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  • 1.
    Bornefalk, Hans
    KTH, School of Engineering Sciences (SCI), Physics.
    Computer-aided detection and novel mammography imaging techniques2006Doctoral thesis, comprehensive summary (Other scientific)
    Abstract [en]

    This thesis presents techniques constructed to aid the radiologists in detecting breast cancer, the second largest cause of cancer deaths for western women. In the first part of the thesis, a computer-aided detection (CAD) system constructed for the detection of stellate lesions is presented. Different segmentation methods and an attempt to incorporate contra-lateral information are evaluated.

    In the second part, a new method for evaluating such CAD systems is presented based on constructing credible regions for the number of false positive marks per image at a certain desired target sensitivity. This method shows that the resulting regions are rather wide and this explains some of the difficulties encountered by other researchers when trying to compare CAD algorithms on different data sets. In this part an attempt to model the clinical use of CAD as a second look is also made and it shows that applying CAD in sequence to the radiologist in a routine manner, without duly altering the decision criterion of the radiologist, might very well result in suboptimal operating points.

    Finally, in the third part two dual-energy imaging methods optimized for contrast-enhanced imaging of breast tumors are presented. The first is based on applying an electronic threshold to a photon-counting digital detector to discriminate between high- and low-energy photons. This allows simultaneous acquisition of the high- and low-energy images. The second method is based on the geometry of a scanned multi-slit system and also allows single-shot contrast-enhanced dual-energy mammography by filtering the x-ray beam that reaches different detector lines differently.

  • 2.
    Bornefalk, Hans
    KTH, School of Engineering Sciences (SCI), Physics.
    Estimation and Comparison of CAD System Performance in Clinical Settings2005In: Academic Radiology, ISSN 1076-6332, E-ISSN 1878-4046, Vol. 12, no 6, p. 687-694Article in journal (Refereed)
    Abstract [en]

    Rationale and Objectives. Computer-aided detection (CAD) systems are frequently compared using free-response receiver operating characteristic (FROC) curves. While there are ample statistical methods for comparing FROC curves, when one is interested in comparing the outcomes of 2 CAD systems applied in a typical clinical setting, there is the additional matter of correctly determining the system operating point. This article shows how the effect of the sampling error on determining the correct CAD operating point can be captured. By incorporating this uncertainty, a method is presented that allows estimation of the probability with which a particular CAD system performs better than another on unseen data in a clinical setting.

    Materials and Methods. The distribution of possible clinical outcomes from 2 artificial CAD systems with different FROC curves is examined. The sampling error is captured by the distribution of possible system thresholds of the classifying machine that yields a specified sensitivity. After introducing a measure of superiority, the probability of one system being superior to the other can be determined.

    Results. It is shown that for 2 typical mammography CAD systems, each trained on independent representative datasets of 100 cases, the FROC curves must be separated by 0.20 false positives per image in order to conclude that there is a 90% probability that one is better than the other in a clinical setting. Also, there is no apparent gain in increasing the size of the training set beyond 100 cases.

    Discussion. CAD systems for mammography are modeled for illustrative purposes, but the method presented is applicable to any computer-aided detection system evaluated with FROC curves. The presented method is designed to construct confidence intervals around possible clinical outcomes and to assess the importance of training set size and separation between FROC curves of systems trained on different datasets.

  • 3.
    Bornefalk, Hans
    KTH, School of Engineering Sciences (SCI), Physics.
    Implications of unchanged detection criteria with CAD as second reader of mammograms2006In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 33, no 4, p. 922-929Article in journal (Refereed)
    Abstract [en]

    In this paper we address the use of computer-aided detection (CAD) systems as second readers in mammography. The approach is based on Bayesian decision theory and its implication for the choice of optimal operating points. The choice of a certain operating point along an ROC curve corresponds to a particular tradeoff between false positives and missed cancers. By minimizing a total risk function given this tradeoff, we determine optimal decision thresholds for the radiologist and CAD system when CAD is used as a second reader. We show that under very general circumstances, the performance of the sequential system is improved if the decision threshold of the latent human decision variable is increased compared to what it would have been in the absence of the CAD system. This means that an initial stricter decision criterion should be applied by the radiologist when CAD is used as a second reader than otherwise. First and foremost, the results in this paper should be interpreted qualitatively, but an attempt is made at quantifying the effect by tuning the model to a prospective study evaluating the use of CAD as a second reader. By making some necessary and plausible assumptions, we are able to estimate the effect of the resulting suboptimal operating point. In this study of 12 860 women, we estimate that a 15% reduction in callbacks for masses could have been achieved with only about a 1.5% relative decrease in sensitivity compared to that without using a stricter initial criterion by the radiologist. For microcalcifications the corresponding values are 7% and 0.2%. (c) 2006 American Association of Physicists in Medicine.

  • 4.
    Bornefalk, Hans
    KTH, School of Engineering Sciences (SCI), Physics.
    Proposal and validation of a method to construct confidence intervals for clinical outcomes around FROC curves for mammography CAD systems2005In: MEDICAL IMAGING 2005: IMAGE PROCESSING, PT 1-3 / [ed] itzpatrick, JM; Reinhardt, JM, BELLINGHAM: SPIE-INT SOC OPTICAL ENGINEERING , 2005, Vol. 5747, p. 675-682Conference paper (Refereed)
    Abstract [en]

    This paper introduces a method for constructing confidence intervals for possible clinical outcomes around the FROC curve of a mammography CAD system. Given the architecture of a CAD classifying machine, there is one and only one system threshold that will yield a desired sensitivity on a certain population. The limited training sample size leads to a sampling error and an uncertainty in determining the optimal system threshold. This leads to an uncertainty in the operating point in the direction along the FROC curve which can be captured by a Bayesian approach where the distribution of possible thresholds is estimated. This uncertainty contributes to a large and spread-out confidence interval which is important to consider when one is intending to make comparisons between CAD algorithms trained on different data sets. The method is validated using a Monte Carlo method designed to capture the effect of correctly determining the system threshold.

  • 5.
    Bornefalk, Hans
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Synthetic Hounsfield units from spectral CT data2012In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 57, no 7, p. N83-N87Article in journal (Refereed)
    Abstract [en]

    Beam-hardening-free synthetic images with absolute CT numbers that radiologists are used to can be constructed from spectral CT data by forming 'dichromatic' images after basis decomposition. The CT numbers are accurate for all tissues and the method does not require additional reconstruction. This method prevents radiologists from having to relearn new rules-of-thumb regarding absolute CT numbers for various organs and conditions as conventional CT is replaced by spectral CT. Displaying the synthetic Hounsfield unit images side-by-side with images reconstructed for optimal detectability for a certain task can ease the transition from conventional to spectral CT.

  • 6.
    Bornefalk, Hans
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Task-based weights for photon counting spectral x-ray imaging2011In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 38, no 11, p. 6065-6073Article in journal (Refereed)
    Abstract [en]

    Purpose: To develop a framework for taking the spatial frequency composition of an imaging taskinto account when determining optimal bin weight factors for photon counting energy sensitivex-ray systems. A second purpose of the investigation is to evaluate the possible improvement comparedto using pixel based weights.Methods: The Fourier based approach of imaging performance and detectability index d0 is appliedto pulse height discriminating photon counting systems. The dependency of d0 on the bin weightfactors is made explicit, taking into account both differences in signal and noise transfer characteristicsacross bins and the spatial frequency dependency of interbin correlations from reabsorbedscatter. Using a simplified model of a specific silicon detector, d0 values for a high and a low frequencyimaging task are determined for optimal weights and compared to pixel based weights.Results: The method successfully identifies bins where a large point spread function degradesdetection of high spatial frequency targets. The method is also successful in determining how todownweigh highly correlated bins. Quantitative predictions for the simplified silicon detectormodel indicate that improvements in the detectability index when applying task-based weightsinstead of pixel based weights are small for high frequency targets, but could be in excess of 10%for low frequency tasks where scatter-induced correlation otherwise degrade detectability.Conclusions: The proposed method makes the spatial frequency dependency of complex correlationstructures between bins and their effect on the system detective quantum efficiency easier toanalyze and allows optimizing bin weights for given imaging tasks. A potential increase in detectabilityof double digit percents in silicon detector systems operated at typical CT energies (100kVp) merits further evaluation on a real system. The method is noted to be of higher relevancefor silicon detectors than for cadmium (zink) telluride detectors.

  • 7.
    Bornefalk, Hans
    KTH, Superseded Departments, Physics.
    Use of phase and certainty information in automatic detection of stellate patterns in mammograms: IMAGE PROCESSING, PTS 1-32004In: MEDICAL IMAGING 2004: IMAGE PROCESSING, PTS 1-3 / [ed] Fitzpatrick, JM; Sonka, M, BELLINGHAM: SPIE-INT SOC OPTICAL ENGINEERING , 2004, Vol. 5370, p. 97-107Conference paper (Refereed)
    Abstract [en]

    Detection of stellate patterns is a very important step in computer-aided detection schemes designed for mammography. We introduce a new way of finding these regions based on the use of quadrature filters. The method allows extraction of a certainty measure for each orientation estimate. This makes the method of finding the areas the spicules seem to emanate from more robust than simply basing it on the orientation estimates themselves. The local phase extracted from the filter outputs allows us to discriminate between orientation estimates from edges and dark lines from those generated by bright line structures, i.e. spicules. This makes the method more specific. We also show how the method can be modified for finding non-spiculated masses in digitized mammograms.

  • 8.
    Bornefalk, Hans
    KTH, School of Engineering Sciences (SCI), Physics.
    Use of Quadrature Filters for Detection of Stellate Lesions in Mammograms2005In: IMAGE ANALYSIS, PROCEEDINGS / [ed] Kalviainen, H; Parkkinen, J; Kaarna, A, 2005, Vol. 3540, p. 649-658Conference paper (Refereed)
    Abstract [en]

    We propose a method for finding stellate lesions in digitized mammograms based on the use of both local phase and local orientation information extracted from quadrature filter outputs. The local phase information allows efficient and fast separation between edges and lines and the local orientation estimates are used to find areas circumscribed by edges and with radiating lines. The method is incorporated in a computer-aided detection system and evaluation by FROG-curve analysis on a data set of 90 mammograms (45 pairs) yields a false positive rate of 0.3 per image at 90% sensitivity.

  • 9.
    Bornefalk, Hans
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    XCOM intrinsic dimensionality for low-Z elements at diagnostic energies2012In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 39, no 2, p. 654-657Article in journal (Refereed)
    Abstract [en]

    Purpose: To determine the intrinsic dimensionality of linear attenuation coefficients (LACs) from XCOM for elements with low atomic number (Z = 1-20) at diagnostic x-ray energies (25-120 keV). H-0(q), the hypothesis that the space of LACs is spanned by q bases, is tested for various q-values. Methods: Principal component analysis is first applied and the LACs are projected onto the first q principal component bases. The residuals of the model values vs XCOM data are determined for all energies and atomic numbers. Heteroscedasticity invalidates the prerequisite of i.i.d. errors necessary for bootstrapping residuals. Instead wild bootstrap is applied, which, by not mixing residuals, allows the effect of the non-i.i.d residuals to be reflected in the result. Credible regions for the eigenvalues of the correlation matrix for the bootstrapped LAC data are determined. If subsequent credible regions for the eigenvalues overlap, the corresponding principal component is not considered to represent true data structure but noise. If this happens for eigenvalues l and l + 1, for any l <= q, H-0(q) is rejected. Results: The largest value of q for which H-0(q) is nonrejectable at the 5%-level is q = 4. This indicates that the statistically significant intrinsic dimensionality of low-Z XCOM data at diagnostic energies is four. Conclusions: The method presented allows determination of the statistically significant dimensionality of any noisy linear subspace. Knowledge of such significant dimensionality is of interest for any method making assumptions on intrinsic dimensionality and evaluating results on noisy reference data. For LACs, knowledge of the low-Z dimensionality might be relevant when parametrization schemes are tuned to XCOM data. For x-ray imaging techniques based on the basis decomposition method (Alvarez and Macovski, Phys. Med. Biol. 21, 733-744, 1976), an underlying dimensionality of two is commonly assigned to the LAC of human tissue at diagnostic energies. The finding of a higher statistically significant dimensionality thus raises the question whether a higher assumed model dimensionality (now feasible with the advent of multibin x-ray systems) might also be practically relevant, i.e., if better tissue characterization results can be obtained.

  • 10.
    Bornefalk, Hans
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Bornefalk-Hermansson, Anna
    On the comparison of FROC curves in mammography CAD systems2005In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 32, no 2, p. 412-417Article in journal (Refereed)
    Abstract [en]

    We present a novel method for assessing the performance of computer-aided detection systems on unseen cases at a given sensitivity level. The sampling error introduced when training the system on a limited data set is captured as the uncertainty in determining the system threshold that would yield a certain predetermined sensitivity on unseen data sets. By estimating the distribution of system thresholds, we construct a confidence interval for the expected number of false positive markings per image at a given sensitivity. We present two alternative procedures for estimating the probability density functions needed for the construction of the confidence interval. The first is based on the common assumption of Poisson distributed number of false positive markings per image. This procedure also relies on the assumption of independence between false positives and sensitivity, an assumption that can be relaxed with the second procedure, which is nonparametric. The second procedure uses the bootstrap applied to the data generated in the leave-one-out construction of the FROC curve, and is a fast and robust way of obtaining the desired confidence interval. Standard FROC curve analysis does not account for the uncertainty in setting the system threshold, so this method should allow for a more fair comparison of different systems. The resulting confidence intervals are surprisingly wide. For our system a conventional FROC curve analysis yields 0.47 false positive markings per image at 90% sensitivity. The 90% confidence interval for the number of false positive markings per image is (0.28, 1.02) with the parametric procedure and (0.27, 1.04) with the nonparametric bootstrap. Due to its computational simplicity and its allowing more fair comparisons between systems, we propose this method as a complement to the traditionally presented FROC curves. (C) 2005 American Association of Physicists in Medicine.

  • 11.
    Bornefalk, Hans
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Photon-counting spectral computed tomography using silicon strip detectors: a feasibility study2010In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 55, no 7, p. 1999-2022Article in journal (Refereed)
    Abstract [en]

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

  • 12.
    Bornefalk, Hans
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Hemmendorff, Magnus
    KTH, School of Engineering Sciences (SCI), Physics.
    Hjarn, Torbjorn
    Dual-energy imaging using a digital scanned multi-slit system for mammography: evaluation of a differential beam filtering technique - art. no. 61422O2006In: Medical Imaging 2006: Physics of Medical Imaging, Pts 1-3 / [ed] Flynn, MJ; Hsieh, J, BELLINGHAM, WA: SPIE-INT SOC OPTICAL ENGINEERING , 2006, Vol. 6142, p. O1422-O1422Conference paper (Refereed)
    Abstract [en]

    This paper describes a method for single exposure contrast-enhanced dual-energy imaging of tumors utilizing a scanned multi-slit system for digital mammography. This photon counting system employs an array of silicon strip detectors in an edge-on geometry. In the multi-slit setup, the line detectors and pre-collimator slits are aligned orthogonal to the scan direction. This geometry is advantageous to dual-energy imaging, since it allows differential filtering of the x-ray beam in the pre-collimator slits. A high-energy image is constructed from those lines where the filter material has been chosen to harden the x-ray beam and the low-energy image from the lines with a filter producing softer beams. Both images are obtained in the same scan, eliminating the need to change tube voltages and anode materials and minimizing the risk of motion artifacts. The method is illustrated on a purpose-built phantom and logarithmic subtraction of the images produces images essentially free of anatomical clutter with the contrast-enhanced targets clearly visible.

  • 13.
    Bornefalk, hans
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Hemmendorff, Magnus
    Hjärn, Torbjörn
    Contrast-enhanced dual-energy mammography using a scanned multi-slit system: valuation of a differential beam filtering technique2007In: Journal of Electronic Imaging (JEI), ISSN 1017-9909, E-ISSN 1560-229X, Vol. 16, no 2, p. 023006-Article in journal (Refereed)
    Abstract [en]

    This paper describes a method for single-exposure, contrast-enhanced dual-energy imaging of tumors utilizing a scanned multislit system for digital mammography. This photon-counting system employs an array of silicon strip detectors mounted in an edge-on geometry. The line detectors and pre- and post-collimator slits are carefully aligned, and the multislit setup allows differential filtering of the x-ray beam in the pre-collimator slits. A high-energy image is constructed from those lines where the filter material has been chosen to harden the x-ray beam and the low-energy image from the lines with a filter producing softer beams. Both images are obtained in the same scan, eliminating the need to change tube voltages and anode materials and minimizing the risk of motion artifacts. The method is illustrated on a purpose-built phantom, and logarithmic subtraction of the images produces images essentially free of anatomical clutter with the contrast-enhanced targets clearly visible.

  • 14.
    Bornefalk, Hans
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Lewin, John H.
    Danielsson, Mats
    Lundqvist, Mats
    KTH, School of Engineering Sciences (SCI), Physics.
    Improved dual-energy imaging with electronic spectrum splittingIn: Medical physics (Lancaster), ISSN 0094-2405Article in journal (Refereed)
  • 15.
    Bornefalk, Hans
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Lundqvist, Mats
    Dual-energy imaging using a photon counting detector with electronic spectrum-splitting - art. no. 61421H2006In: Medical Imaging 2006: Physics of Medical Imaging, Pts 1-3 / [ed] Flynn, MJ; Hsieh, J, 2006, Vol. 6142, p. H1421-H1421Conference paper (Refereed)
    Abstract [en]

    This paper presents a dual-energy imaging technique optimized for contrast-enhanced mammography using a photon counting detector. Each photon pulse is processed separately in the detector and the addition of an electronic threshold near the middle of the energy range of the x-ray spectrum allows discrimination of high and low energy photons. This effectively makes the detector energy sensitive, and allows the acquisition of high- and low-energy images simultaneously. These high- and low-energy images can be combined to dual-energy images where the anatomical clutter has been suppressed. By setting the electronic threshold close to 33.2 keV (the k-edge of iodine) the system is optimized for dual-energy contrast-enhanced imaging of breast tumors. Compared to other approaches, this method not only eliminates the need for separate exposures that might lead to motion artifacts, it also eliminates the otherwise deteriorating overlap between high- and low-energy spectra. We present phantom dual-energy images acquired on a prototype system to illustrate that the technique is already operational, albeit in its infancy. We also present a theoretical estimation of the potential gain in tumor signal-difference-to-noise ratio when using this electronic spectrum-splitting method as opposed to acquiring the high- and low-energy images separately with double exposures with separate x-ray spectra. Assuming ideal energy sensitive photon counting detectors, we arrive at the conclusion that the signal-difference-to-noise ratio could be increased by 145% at constant dose. We also illustrate our results on synthetic images.

  • 16.
    Bornefalk, Hans
    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.
    Theoretical Comparison of the Iodine Quantification Accuracy of Two Spectral CT Technologies2014In: IEEE Transactions on Medical Imaging, ISSN 0278-0062, E-ISSN 1558-254X, Vol. 33, no 2, p. 556-565Article in journal (Refereed)
    Abstract [en]

    We compare the theoretical limits of iodine quantification for the photon counting multibin and dual energy technologies. Dual energy systems by necessity have to make prior assumptions in order to quantify iodine. We explicitly allow the multibin system to make the same assumptions and also allow them to be wrong. We isolate the effect of technology from imperfections and implementation issues by assuming both technologies to be ideal, i.e., without scattered radiation, unity detection efficiency and perfect energy response functions, and by applying the Cramer-Rao lower bound methodology to assess the quantification accuracy. When priors are wrong the maximum likelihood estimates will be biased and the mean square error of the quantification error is a more appropriate figure of merit. The evaluation assumes identical X-ray spectra for both methodologies and for that reason a sensitivity analysis is performed with regard to the assumed X-ray spectrum. We show that when iodine is quantified over regions of interest larger than 6 cm, multibin systems benefit by independent estimation of three basis functions. For smaller regions of interest multibin systems can increase quantification accuracy by making the same prior assumptions as dual energy systems.

  • 17.
    Bornefalk, Hans
    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.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Allowable forward model misspecification for accurate basis decomposition in a silicon detector based spectral CT2015In: IEEE Transactions on Medical Imaging, ISSN 0278-0062, E-ISSN 1558-254X, Vol. 34, no 3, p. 788-795Article in journal (Refereed)
    Abstract [en]

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

  • 18.
    Bornefalk, Hans
    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.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Necessary forward model specification accuracy for basis material decomposition in spectral CT2014In: Medical Imaging 2014: Physics of Medical Imaging, SPIE - International Society for Optical Engineering, 2014, p. 90332I-Conference paper (Refereed)
    Abstract [en]

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

  • 19.
    Bornefalk, Hans
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Persson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Xu, Cheng
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Karlsson, Staffan
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Svensson, Christer
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Effect of Temperature Variation on the Energy Response of a Photon Counting Silicon CT Detector2013In: IEEE Transactions on Nuclear Science, ISSN 0018-9499, E-ISSN 1558-1578, Vol. 60, no 2, p. 1442-1449Article in journal (Refereed)
    Abstract [en]

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

  • 20.
    Bornefalk, Hans
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Xu, Cheng
    Svensson, Christer
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Design considerations to overcome cross talk in a photon counting silicon strip detector for computed tomography2010In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 621, no 1-3, p. 371-378Article in journal (Refereed)
    Abstract [en]

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

  • 21.
    Bornefalk, Hans
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Xu, Cheng
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Svensson, Christer
    Division of Electronic Devices, Linköping University.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Simulation study of an energy sensitive photon counting silicon strip detector for computed tomography: identifying strengths and weaknesses and developing work-arounds2010In: MEDICAL IMAGING 2010: PHYSICS OF MEDICAL IMAGING / [ed] Samei E; Pelc NJ, 2010, Vol. 7622Conference paper (Refereed)
    Abstract [en]

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

  • 22.
    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.

  • 23.
    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)
  • 24.
    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.

  • 25.
    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.

  • 26.
    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.

  • 27.
    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.

  • 28.
    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.

  • 29.
    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)
  • 30.
    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.

  • 31.
    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.

  • 32.
    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.

  • 33.
    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.

  • 34.
    Persson, Mats
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Huber, Ben
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Karlsson, Staffan
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Liu, Xuejin
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Chen, Han
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Xu, Cheng
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Yveborg, Moa
    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.
    Energy-resolved CT imaging with a photon-counting silicon-strip detector2014In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 59, no 22, p. 6709-6727Article in journal (Refereed)
    Abstract [en]

    Photon-counting detectors are promising candidates for use in the next generation of x-ray computed tomography (CT) scanners. Among the foreseen benefits are higher spatial resolution, better trade-off between noise and dose and energy discriminating capabilities. Silicon is an attractive detector material because of its low cost, mature manufacturing process and high hole mobility. However, it is sometimes overlooked for CT applications because of its low absorption efficiency and high fraction of Compton scatter. The purpose of this work is to demonstrate that silicon is a feasible material for CT detectors by showing energy-resolved CT images acquired with an 80 kVp x-ray tube spectrum using a photon-counting silicon-strip detector with eight energy thresholds developed in our group. We use a single detector module, consisting of a linear array of 50 0.5 x 0.4 mm detector elements, to image a phantom in a table-top lab setup. The phantom consists of a plastic cylinder with circular inserts containing water, fat and aqueous solutions of calcium, iodine and gadolinium, in different concentrations. By using basis material decomposition we obtain water, calcium, iodine and gadolinium basis images and demonstrate that these basis images can be used to separate the different materials in the inserts. We also show results showing that the detector has potential for quantitative measurements of substance concentrations.

  • 35.
    Persson, Mats
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Huber, Ben
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Karlsson, Staffan
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Liu, Xuejin
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Chen, Han
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Xu, Cheng
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Yveborg, Moa
    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.
    Energy-resolved CT imaging with a photon-counting silicon-strip detector2014In: Medical Imaging 2014: Physics of Medical Imaging, SPIE - International Society for Optical Engineering, 2014, p. 90333L-Conference paper (Refereed)
    Abstract [en]

    Photon-counting detectors are promising candidates for use in the next generation of x-ray CT scanners. Among the foreseen benefits are higher spatial resolution, better trade-off between noise and dose, and energy discriminating capabilities. Silicon is an attractive detector material because of its low cost, mature manufacturing process and high hole mobility. However, it is sometimes claimed to be unsuitable for use in computed tomography because of its low absorption efficiency and high fraction of Compton scatter. The purpose of this work is to demonstrate that high-quality energy-resolved CT images can nonetheless be acquired with clinically realistic exposure parameters using a photon-counting silicon-strip detector with eight energy thresholds developed in our group. We use a single detector module, consisting of a linear array of 50 0.5 × 0.4 mm detector elements, to image a phantom in a table-top lab setup. The phantom consists of a plastic cylinder with circular inserts containing water, fat and aqueous solutions of calcium, iodine and gadolinium, in different concentrations. We use basis material decomposition to obtain water, calcium, iodine and gadolinium basis images and demonstrate that these basis images can be used to separate the different materials in the inserts. We also show results showing that the detector has potential for quantitative measurements of substance concentrations.

  • 36.
    Persson, Mats
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Meyer, Bettina
    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.
    Quantification of ring artifact visibility in CT2012In: Medical Imaging 2012: Physics Of Medical Imaging, SPIE - International Society for Optical Engineering, 2012, Vol. 8313, p. 83132J-Conference paper (Refereed)
    Abstract [en]

    Ring artifacts appear in computed tomography images if there are too large inhomogeneities between different detector elements. The question of how large inhomogeneities are acceptable is gaining in importance due to the development of energy discriminating photon counting CT, where detector homogeneity is an important design parameter. We propose using the systematic-to-statistical error quotient q, defined as the variance of the expected log-normalized count number between detector elements (dels) divided by the variance of log-normalized count numbers measured with the same del, as a metric of ring artifact visibility. With a simple observer study using simulated images, it is shown that rings are visible in the reconstructed image if q exceeds a threshold which lies close to 1.2·10 -3 for 1500 detector elements and 2000 projection angles. It is also shown by visual inspection of simulated images that the threshold value is, to a good approximation, inversely proportional to the number of angle measurements and independent of the number of detector elements. The results suggest that a simple oberver study, together with these scaling relationships, is sufficient for establishing sinogram homogeneity requirements for a particular reconstruction method.

  • 37.
    Xu, Cheng
    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.
    Persson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Karlsson, Staffan
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Svensson, Christer
    Bornefalk, Hans
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Energy resolution of a segmented silicon strip detector for photon-counting spectral CT2013In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 715, p. 11-17Article in journal (Refereed)
    Abstract [en]

    We investigated the energy resolution of a segmented silicon strip detector for photon-counting spectral computed tomography (CT). The detector response to different monochromatic photon energies and various photon fluxes was characterized at the Elettra synchrotron. An RMS energy resolution of 1.50 keV has been demonstrated for 22 keV photons at zero flux, and it deteriorated as a function of input count rate at a rate of 5.13 eV mm2 /Mcps. The charge sharing effect has been evaluated. The results show that around 11.1% of the interacting photons experience charge sharing for 22 keV photons and 15.3% for 30 keV.

  • 38.
    Xu, Cheng
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Bornefalk, Hans
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Evaluation of Energy Loss and Charge Sharing in Cadmium Telluride Detectors for Photon-Counting Computed Tomography2011In: IEEE Transactions on Nuclear Science, ISSN 0018-9499, E-ISSN 1558-1578, Vol. 58, no 3, p. 614-625Article in journal (Refereed)
    Abstract [en]

    We present estimates of energy loss and charge sharing for a pixelated cadmium telluride (CdTe) detector used for photon-counting spectral computed tomography (CT). In a photon-counting pixelated CdTe detector, several physical effects lead to detected events with reduced energies, including Compton scattering, fluorescence emission, charge diffusion, trapping of charge carriers and slow-hole-motion-induced incomplete charge collection. Charge sharing is the result of the lost energy being collected by adjacent pixels. We simulated the photon transport and the charge-collection process with a Monte Carlo-based simulation and evaluated these effects on the detector performance. The trapping effect and poor hole collection have been studied together using an analytical model. We also investigated the detector response under the influence of only the fluorescence effect. We conclude that the charge sharing effects should be taken into account when the pixel is smaller than 1 mm(2). A straightforward way to decrease the double counting of X-rays from events with charge sharing is to increase the electronic threshold. However, increasing the threshold comes at the cost of losing low-energy events, which is undesirable, at least in applications such as pediatric imaging.

  • 39.
    Xu, Cheng
    et al.
    KTH, School of Engineering Sciences (SCI), Physics. KTH, School of Biotechnology (BIO), Centres, Albanova VinnExcellence Center for Protein Technology, ProNova.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics. KTH, School of Biotechnology (BIO), Centres, Albanova VinnExcellence Center for Protein Technology, ProNova.
    Bornefalk, Hans
    KTH, School of Engineering Sciences (SCI), Physics. KTH, School of Biotechnology (BIO), Centres, Albanova VinnExcellence Center for Protein Technology, ProNova.
    Validity of spherical approximations of initial charge cloud shape in silicon detectors2011In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 648, no SUPPL. 1, p. 190-193Article in journal (Refereed)
    Abstract [en]

    Spherical approximation has been used extensively in low-energy X-ray imaging to represent the initial charge cloud produced by photon interactions in silicon detectors, mainly because of its simplicity. However, for high-energy X-rays, where the initial charge distribution is as important as the diffusion process, the spherical approximation will not result in a realistic detector response. In this paper, we present a bubble-line model that simulates the initial charge cloud in silicon detectors for photons in the energy range of medical imaging. An initial charge cloud can be generated by sampling the center of gravity and the track size from statistical distributions derived from Monte Carlo generated tracks and by distributing a certain proportion of photon energy into a bubble (68%) and a line portion uniformly. The simulations of detector response demonstrate that the new model simulates the detector response accurately and corresponds well to Monte Carlo simulation.

  • 40.
    Xu, Cheng
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Karlsson, Staffan
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Svensson, Christer
    Bornefalk, Hans
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Performance characterization of a silicon strip detector for spectral computed tomography utilizing a laser testing system2011In: MEDICAL IMAGING 2011: PHYSICS OF MEDICAL IMAGING / [ed] Pelc, NJ; Samei, E; Nishikawa, RM, 2011, Vol. 7961Conference paper (Refereed)
    Abstract [en]

    A new silicon strip detector with sub-millimeter pixel size operated in single photon-counting mode has been developed for use in spectral computed tomography (CT). An ultra fast application specific integrated circuit (ASIC) specially designed for fast photon-counting application is used to process the pulses and sort them into eight energy bins. This report characterizes the ASIC and detector in terms of thermal noise (0.77 keV RMS), energy resolution when electron-hole pairs are generated in the detector diode (1.5 keV RMS) and Poissonian count rate with retained count rate linearity and energy resolution (200 Mcps.mm(-2)). The performance of the photon-counting detector has been tested using a picosecond pulsed laser system to inject energy into the detector, simulating x-ray interactions. The laser testing results indicate a good energy-discriminating capability of the detector, assigning the pulses to higher and higher energy bins as the intensity of the laser pulses are increased.

  • 41.
    Xu, Cheng
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Karlsson, Staffan
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Svensson, Christer
    Bornefalk, Hans
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Preliminary evaluation of a silicon strip detector for photon-counting spectral CT2012In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 677, p. 45-51Article in journal (Refereed)
    Abstract [en]

    An edge-on silicon strip detector designed for photon-counting spectral computed tomography (CT) is presented. Progress on the development of an application specific integrated circuit (ASIC) to process the pulses and sort them into energy bins is reported upon. The ASIC and detector are evaluated in terms of electronic noise, energy resolution, count rate linearity under high-frequency periodic pulses, threshold variation and gain. The high-frequency periodic pulses are injected both by means of an external pulse generator and a pulsed laser illuminating the silicon diode. The pulsed laser system has similar to 100 ps pulse width and thus generates near instantaneous pulses in the diode, thus mimicking real X-ray conversions. The evaluation shows a low thermal noise level of 0.77 key RMS, an energy resolution of 1.5 keV RMS when electron-hole pairs are generated in the detector diode by the laser injection. The test results furthermore indicate a good energy-discriminating capability of the detector with the thresholds spread out, assigning the external pulses to higher and higher energy bins as the pulse intensity is increased.

  • 42.
    Xu, Cheng
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Persson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Chen, Han
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Karlsson, Staffan
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Svensson, Christer
    Bornefalk, Hans
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Evaluation of a Second-Generation Ultra-Fast Energy-Resolved ASIC for Photon-Counting Spectral CT2013In: IEEE Transactions on Nuclear Science, ISSN 0018-9499, E-ISSN 1558-1578, Vol. 60, no 1, p. 437-445Article in journal (Refereed)
    Abstract [en]

    A second-generation ultra-fast energy-resolved application specific integrated circuit (ASIC) has been developed for photon-counting spectral computed tomography (CT). The energy resolution, threshold dispersion and gain of the ASIC were characterized with synchrotron radiation at Diamond Light Source. The standard deviation of threshold offsets at zero keV is 0.89 keV. An RMS energy resolution of 1.09 keV has been demonstrated for 15 keV photon energy at a count rate of 40 kcps, and it deteriorates at a rate of 0.29 keV/Mcps with the increase of output cout rate. The count rate performance of the ASIC has also been evaluated with 120 kV polychromatic x-rays produced by a tungsten anode tube and the results are presented.

  • 43.
    Xu, Cheng
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Yveborg, Moa
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Chen, Han
    KTH, School of Engineering Sciences (SCI), Physics.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Karlsson, Staffan
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Svensson, C.
    Bornefalk, Hans
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Evaluation of an ultra-fast photon-counting energy-resolved ASIC for spectral CT2012In: Progress in Biomedical Optics and Imaging - Proceedings of SPIE, SPIE - International Society for Optical Engineering, 2012, Vol. 8313, p. 83130Y-Conference paper (Refereed)
    Abstract [en]

    We have developed an ultra-fast photon-counting energy-resolved application specific integrated circuit (ASIC) for spectral computed tomography (CT). A comprehensive characterization has been carried out to investigate the performance of the ASIC in terms of energy resolution under different photon flux rates and the count rate linearity in photon-counting mode. An energy resolution of 4.7 % has been achieved for 59.5 keV low flux photons. The count rate performance of the ASIC was measured with 120 kVp polychromatic x-rays. The results indicate that the count rate linearity can be kept for a flux rate up to 150 Mphotons s -1 mm -2 with retained energy information, and this value is increased to be 250 Mphotons s -1 mm -2 in photon-counting mode.

  • 44. Yao, Y.
    et al.
    Bornefalk, Hans
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Hsieh, S. S.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Pelc, N. J.
    Use of depth information from in-depth photon counting detectors for x-ray spectral imaging: A preliminary simulation study2014In: Medical Imaging 2014: Physics of Medical Imaging, SPIE - International Society for Optical Engineering, 2014, p. 90333E-Conference paper (Refereed)
    Abstract [en]

    Purpose: Photon counting x-ray detectors (PCXD) may improve dose-efficiency but are hampered by limited count rate. They generally have imperfect energy response. Multi-layer ("in-depth") detectors have been proposed to enable higher count rates but the potential benefit of the depth information has not been explored. We conducted a simulation study to compare in-depth detectors against single layer detectors composed of common materials. Both photon counting and energy integrating modes were studied. Methods: Polyenergetic transmissions were simulated through 25cm of water and 1cm of calcium. For PCXD composed of Si, GaAs or CdTe a 120kVp spectrum was used. For energy integrating x-ray detectors (EIXD) made from GaAs, CdTe or CsI, spectral imaging was done using 80 and 140kVp and matched dose. Semi-ideal and phenomenological energy response models were used. To compare these detectors, we computed the Cramér-Rao lower bound (CRLB) of the variance of basis material estimates. Results: For PCXDs with perfect energy response, depth data provides no additional information. For PCXDs with imperfect energy response and for EIXDs the improvement can be significant. E.g., for a CdTe PCXD with realistic energy response, depth information can reduce the variance by 50%. The improvement depends on the x-ray spectrum. For a semi-ideal Si detector and a narrow x-ray spectrum the depth information has minimal advantage. For EIXD, the in-depth detector has consistent variance reduction (15% and 17%19% for water and calcium, respectively). Conclusions: Depth information is beneficial to spectral imaging for both PCXD and EIXD. The improvement depends critically on the detector energy response.

  • 45.
    Yao, Yuan
    et al.
    Stanford Univ, Dept Radiol, Stanford, CA 94305 USA. Stanford Univ, Dept Bioengn, Stanford, CA 94305 USA..
    Bornefalk, Hans
    KTH, School of Engineering Sciences (SCI), Physics.
    Hsieh, Scott S.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging. Royal Inst Technol, Dept Phys, Stockholm, Sweden..
    Pelc, Norbert J.
    UTILIZATION OF IN-DEPTH PHOTON COUNTING DETECTORS TOWARDS X-RAY SPECTRAL IMAGING: THE BENEFITS FROM THE DEPTH INFORMATION2014In: 2014 IEEE 11th International Symposium on Biomedical Imaging (ISBI), IEEE , 2014, p. 1156-1159Conference paper (Refereed)
    Abstract [en]

    The in-depth photon counting x-ray detector (PCXD) is a multi-layer detector arrangement which has been introduced to tackle photon count rate limitations of current systems. The capability of resolving photon detections along the detector's depth direction enables multiple measurements in a single scan with energy information that could be potentially utilized for x-ray spectral imaging. The benefit of this depth information has not been explored. We conducted a simulation study to evaluate the performance of in-depth PCXDs for dual material decomposition and compared it against single layer detectors. Common semiconductor materials (Si, GaAs and CdTe) were assessed, with imperfect energy response modeled. We demonstrate that depth information is useful if spectral distortion is present. The benefits depend on how the detector is segmented in the depth direction.

  • 46.
    Yveborg, Moa
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Bornefalk, Hans
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Performance evaluation of a sub-millimetre spectrally resolved CT system on high- and low-frequency imaging tasks: a simulation2012In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 57, no 8, p. 2373-2391Article in journal (Refereed)
    Abstract [en]

    We are developing a photon-counting silicon strip detector with 0.4 x 0.5 mm(2) detector elements for clinical CT applications. Except for the limited detection efficiency of approximately 0.8 for a spectrum of 80 kVp, the largest discrepancies from ideal spectral behaviour have been shown to be Compton interactions in the detector and electronic noise. Using the framework of cascaded system analysis, we reconstruct the 3D MTF and NPS of a silicon strip detector including the influence of scatter and charge sharing inside the detector. We compare the reconstructed noise and signal characteristics with a reconstructed 3D MTF and NPS of an ideal energy-integrating detector system with unity detection efficiency, no scatter or charge sharing inside the detector, unity presampling MTF and 1 x 1 mm(2) detector elements. The comparison is done by calculating the dose-normalized detectability index for some clinically relevant imaging tasks and spectra. This work demonstrates that although the detection efficiency of the silicon detector rapidly drops for the acceleration voltages encountered in clinical computed tomography practice, and despite the high fraction of Compton interactions due to the low atomic number, silicon detectors can perform on a par with ideal energy-integrating detectors for routine imaging tasks containing low-frequency components. For imaging tasks containing high-frequency components, the proposed silicon detector system can perform approximately 1.1-1.3 times better than a fully ideal energy-integrating system.

  • 47.
    Yveborg, Moa
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Bornefalk, Hans
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Task based weights for spectral computed tomography2012In: Progress in Biomedical Optics and Imaging - Proceedings of SPIE, SPIE - International Society for Optical Engineering, 2012, Vol. 8313, p. 831334-Conference paper (Refereed)
    Abstract [en]

    In photon counting multi-bin CT, both projection based weighting and image based weighting (designed to maximize the signal-difference-to-noise ratio SDNR) in the final image, are pixel based and do not account for any spatial frequency dependency of signal and noise among the bins. The same weighting scheme will be used when imaging objects with a large fraction of high spatial frequencies and those with predominantly low spatial frequency content. Any effect on the detectability of a certain target due to correlation between detector elements and bins that might arise in pulse height discriminating systems will not be captured using such an approach. We show how to take the spatial frequency dependency of signal and noise transfer for each bin, and the spatial frequency composition of a target, into account when determining optimal task based weights for photon-counting multi-bin CT imaging using the 2D slice detectability index by applying cascaded system analysis to a spectrally resolved photon counting CT detector system with multiple bins.

  • 48.
    Yveborg, Moa
    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.
    Bornefalk, Hans
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Theoretical comparison of a dual energy system and photon counting silicon detector used for material quantification in spectral CT2015In: IEEE Transactions on Medical Imaging, ISSN 0278-0062, E-ISSN 1558-254X, Vol. 34, no 3, p. 796-806Article in journal (Refereed)
    Abstract [en]

    Any method using dual energy computed tomography (CT) has to make prior assumptions in order to quantify k-edge contrast agents. This work estimates the mean square error (MSE) in contrast agent quantification employing a method based on assigning each reconstructed voxel a ratio of soft tissue and fat using dual energy CT. The results are compared to the MSE using a photon counting silicon detector with multiple bins. The square root of the MSEs of the quantifications of iodine and gadolinium for an object consisting of soft tissue and fat using the silicon detector and dual energy CT range from below 2% and 1% of the contrast agent content for 100 ${rm mg}/{rm cm}^{3}$ of iodine and gadolinium, up to approximately 10% and 13%, and 6% and 4%, for 5 ${rm mg}/{rm cm}^{3}$ of iodine and gadolinium, respectively. When adding bone with a voxel volume fraction of 2.2%, the square root of the MSEs of the quantifications of iodine and gadolinium using dual energy CT increases to 25% and 6%, respectively, for 5 ${rm mg}/{rm cm}^{3}$ of contrast agent. In conclusion, results indicate that the noise levels of the material quantification using the silicon detector are higher than the noise levels using a dual energy CT when the composition of the object is known. However, using a dual energy CT increases the risk of model specification error and subsequently a large bias in contrast agent quantification, a problem which does not exist when using a multi-bin CT where the number of energy bins is larger than two.

  • 49.
    Yveborg, Moa
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Danielsson, Mats E.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Bornefalk, Hans
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Performance evaluation of a sub-millimeter spectrally resolved CT system on pediatric imaging tasks: a simulation2011In: MEDICAL IMAGING 2011: PHYSICS OF MEDICAL IMAGING / [ed] Pelc, NJ; Samei, E; Nishikawa, RM, 2011, Vol. 7961Conference paper (Refereed)
    Abstract [en]

    We are developing a photon counting silicon strip detector with 0.4x0.5 mm(2) square detector elements for clinical CT applications. Except the somewhat limited detection efficiency at higher kVp's the largest discrepancies from ideal spectral behavior have been shown to be Compton interactions in the detector combined with electronic noise. Using the framework of cascaded systems analysis, we reconstruct the 3D MTF and NPS of a silicon strip detector using "optimal" projection based weighting, including the influence of scatter and charge sharing inside the detector. We compare the reconstructed noise and signal characteristics with a reconstructed 3D MTF and NPS of an ideal energy integrating detector by calculating the detectability index for several clinically relevant imaging task. This work demonstrates that although the detection efficiency of the silicon detector rapidly drops for the acceleration voltages encountered in clinical computed tomography practice and the high fraction of Compton interactions due to the low atomic number, silicon detectors can perform on par with ideal energy integrating detectors for routine imaging tasks contaning low frequency components. For imaging task containing high frequency components, silicon detectors can perform approximately 1.4 - 1.8 times better than a fully ideal energy integrating system with unity detection, no scatter or charge sharing inside the detector and 1x1 mm(2) square detector elements.

  • 50.
    Yveborg, Moa
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Hans, Bornefalk
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Mats, Persson
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Optimal frequency-based weighting for spectral x-ray projection imaging2015In: IEEE Transactions on Medical Imaging, ISSN 0278-0062, E-ISSN 1558-254X, Vol. 34, no 3, p. 779-787Article in journal (Refereed)
    Abstract [en]

    The purpose of this work is to derive a weighting scheme that maximizes the frequency-dependent ideal observer signal-difference-to-noise ratio, commonly referred to as detectability index or Hotelling-SDNR, for spectral X-ray projection imaging. Starting from basic statistical decision theory, optimal frequency-dependent weights are derived for a multiple-bin system and the Hotelling-SDNR calculated. A 28% increase in detectability index is found for high frequency objects when applying optimal frequency-dependent weights instead of pixel-based weights to a simplified model of a silicon detector, decreasing towards 0% for low frequency objects. Simulation results indicate a potentially large increase in detectability for high-frequency object imaging using silicon detectors, thus meriting further evaluations on a real system.

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