneity of the detector module. The detector shows a linear pulse-height response in the energy range from 40 keV to 120 keV. The gain variation among the channels is below 4%, whereas the variation of the offsets is on the order of 1 keV. We find an absolute energy resolution (sigma(E)) that degrades from 1.5 keV to 1.9 keV with increasing x-ray energy from 40 keV to 100 keV. With increasing input count rate, sigma(E) degrades by approximately 4 . 10(-3) keV Mcps(-1) mm(2), which is, within error bars, the same for the different energies. The effect of charge sharing on the width of the response peak is found to be negligible.

  • 70.
    Liu, Xuejin
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Bornefalk, Hans
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Chen, Han
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Danielsson, 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.
    Persson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Xu, Cheng
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Huber, Ben
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Characterization of a silicon strip detector for photon-counting spectral CT using monoenergetic photons from 40 keV to 120 keV2014In: Medical Imaging 2014: Physics of Medical Imaging, SPIE - International Society for Optical Engineering, 2014, p. 90333O-Conference paper (Refereed)
    Abstract [en]

    Background: We are developing a segmented silicon strip detector that operates in photon-counting mode and allows pulse-height discrimination with 8 adjustable energy bins. In this work, we determine the energy resolution of the detector using monoenergetic x-ray radiation from 40 keV to 120 keV. We further investigate the effects of pulse pileup and charge sharing between detector channels that may lead to a decreased energy resolution. Methods: For each incident monochromatic x-ray energy, we obtain count spectra at different photon fluxes. These spectra corresponds to the pulse-height response of the detector and allow the determination of energy resolution and charge-sharing probability. The energy resolution, however, is influenced by signal pileup and charge sharing. Both effects are quantified using Monte Carlo simulations of the detector that aim to reproduce the conditions during the measurements. Results: The absolute energy resolution is found to increase from 1.7 to 2.1 keV for increasing energies 40 keV to 120 keV at the lowest measured photon flux. The effect of charge sharing is found to increase the absolute energy resolution by a factor of 1.025 at maximum. This increase is considered as negligibly small. The pileup of pulses leads to a deterioration rate of the energy resolution of 4 · 10-3 keV Mcps-1 mm2, corresponding to an increase of 0.04keV per 10 Mcps increase of the detected count rate.

  • 71.
    Liu, Xuejin
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Chen, Han
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Bornefalk, Hans
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Danielsson, 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.
    Persson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Xu, Cheng
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Huber, Ben
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Energy Calibration of a Silicon-Strip Detector for Photon-Counting Spectral CT by Direct Usage of the X-ray Tube Spectrum2015In: IEEE Transactions on Nuclear Science, ISSN 0018-9499, E-ISSN 1558-1578, Vol. 62, no 1, p. 68-75Article in journal (Refereed)
    Abstract [en]

    The variation among energy thresholds in a multibin detector for photon-counting spectral CT can lead to ring artefacts in the reconstructed images. Calibration of the energy thresholds can be used to achieve homogeneous threshold settings or to develop compensation methods to reduce the artefacts. We have developed an energy-calibrationmethod for the different comparator thresholds employed in a photon-counting silicon-strip detector. In our case, this corresponds to specifying the linear relation between the threshold positions in units of mV and the actual deposited photon energies in units of keV. This relation is determined by gain and offset values that differ for different detector channels due to variations in the manufacturing process. Typically, the calibration is accomplished by correlating the peak positions of obtained pulse-height spectra to known photon energies, e. g. with the aid of mono-energetic x rays from synchrotron radiation, radioactive isotopes or fluorescence materials. Instead of mono-energetic x rays, the calibrationmethod presented in this papermakes use of a broad x-ray spectrum provided by commercial x-ray tubes. Gain and offset as the calibration parameters are obtained by a regression analysis that adjusts a simulated spectrum of deposited energies to ameasured pulse-height spectrum. Besides the basic photon interactions such as Rayleigh scattering, Compton scattering and photo-electric absorption, the simulation takes into account the effect of pulse pileup, charge sharing and the electronic noise of the detector channels. We verify the method for different detector channels with the aid of a table-top setup, where we find the uncertainty of the keV-value of a calibrated threshold to be between 0.1 and 0.2 keV.

  • 72.
    Liu, Xuejin
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Chen, Han
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Bornefalk, Hans
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Danielsson, 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.
    Persson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Xu, Cheng
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Huber, Ben
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Modelling the channel-wise count response of a photon-counting spectral CT detector to a broad x-ray spectrum2015In: Medical Imaging 2015: Physics of Medical Imaging, 2015, Vol. 9412, article id 941215Conference paper (Refereed)
    Abstract [en]

    Variations among detector channels in CT very sensitively lead to ring artefacts in the reconstructed images. For material decomposition in the projection domain, the variations can result in intolerable biases in the material line integral estimates. A typical way to overcome these effects is to apply calibration methods that try to unify spectral responses from different detector channels to an ideal response from a detector model. However, the calibration procedure can be rather complex and require excessive calibration measurements for a multitude of combinations of x-ray shapes, tissue combinations and thicknesses. In this paper, we propose a channel-wise model for a multibin photon-counting detector for spectral CT. Predictions of this channel-wise model match well with their physical performances, which can thus be used to eliminate ring artefacts in CT images and achieve projection-basis material decomposition. In an experimental validation, image data show significant improvement with respect to ring artefacts compared to images calibrated with flat-fielding data. Projection-based material decomposition gives basis material images showing good separation among individual materials and good quantification of iodine and gadolinium contrast agents. The work indicates that the channel-wise model can be used for quantitative CT with this detector.

  • 73.
    Liu, Xuejin
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Persson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Bornefalk, Hans
    KTH, School of Engineering Sciences (SCI), Physics.
    Karlsson, Staffan
    KTH, School of Engineering Sciences (SCI), Physics.
    Xu, Cheng
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Huber, Ben
    KTH, School of Engineering Sciences (SCI), Physics.
    Spectral response model for a multibin photon-counting spectral computed tomography detector and its applications2015In: Journal of Medical Imaging, ISSN 2329-4302, E-ISSN 2329-4310, Vol. 2, no 3, article id 033502Article in journal (Refereed)
    Abstract [en]

    Variations among detector channels in computed tomography can lead to ring artifacts in the reconstructed images and biased estimates in projection-based material decomposition. Typically, the ring artifacts are corrected by compensation methods based on flat fielding, where transmission measurements are required for a number of material-thickness combinations. Phantoms used in these methods can be rather complex and require an extensive number of transmission measurements. Moreover, material decomposition needs knowledge of the individual response of each detector channel to account for the detector inhomogeneities. For this purpose, we have developed a spectral response model that binwise predicts the response of a multibin photon-counting detector individually for each detector channel. The spectral response model is performed in two steps. The first step employs a forward model to predict the expected numbers of photon counts, taking into account parameters such as the incident x-ray spectrum, absorption efficiency, and energy response of the detector. The second step utilizes a limited number of transmission measurements with a set of flat slabs of two absorber materials to fine-tune the model predictions, resulting in a good correspondence with the physical measurements. To verify the response model, we apply the model in two cases. First, the model is used in combination with a compensation method which requires an extensive number of transmission measurements to determine the necessary parameters. Our spectral response model successfully replaces these measurements by simulations, saving a significant amount of measurement time. Second, the spectral response model is used as the basis of the maximum likelihood approach for projection-based material decomposition. The reconstructed basis images show a good separation between the calcium-like material and the contrast agents, iodine and gadolinium. The contrast agent concentrations are reconstructed with more than 94% accuracy.

  • 74.
    Liu, Xuejin
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Persson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Bornefalk, Hans
    KTH, School of Engineering Sciences (SCI), Physics.
    Karlsson, Staffan
    KTH, School of Engineering Sciences (SCI), Physics.
    Xu, Cheng
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Huber, Ben
    KTH, School of Engineering Sciences (SCI), Physics.
    Spectral response model for a multibin photon-counting spectral computed tomography detector and its applications (vol 2, 033502, 2015)2016In: Journal of Medical Imaging, ISSN 2329-4302, E-ISSN 2329-4310, Vol. 3, no 4, article id 049801Article in journal (Refereed)
  • 75.
    Merzan, Debprah
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Optimization of Automatic ExposureControl of a Photon CountingMammography System2014Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
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  • 76.
    Mi, Wujun
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    A Stacked Prism Lens Concept for Next-Generation Hard X-Ray Telescopes2019Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Over the past half century, the focusing X-ray telescope has played a very prominent role in X-ray astronomy at the frontier of fundamental physics. The finer angular resolution and increased effective area have enabled more and more exciting discoveries and detailed studies of the high-energy universe, including the cosmic X-ray background (CXB) radiation, black holes in active galactic nuclei (AGN), galaxy clusters, supernova remnants, and so on. At present, nearly all the state-of-the-art focusing X-ray telescopes are based on Wolter-I optics or its variations, for which the throughput is severely restricted by the mirror’s surface roughness, figure error, alignment error, and so on.

    Within the course of this work, we have developed a novel point-focusing refractive lens, the stacked prism lens (SPL), which is built by stacking disks embedded with various number of prismatic rings. As a Fresnel-like X-ray lens, it could provide a significantly higher efficiency and larger effective aperture than the conventional compound refractive lenses (CRLs). The aim of this thesis is to demonstrate the feasibility of the stacked prism lens and investigate the application to a next-generation hard X-ray telescope.

    First, SU-8 prototype lenses are fabricated by focused ultraviolet (UV) lithography, for which a UV lens is used as a photomask to form 3D patterns in the photoresist. The UV lens is homemade by grayscale electron beam lithography (EBL), and a proximity effect correction (PEC) method based on multivariate adaptive regression splines (MARS) ensures accurate control of the desired UV lens profile. The details of the whole fabrication process are described, and the fabrication results are discussed. Following that, the completed stacked prism lenses are characterized in the synchrotron radiation facility, and the results show the expected performance.

    Finally, a hard X-ray focusing telescope concept based on the proposed stacked prism lens array is presented. The performance, in terms of angular resolution, effective collecting area, field of view (FOV), mass and so on, is investigated by self-developed simulation software based on ray-tracing method and compared with the current Wolter telescopes. The results suggest that the proposed stacked prism lens is a promising candidate for next-generation hard X-ray telescope with high angular resolution and large effective collecting area.

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  • 77.
    Mi, Wujun
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Karlsson, Staffan
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Holmberg, Anders
    KTH, School of Engineering Sciences (SCI), Applied Physics, Biomedical and X-ray Physics.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Nillius, Peter
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Fabrication of circular sawtooth gratings using focused UV lithography2016In: Journal of Micromechanics and Microengineering, ISSN 0960-1317, E-ISSN 1361-6439, Vol. 26, no 3, article id 035001Article in journal (Refereed)
    Abstract [en]

    AbstractThis paper presents a novel micro-fabrication method using focused ultraviolet (UV) light to manufacture three-dimensional sawtooth structures in ultra-thick negative photoresist to fabricate a novel multi-prism x-ray lens. The method uses a lens to shape the UV beam instead of the photomask conventionally used in UV lithography. Benefits of this method include the ability to manufacture sawtooth structures in free form, for example in circular shapes as well as arrays of these shapes, and in resist that is up to 76 μm thick.To verify the method, initially a simple simulation based on Fourier optics was done to predict the exposure energy distribution in the photoresist. Furthermore, circular sawtooth gratings were manufactured in a 76 μm SU-8 resist. The UV lens was fabricated using electron beam lithography and then used to expose the SU-8 with UV light. This paper details the complete developed process, including pre-exposure with an e-beam and cold development, which creates stable sawtooth structures. The measured profile was compared to the ideal sawtooth and the simulation. The main discrepancy was in the smallest feature size, the sawtooth tips, which were wider than the desired structures, as would be expected by simulation.

  • 78.
    Mi, Wujun
    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.
    Efficient proximity effect correction method based on multivariate adaptive regression splines for grayscale e-beam lithography2014In: Journal of Vacuum Science & Technology B, ISSN 1071-1023, E-ISSN 1520-8567, Vol. 32, no 3, article id 031602Article in journal (Refereed)
    Abstract [en]

    Grayscale electron beam lithography is an important technique to manufacture three-dimensional (3D) micro- and nano-structures, such as diffractive optical devices and Fresnel lenses. However, the proximity effect due to the scattering of electrons may cause significant error to the desired 3D structure. Conventional proximity correction methods depend on the exposure energy distribution which sometimes is difficult to obtain. In this study, the authors develop a novel proximity effect correction method based on multivariate adaptive regression splines, which takes exposure energy and development into consideration simultaneously. To evaluate the method, a Fresnel lens was fabricated through simulation and experiment. The measurements demonstrate the feasibility and validity of the method.

  • 79.
    Mi, Wujun
    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.
    Pearce, Mark
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. The Oskar Klein Centre for Cosmoparticle Physics, AlbaNova University Centre, Stockholm, Sweden.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    A stacked prism lens concept for next-generation hard X-ray telescopes2019In: Nature Astronomy, E-ISSN 2397-3366Article in journal (Refereed)
    Abstract [en]

    Effective collecting area, angular resolution, field of view and energy response are fundamental attributes of X-ray telescopes. The performance of state-of-the-art telescopes is currently restricted by Wolter optics, especially for hard X-rays. Here we report the development of a stacked prism lens (SPL), which is lightweight and modular and has the potential for a significant improvement in effective area, while retaining high angular resolution. The proposed optics are built by stacking disks embedded with prismatic rings, created with photoresist by focused ultraviolet lithography. We demonstrate the SPL approach using a prototype lens that was manufactured and characterized at a synchrotron radiation facility. The design of a potential satellite-borne X-ray telescope is outlined and the performance is compared with contemporary missions.

  • 80.
    Nillius, Peter
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Geometric scattering in prism-array lenses for hard x-rays: Measurements, simulations and models2012In: X-Ray Optics And Microanalysis, American Institute of Physics (AIP), 2012, Vol. 1437, p. 111-115Conference paper (Refereed)
    Abstract [en]

    This work investigates the properties of off-axis focusing of prism-array lenses. Raytracing simulations are in agreement with measurements on a planar silicon prism-array lens at 13.4 keV. The simulations show that refractions and reflections on the paraxial side of the prisms cause scattering. This geometric scattering is the main impacting factor when focusing sources that are off the optical axis. For low-attenuating materials it also limits the effective aperture on axis. A new analytical model that is able to predict the amount of scattering is presented. Such a model is for example useful when optimising optical systems, but also to understand the limits and possibilities prism-array lenses.

  • 81.
    Nillius, Peter
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Theoretical Bounds and Optimal Configurations for Multi-Pinhole SPECT2009In: 2008 IEEE NUCLEAR SCIENCE SYMPOSIUM AND MEDICAL IMAGING CONFERENCE (2008 NSS/MIC), 2009, p. 5020-5022Conference paper (Refereed)
    Abstract [en]

    The pinhole geometry in SPECT has an inherent trade-off between resolution and sensitivity. High resolution requires a small aperture which on the other hand directly reduces the rate of detected photons. Recent systems overcome this to some extent by using multiple pinholes spread out around the imaging object, effectively increasing the sensitivity with a factor equal to the number of pinholes. The images of each pinhole must fit on the detector without overlap. This creates another trade-off between resolution, sensitivity and the field-of-view (FOV) of the system. The present work analytically analyzes the properties of the multi-pinhole SPECT geometry. Optimal configurations are identified and characterized. One of the main results is that there exists a theoretical upper bound for the sensitivity given the resolution and the FOV. This upper bound is proportional to the square of the resolution and inversely proportional to the square FOV diameter. This means that if we want to improve the resolution by a factor of ten, the sensitivity will go down a factor 100, unless we decrease the FOV an equal amount. The bound can not be broken even if the detector sphere is infinitely large. One important parameter when designing a system is how close it will be to its theoretical bound. The closer to the bound the more extreme the system will be, in terms of size and number of pinholes. A moderate distance away from the bound the proportions of the system are more realistic.

  • 82.
    Nillius, Peter
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Theoretical Bounds and System Design for Multipinhole SPECT2010In: IEEE Transactions on Medical Imaging, ISSN 0278-0062, E-ISSN 1558-254X, Vol. 29, no 7, p. 1390-1400Article in journal (Refereed)
    Abstract [en]

    The pinhole camera in single photon emission computed tomography (SPECT) has an inherent trade-off between resolution and sensitivity. Recent systems overcome this to some extent by utilizing multiple pinholes distributed around the imaging object. The present work is a theoretical study on how to optimally construct such systems. We use an analytic model to analyze the multipinhole SPECT geometry and identify the underlying trade-offs. One of the results is the derivation of the upper bound for the sensitivity, given the geometric resolution and field-of-view (FOV). Reaching this bound requires an infinitely large detector. However, a sensitivity very close to the upper bound can be achieved by a system with realistic proportions. We show that it is usually possible to get a sensitivity that is 95%-99% of the upper bound. Further analysis reveals a trade-off between sensitivity, magnification, and the number of pinholes. Based on this new theory, we develop a strategy for multipinhole SPECT design, from which a number of example systems are computed. Penetration in the pinhole knife edge is accounted for by using the resolution and sensitivity equivalent apertures.

  • 83.
    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, p. 555-557Article 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

  • 84.
    Nillius, Peter
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Klamra, Wlodek
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Sibczynski, Pawel
    Sharma, Diksha
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Badano, Aldo
    Light output measurements and computational models of microcolumnar CsI scintillators for x-ray imaging2015In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 42, no 2, p. 600-605Article in journal (Refereed)
    Abstract [en]

    Purpose: The authors report on measurements of light output and spatial resolution of microcolumnar CsI:Tl scintillator detectors for x-ray imaging. In addition, the authors discuss the results of simulations aimed at analyzing the results of synchrotron and sealed-source exposures with respect to the contributions of light transport to the total light output. Methods: The authors measured light output from a 490-mu m CsI:Tl scintillator screen using two setups. First, the authors used a photomultiplier tube (PMT) to measure the response of the scintillator to sealed-source exposures. Second, the authors performed imaging experiments with a 27-keV monoenergetic synchrotron beam and a slit to calculate the total signal generated in terms of optical photons per keV. The results of both methods are compared to simulations obtained with hybrid MANTIS, a coupled x-ray, electron, and optical photon Monte Carlo transport package. The authors report line response (LR) and light output for a range of linear absorption coefficients and describe a model that fits at the same time the light output and the blur measurements. Comparing the experimental results with the simulations, the authors obtained an estimate of the absorption coefficient for the model that provides good agreement with the experimentally measured LR. Finally, the authors report light output simulation results and their dependence on scintillator thickness and reflectivity of the backing surface. Results: The slit images from the synchrotron were analyzed to obtain a total light output of 48 keV(-1) while measurements using the fast PMT instrument setup and sealed-sources reported a light output of 28 keV-1. The authors attribute the difference in light output estimates between the two methods to the difference in time constants between the camera and PMT measurements. Simulation structures were designed to match the light output measured with the camera while providing good agreement with the measured LR resulting in a bulk absorption coefficient of 5x10(-5) mu m(-1). Conclusions: The combination of experimental measurements for microcolumnar CsI:Tl scintillators using sealed-sources and synchrotron exposures with results obtained via simulation suggests that the time course of the emission might play a role in experimental estimates. The procedure yielded an experimentally derived linear absorption coefficient for microcolumnar Cs:Tl of 5x10(-5) mu m(-1). To the author's knowledge, this is the first time this parameter has been validated against experimental observations. The measurements also offer insight into the relative role of optical transport on the effective optical yield of the scintillator with microcolumnar structure.

  • 85.
    Nillius, Peter
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Sullivan, Josephine
    KTH, School of Computer Science and Communication (CSC), Computer Vision and Active Perception, CVAP.
    Argyros, Antonis
    Shading models for illumination and reflectance invariant shape detectors2008In: 2008 IEEE Conference On Computer Vision And Pattern Recognition: Vols 1-12, 2008, p. 3353-3360Conference paper (Refereed)
    Abstract [en]

    Many objects have smooth surfaces of a fairly uniform color, thereby exhibiting shading patterns that reveal information about its shape, an important clue to the nature of the object. This papers explores extracting this information from images, by creating shape detectors based on shading. Recent work has derived low-dimensional models of shading that can handle realistic unknown lighting conditions and surface reflectance properties. We extend this theory by also incorporating variations in the surface shape. In doing so it enables the creation of very general models for the 2D appearance of objects, not only coping with variations in illumination and BRDF but also in shape alterations such as small scale and pose changes. Using this framework we propose a scheme to build shading models that can be used for shape detection in a bottom up fashion without any a priori knowledge about the scene. From the developed theory we construct detectors for two basic shape primitives, spheres and cylinders. Their performance is evaluated by extensive synthetic experiments as well as experiments on real images.

  • 86. 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, p. 83130I-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.

  • 87.
    Norrlid, Lilian del Risco
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Fredenberg, Erik
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Hemmendorff, Magnus
    Jackowski, Christian
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Imagine of small children prototype for photon counting tomosynthesis2009In: Medical Imaging 2009: Physics of Medical Imaging, 2009, Vol. 7258, p. 72581-9Conference paper (Refereed)
    Abstract [en]

    We present data on a first prototype for photon counting tomosynthesis imaging of small children, which we call photoncounting tomosynthesis (PCT). A photon counting detector can completely eliminate electronic noise, which makes it ideal for tomosynthesis because of the low dose in each projection. Another advantage is that the detector allows for energy sensitivity in later versions, which will further lower the radiation dose. In-plane resolution is high and has been measured to be 5lp/mm, at least 4 times better than in CT, while the depth resolution was significantly lower than typical CT resolution. The image SNR decreased from 30 to 10 for a detail of 10 mm depth in increasing thickness of PMMA from 10 to 80 mm. The air kerma measured for PCT was 5.2 mGy, which leads to an organ dose to the brain of approximately 0.7 mGy. This dose is 96 % lower than a typical CT dose. PCT can be appealing for pediatric imaging since young children have an increased sensitivity to radiation induced cancers. We have acquired post mortem images of a newborn with the new device and with a state-of-the-art CT and compared the diagnostic information and dose levels of the two modalities. The results are promising but more work is needed to provide input to a next generation prototype that would be suitable for clinical trials.

  • 88.
    Ostling, Janina
    et al.
    -.
    Wallmark, M.
    -.
    Brahme, Anders
    -.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Iacobaeus, Christian
    -.
    Fonte, P.
    -.
    Peskov, Vladimir N
    -.
    Novel detector for portal imaging in radiation therapy2000In: Medical Imaging 2000, 2000, p. 84-95Conference paper (Refereed)
  • 89.
    Persson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Reconstruction of spectral CT images2011Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Abstract

    X-ray computed tomography (CT) is a medical imaging modality that allows reconstruction of

    the internal stucture of the human body from a large number of x-ray attenuation measurements.

    In recent years, the development of spectral CT, in which the energy dependence of the x-ray

    attenuation coe-cient is utilized, has attracted considerable interest. This thesis is concerned with

    a spectral CT system based on a photon-counting silicon strip detector which is being developed

    in our group.

    A computer model for photon counting spectral CT was developed and used for simulating

    the proposed CT system as well as a laboratory CT setup designed for testing purposes. The

    simulations were used to compare the performance of two reconstruction methods for spectral CT,

    image-based energy weighting and basis material decomposition. The study shows that the two

    methods perform equally well when the number of x-ray photons in each measurement is high, while

    basis material decomposition performs signcantly worse than energy weighting for data with few

    photons in each measurement.

    Also included in the thesis is a simulation study of the detrimental eects of electronic noise

    and thresold variations on image quality. It is shown that a proposed electronic noise reduction of

    20% in the readout channels gives an improvement in mean

     

    SDNR2

    over many channels of only

    1.8%. In addition, a scheme for countering electronic noise contamination of the lowest energy bins

    due to threshold variations is discussed.

    The nal chapter of the thesis describes an experiment which demonstrates that the CT detector

    design studied here can indeed be used to obtain high-quality images.

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  • 90.
    Persson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Spectral Computed Tomography with a Photon-Counting Silicon-Strip Detector2016Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Computed tomography (CT) is a widely used medical imaging modality. By rotating an x-ray tube and an x-ray detector around the patient, a CT scanner is able to measure the x-ray transmission from all directions and form an image of the patient’s interior. CT scanners in clinical use today all use energy-integrating detectors, which measure the total incident energy for each measurement interval. A photon-counting detector, on the other hand, counts the number of incoming photons and can in addition measure the energy of each photon by comparing it to a number of energy thresholds. Using photon- counting detectors in computed tomography could lead to improved signal-to-noise ratio, higher spatial resolution and improved spectral imaging which allows better visualization of contrast agents and more reliable quantitative measurements. In this Thesis, the feasibility of using a photon-counting silicon-strip detector for CT is investigated. In the first part of the Thesis, the necessary performance requirements on such a detector is investigated in two different areas: the detector element homogeneity and the capability of handling high photon fluence rates. A metric of inhomogeneity is proposed and used in a simulation study to evaluate different inhomogeneity compensation methods. Also, the photon fluence rate incident on the detector in a scanner in clinical use today is investigated for different patient sizes through dose rate measurements together with simulations of transmission through patient im- ages. In the second part, a prototype detector module is used to demonstrate new applications enabled by the energy resolution of the detector. The ability to generate material-specific images of contrast agents with iodine and gadolinium is demonstrated. Furthermore, it is shown theoretically and ex- perimentally that interfaces in the image can be visualized by imaging the so-called nonlinear partial volume effect. The results suggest that the studied silicon-strip detector is a promising candidate for photon-counting CT.  

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  • 91.
    Persson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Spectral x-ray imaging2014Patent (Other (popular science, discussion, etc.))
  • 92.
    Persson, Mats
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Adler, Jonas
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Mathematics (Div.).
    Spectral CT reconstruction with anti-correlated noise model and joint prior2017Conference paper (Other academic)
    Abstract [en]

    Spectral CT allows reconstructing a set of material selective basis images which can be used for material quantification. These basis images can be reconstructed independently of each other or treated as a joint reconstruction problem. In this work, we investigate the effect of two ways of introducing coupling between the basis images: using an anti-correlated noise model and regularizing the basis images with a joint prior. We simulate imaging of a FORBILD Head phantom with an ideal photon-counting detector and reconstruct the resulting basis sinograms with and without these two kinds of coupling. The results show that the anti-correlated noise model gives better spatial resolution than the uncorrelated noise model at the same noise level, but also introduces artifacts. If anti-correlations are introduced also in the prior, these artifacts are reduced and the resolution is improved further.

  • 93.
    Persson, Mats
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Bornefalk, Hans
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    A Framework for Evaluating Threshold Variation Compensation Methods in Photon Counting Spectral CT2012In: IEEE Transactions on Medical Imaging, ISSN 0278-0062, E-ISSN 1558-254X, Vol. 31, no 10, p. 1861-1874Article in journal (Refereed)
    Abstract [en]

    One of the challenges in the development of photon counting spectral computed tomography (CT) detectors is that the location of the energy thresholds tends to vary among detector elements. If not compensated for, this threshold variation leads to ring artifacts in the reconstructed images. In this paper, a framework is presented for the systematic comparison of different methods of compensating for inhomogeneities among detector elements in photon counting CT with multiple energy bins. Furthermore, we propose the use of an affine minimum mean square error estimator, calibrated against transmission measurements on different combinations of two materials, for inhomogeneity compensation. Using the framework developed here, this method is compared to two other compensation schemes, flatfielding using an air scan and signal-to-thickness calibration using a step wedge calibrator, in a simulation study. The results show that for all but the lowest studied level of threshold spread, the proposed method is superior to signal-to-thickness calibration, which in turn is superior to flatfielding. We also demonstrate that the effects of threshold variation can be countered to a large extent by substructuring each detector element into depth segments.

  • 94.
    Persson, Mats
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Bujila, Robert
    Unit of X-ray Physics, Section of Imaging Physics Solna, Department of Medical Physics,Karolinska University Hospital.
    Nowik, Patrik
    Unit of X-ray Physics, Section of Imaging Physics Solna, Department of Medical Physics,Karolinska University Hospital.
    Andersson, Henrik
    Unit of X-ray Physics, Section of Imaging Physics Solna, Department of Medical Physics, Karolinska University Hospital.
    Kull, Love
    Medical Radiation Physics, Sunderby Hospital.
    Andersson, Jonas
    Department of Radiation Sciences, Radiation Physics, Umeå University.
    Bornefalk, Hans
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Upper limits of the photon fluence rate on CT detectors: case study on a commercial scannerIn: Article in journal (Other academic)
    Abstract [en]

    Purpose: The highest photon fluence rate that a CT detector must be able to measure is animportant parameter. We calculate the maximum transmitted fluence rate in a commercial CT scanner as a function of patient size for standard head, chest and abdomen protocols.Method: We scanned an anthropomorphic phantom (Kyoto Kagaku PBU-60) with the reference CT protocols provided by AAPM on a GE LightSpeed VCT scanner and noted the tube currentapplied with the tube current modulation (TCM) system. By rescaling this tube current usingpublished measurements on the tube current modulation of a GE scanner we could estimate the tube current that these protocols would have resulted in for other patient sizes. An ECG gatedchest protocol was also simulated. Using measured dose rate profiles along the bowtie filters, wesimulated imaging of anonymized patient images with a range of sizes on a GE VCT scanner andcalculated the maximum transmitted fluence rate. In addition, the 99th and the 95th percentilesof the transmitted fluence rate distribution behind the patient are calculated and the effect of omitting projection lines passing just below the skin line is investigated.Results: The highest transmitted fluence rates on the detector for the AAPM reference protocolswith centered patients are found for head and chest images of small patients, with a maximumof 7.1 · 107 mm−2 s−1 for head and 9.6 · 107 mm−2 s−1 for chest. Miscentering the head by 50 mm downwards increases the maximum transmitted fluence rate to 3.9 · 108 mm−2 s−1 . The ECG gatedchest protocol gives fluence rates up to 2.3 · 108 − 2.4 · 108 mm−2 s−1 depending on miscentering.Conclusion: The fluence rate on a CT detector reaches 1 · 108 − 4 · 108 mm−2 s−1 in standardimaging protocols, with the highest rates occurring for ECG gated chest and miscentered headscans. These results will be useful to developers of CT detectors, in particular photon countingdetectors.

  • 95.
    Persson, Mats
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging. Royal Inst Technol, Dept Phys, SE-10691 Stockholm, Sweden..
    Bujila, Robert
    Karolinska Univ Hosp, Unit Xray Phys, Sect Imaging Phys Solna, Dept Med Phys, SE-17176 Stockholm, Sweden..
    Nowik, Patrik
    Karolinska Univ Hosp, Unit Xray Phys, Sect Imaging Phys Solna, Dept Med Phys, SE-17176 Stockholm, Sweden..
    Andersson, Henrik
    Karolinska Univ Hosp, Unit Xray Phys, Sect Imaging Phys Solna, Dept Med Phys, SE-17176 Stockholm, Sweden..
    Kull, Love
    Sunderby Hosp, Med Radiat Phys, SE-97180 Lulea, Sweden..
    Andersson, Jonas
    Umea Univ, Radiat Phys, Dept Radiat Sci, SE-90185 Umea, Sweden..
    Bornefalk, Hans
    KTH, School of Engineering Sciences (SCI), Physics. Royal Inst Technol, Dept Phys, SE-10691 Stockholm, Sweden..
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging. Royal Inst Technol, Dept Phys, SE-10691 Stockholm, Sweden..
    Upper limits of the photon fluence rate on CT detectors: Case study on a commercial scanner2016In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 43, no 7, p. 4398-4411Article in journal (Refereed)
    Abstract [en]

    Purpose: The highest photon fluence rate that a computed tomography (CT) detector must be able to measure is an important parameter. The authors calculate the maximum transmitted fluence rate in a commercial CT scanner as a function of patient size for standard head, chest, and abdomen protocols. Methods: The authors scanned an anthropomorphic phantom (Kyoto Kagaku PBU-60) with the reference CT protocols provided by AAPM on a GE LightSpeed VCT scanner and noted the tube current applied with the tube current modulation (TCM) system. By rescaling this tube current using published measurements on the tube current modulation of a GE scanner [N. Keat, "CT scanner automatic exposure control systems," MHRA Evaluation Report 05016, ImPACT, London, UK, 2005], the authors could estimate the tube current that these protocols would have resulted in for other patient sizes. An ECG gated chest protocol was also simulated. Using measured dose rate profiles along the bowtie filters, the authors simulated imaging of anonymized patient images with a range of sizes on a GE VCT scanner and calculated the maximum transmitted fluence rate. In addition, the 99th and the 95th percentiles of the transmitted fluence rate distribution behind the patient are calculated and the effect of omitting projection lines passing just below the skin line is investigated. Results: The highest transmitted fluence rates on the detector for the AAPM reference protocols with centered patients are found for head images and for intermediate-sized chest images, both with a maximum of 3.4 . 10(8) mm(-2) s-1, at 949 mm distance from the source. Miscentering the head by 50 mm downward increases the maximum transmitted fluence rate to 5.7 . 10(8) mm(-2) s(-1). The ECG gated chest protocol gives fluence rates up to 2.3 . 10(8)-3.6 . 10(8) mm(-2) s(-1) depending on miscentering. Conclusions: The fluence rate on a CT detector reaches 3 . 10(8)-6 . 10(8) mm(-2) s(-1) in standard imaging protocols, with the highest rates occurring for ECG gated chest and miscentered head scans. These results will be useful to developers of CT detectors, in particular photon counting detectors. (C) 2016 American Association of Physicists in Medicine.

  • 96.
    Persson, Mats
    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.
    Resolution improvement in x-ray imaging with an energy-resolving detector2017In: Medical Imaging 2017: Physics of Medical Imaging, SPIE - International Society for Optical Engineering, 2017, Vol. 10132, article id 101321DConference paper (Refereed)
    Abstract [en]

    In x-ray imaging, improving spatial resolution is an important goal, but developing detectors with smaller pixels is technically challenging. We demonstrate a technique for improving the spatial resolution by utilizing the fact that linear attenuation coefficients of all substances within the human body can be expressed, to a good approximation, as a linear combination of two basis functions, or three if there is iodine contrast present in the image. When the x rays pass an interface parallel to the beam direction, the exponential attenuation law makes the linear attenuation coefficient measured by the detector a nonlinear combination of the linear attenuation coefficients on each side of the interface. This so-called nonlinear partial volume effect causes the spectral response to be dependent on the steepness of interfaces in the imaged volume. In this work, we show how this effect can be used to improve the spatial resolution in spectral projection x-ray imaging and quantify the achievable resolution improvement. We simulate x-ray transmission imaging of sharp and gradual changes in the projected path length of iodine contrast with an ideal energy-resolving photon-counting detector and demonstrate that the slope of the transition can be determined from the registered spectrum. We simulate piecewise-linear transitions and show that the algorithm is able to reproduce the transition profile on a subpixel scale. The FWHM resolution of the method is 5-30 % of the pixel width. The results show that an energy-resolving detector can be used to improve spatial resolution when imaging interfaces of highly attenuating objects.

  • 97.
    Persson, Mats
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Fu, Lin
    GE Research.
    Edic, Pete
    GE Research.
    De Man, Bruno
    GE Research.
    A new series expansion method and its application tophoton-counting CT reconstruction2020In: Proceedings of SPIE, Bellingham, WA: SPIE - International Society for Optical Engineering, 2020, Vol. 11312, article id 113121HConference paper (Other academic)
    Abstract [en]

    The introduction of photon-counting detectors in x-ray computed tomography raises the question of how reconstructionalgorithms should be adapted to photon-counting measurement data. The transition from energyintegratingto photon-counting detectors introduces new eects into the data model, such as pure Poisson statisticsand increased cross talk between detector pixels, (e.g. due to charge sharing), but it is still not known indetail how these eects can be treated accurately by the reconstruction algorithm. In this work, we proposea new reconstruction method based on penalized-likelihood reconstruction that incorporates these eects. Bystarting from a simple, easily-solved reconstruction problem and adding correction terms for the additional physicaleects, we obtain a series expansion for the solution to the image reconstruction problem. This approachserves the twofold purpose of (1) yielding a new, potentially faster method of incorporating complex detectormodels in the reconstruction process and (2) providing insight into the impact of the non-ideal physical eects onthe reconstructed image. We investigate the potential for reconstructing images from simulated photon-countingenergy-resolving CT data with the new algorithm by including correction terms representing pure Poisson statisticsand interpixel cross talk; and we investigate the impact of these physical eects on the reconstructed images.Results indicate that using two correction terms gives good agreement with the converged solution, suggestingthat the new method is feasible in practice. This new approach to image reconstruction can help in developingimproved reconstruction algorithms for photon-counting CT.

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  • 98.
    Persson, Mats
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Grönberg, Fredrik
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Bias-variance tradeoff in anticorrelated noise reduction for spectral CT2017In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 44, no 9, p. E242-E254Article in journal (Refereed)
    Abstract [en]

    Purpose: In spectral CT, basis material decomposition is commonly used to generate a set of basis images showing the material composition at each point in the field of view. The noise in these images typically contains anticorrelations between the different basis images, which leads to increased noise in each basis image. These anticorrelations can be removed by changing the basis functions used in the material decomposition, but the resulting basis images can then no longer be used for quantitative measurements. Recent studies have demonstrated that reconstruction methods which take the anticorrelations into account give reduced noise in the reconstructed image. The purpose of this work is to analyze an analytically solvable denoising model problem and investigate its effect on the noise level and bias in the image as a function of spatial frequency. Method: A denoising problem with a quadratic regularization term is studied as a mathematically tractable model for such a reconstruction method. An analytic formula for the resulting image in the spatial frequency domain is presented, and this formula is applied to a simple mathematical phantom consisting of an iodinated contrast agent insert embedded in soft tissue. We study the effect of the denoising on the image in terms of its transfer function and the visual appearance, the noise power spectrum and the Fourier component correlation coefficient of the resulting image, and compare the result to a denoising problem which does not model the anticorrelations in the image. Results: Including the anticorrelations in the noise model of the denoising method gives 3-40% lower noise standard deviation in the soft-tissue image while leaving the iodine standard deviation nearly unchanged (0-1% difference). It also gives a sharper edge-spread function. The studied denoising method preserves the noise level and the anticorrelated structure at low spatial frequencies but suppresses the noise and removes the anticorrelations at higher spatial frequencies. Cross-talk between images gives rise to artifacts at high spatial frequencies. Conclusions: Modeling anticorrelations in a denoising problem can decrease the noise level in the basis images by removing anticorrelations at high spatial frequencies while leaving low spatial frequencies unchanged. In this way, basis image cross-talk does not lead to low spatial frequency bias but it may cause artifacts at edges in the image. This theoretical insight will be useful for researchers analyzing and designing reconstruction algorithms for spectral CT.

  • 99.
    Persson, Mats
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Grönberg, Fredrik
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Spatial-frequency-domain study of anticorrelated noise reduction in spectral CT2016In: CT-Meeting 2016, Proceedings, 2016, p. 283-286Conference paper (Refereed)
  • 100.
    Persson, Mats
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging. Stanford Univ, Dept Bioengn, Stanford, CA 94305 USA..
    Holmin, Staffan
    Karolinska Inst, Dept Clin Neurosci, Stockholm, Sweden.;Karolinska Univ Hosp, Dept Neuroradiol, Stockholm, Sweden..
    Karlsson, Staffan
    Bornefalk, Hans
    KTH, School of Engineering Sciences (SCI), Physics.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Subpixel x-ray imaging with an energy-resolving detector2018In: Journal of Medical Imaging, ISSN 2329-4302, E-ISSN 2329-4310, Vol. 5, no 1, article id 013507Article in journal (Refereed)
    Abstract [en]

    The detector pixel size can be a severe limitation in x-ray imaging of fine details in the human body. We demonstrate a method of using spectral x-ray measurements to image the spatial distribution of the linear attenuation coefficient on a length scale smaller than one pixel, based on the fact that interfaces parallel to the x-ray beam have a unique spectral response, which distinguishes them from homogeneous materials. We evaluate the method in a simulation study by simulating projection imaging of the border of an iodine insert with 200 mg/ml I in a soft tissue phantom. The results show that the projected iodine profile can be recovered with an RMS resolution of 5% to 34% of the pixel size, using an ideal energy-resolving detector. We also validate this method in an experimental study by imaging an iodine insert in a polyethylene phantom using a photon-counting silicon-strip detector. The results show that abrupt and gradual transitions can be distinguished based on the transmitted x-ray spectrum, in good agreement with simulations. The demonstrated method may potentially be used for improving visualization of blood vessel boundaries, e.g., in acute stroke care.

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