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  • 1.
    Berggren, Karl
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Cascaded systems analysis of shift-variant image quality in slit-scanning breast tomosynthesis2018In: Medical PhysicsArticle in journal (Refereed)
  • 2.
    Berggren, Karl
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Spectral image quality and applications in breast tomosynthesis2018Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    In the 1970s, it was determined that screening mammography is an efficient tool in fighting the increasing number of women dying from breast cancer, and many countries have established screening programs since then. Mammography systems have improved substantially over the years with one of the major advances being the transition from x-ray film to digital x-ray detectors. Following this development, the number of women dying from breast cancer has decreased, but there is still much room for improvement. One technology that is changing the breast imaging landscape is breast tomosynthesis; tomographic imaging with in-plane resolution similar to that of mammography, albeit limited height resolution. Breast tomosynthesis is commonly implemented with flat-panel detectors, but line detectors in a slit-scanning geometry can also be used. The latter configuration allows for more complex detector technologies, such as spectral photon-counting detectors that enable single-shot spectral imaging. The combination of spectral imaging and tomosynthesis opens up for a range of new applications, but the slit scanning geometry, which differs substantially from that of flat-panel tomosynthesis systems, and the factors affecting image quality have not been well understood. This thesis aims at filling this gap. Image quality and the parameters that influence image quality in spectral photon-counting slit-scanning breast tomosynthesis are characterized and analyzed using cascaded-systems modelling and linear image quality metrics. In addition, the thesis goes into characterizing the x-ray properties of breast tissue, an important input parameter for accurate material decomposition of in-vivo tissue. Material decomposition with spectral imaging opens up a range of applications, such as accurate measurement of volumetric breast density and spectral lesion characterization for decision support as part of mammography screening, and contrast-enhanced K-edge imaging for diagnostics. Tomosynthesis combined with material decomposition has the potential to improve these methods further by, for instance, separating lesions or regions of interest from surrounding fibro-glandular tissue in quantitative 3D maps of breast tissue.

  • 3.
    Berggren, Karl
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging. Philips Mammorgaphy Solutions.
    Cederström, Björn
    Philips Mammography Solutions.
    Lundqvist, Mats
    Philips Mammography Solutions.
    Fredenberg, Erik
    Philips Research.
    Characterization of photon-counting multislit breast tomosynthesis2018In: Medical Physics, E-ISSN 2473-4209Article in journal (Refereed)
    Abstract [en]

    Purpose: It has been shown that breast tomosynthesis may improve sensitivity and specificity compared to two-dimensional mammography, resulting in increased detection-rate of cancers or lowered call-back rates. The purpose of this study is to characterize a spectral photon-counting multislit breast tomosynthesis system that is able to do single-scan spectral imaging with multiple collimated x-ray beams. The system differs in many aspects compared to conventional tomosynthesis using energyintegrating flat-panel detectors. Methods: The investigated system was a prototype consisting of a dual-threshold photon-counting detector with 21 collimated line detectors scanning across the compressed breast. A review of the system is done in terms of detector, acquisition geometry, and reconstruction methods. Three reconstruction methods were used, simple back-projection, filtered back-projection and an iterative algebraic reconstruction technique. The image quality was evaluated by measuring the modulation transfer-function (MTF), normalized noise-power spectrum, detective quantum-efficiency (DQE), and artifact spread-function (ASF) on reconstructed spectral tomosynthesis images for a total-energy bin (defined by a low-energy threshold calibrated to remove electronic noise) and for a high-energy bin (with a threshold calibrated to split the spectrum in roughly equal parts). Acquisition was performed using a 29 kVp W/Al x-ray spectrum at a 0.24 mGy exposure. Results: The difference in MTF between the two energy bins was negligible, that is, there was no energy dependence on resolution. The MTF dropped to 50% at 1.5 lp/mm to 2.3 lp/mm in the scan direction and 2.4 lp/mm to 3.3 lp/mm in the slit direction, depending on the reconstruction method. The full width at half maximum of the ASF was found to range from 13.8 mm to 18.0 mm for the different reconstruction methods. The zero-frequency DQE of the system was found to be 0.72. The fraction of counts in the high-energy bin was measured to be 59% of the total detected spectrum. Scantimes ranged from 4 s to 16.5 s depending on voltage and current settings. Conclusions: The characterized system generates spectral tomosynthesis images with a dual-energy photon-counting detector. Measurements show a high DQE, enabling high image quality at a low dose, which is beneficial for low-dose applications such as screening. The single-scan spectral images open up for applications such as quantitative material decomposition and contrast-enhanced tomosynthesis. 

  • 4.
    Berggren, Karl
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging. Philips Mammography Solutions.
    Cederström, Björn
    Philips Mammography Solutions.
    Lundqvist, Mats
    Philips.
    Fredenberg, Erik
    Philips Research.
    Technical Note: Comparison of first‐ and second‐generation photon‐counting slit‐scanning tomosynthesis systems2018In: Medical PhysicsArticle in journal (Refereed)
    Abstract [en]

    Purpose: Digital breast tomosynthesis (DBT) is an emerging tool for breast-cancer screening and diagnostics. The purpose of this study is to present a second-generation photon-counting slitscanning DBT system and compare it to the first-generation system in terms of geometry and image quality. The study presents the first image-quality measurements on the second-generation system. Method: The geometry of the new system is based on a combined rotational and linear motion, in contrast to a purely rotational scan motion in the first generation. In addition, the calibration routines have been updated. Image quality was measured in the center of the image field in terms of in-slice modulation transfer function (MTF), artifact spread function (ASF), and in-slice detective quantum efficiency (DQE). Images were acquired using a W/Al 29 kVp spectrum at 13 mAs with 2 mm Al additional filtration and reconstructed using simple back-projection. Result: The in-slice 50% MTF was improved in the chest-mammilla direction, going from 3.2 to 3.5 lp/mm, and the zero-frequency DQE increased from 0.71 to 0.77. The MTF and ASF were otherwise found to be on par for the two systems. The new system has reduced in-slice variation of the tomographic angle. Conclusions: The new geometry is less curved, which reduces in-slice tomographic-angle variation, and increases the maximum compression height, making the system accessible for a larger population. The improvements in MTF and DQE were attributed to the updated calibration procedures. We conclude that the second-generation system maintains the key features of the photon-counting system while maintaining or improving image quality and improving the maximum compression height. 

  • 5.
    Berggren, Karl
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging. Philips Healthcare, Sweden.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Fredenberg, Erik
    Philips Healthcare, Sweden.
    Rayleigh imaging in spectral mammography2016In: MEDICAL IMAGING 2016: PHYSICS OF MEDICAL IMAGING, 2016, article id 97830AConference paper (Refereed)
    Abstract [en]

    Spectral imaging is the acquisition of multiple images of an object at different energy spectra. In mammography, dual-energy imaging (spectral imaging with two energy levels) has been investigated for several applications, in particular material decomposition, which allows for quantitative analysis of breast composition and quantitative contrast-enhanced imaging. Material decomposition with dual-energy imaging is based on the assumption that there are two dominant photon interaction effects that determine linear attenuation: the photoelectric effect and Compton scattering. This assumption limits the number of basis materials, i.e. the number of materials that are possible to differentiate between, to two. However, Rayleigh scattering may account for more than 10% of the linear attenuation in the mammography energy range. In this work, we show that a modified version of a scanning multi-slit spectral photon-counting mammography system is able to acquire three images at different spectra and can be used for triple-energy imaging. We further show that triple-energy imaging in combination with the efficient scatter rejection of the system enables measurement of Rayleigh scattering, which adds an additional energy dependency to the linear attenuation and enables material decomposition with three basis materials. Three available basis materials have the potential to improve virtually all applications of spectral imaging.

  • 6.
    Berggren, Karl
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Eriksson, Mikael
    Hall, Per
    Wallis, Matthew
    Fredenberg, Erik
    In-vivo measurement of the effective atomic number of breast skin using spectral mammography2018In: Article in journal (Refereed)
  • 7.
    Berggren, Karl
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging. Philips Healthcare, S-17141 Solna, Sweden.
    Lundqvist, Mats
    Cederstrom, Bjorn
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Fredenberg, Erik
    Physical characterization of photon-counting tomosynthesis2015Conference paper (Refereed)
    Abstract [en]

    Tomosynthesis is emerging as a next generation technology in mammography. Combined with photon-counting detectors with the ability for energy discrimination, a novel modality is enabled - spectral tomosynthesis. Further advantages of photon-counting detectors in the context of tomosynthesis include elimination of electronic noise, efficient scatter rejection (in some geometries) and no lag. Fourier-based linear-systems analysis is a well-established method for optimizing image quality in two-dimensional x-ray systems. The method has been successfully adapted to three-dimensional imaging, including tomosynthesis, but several areas need further investigation. This study focuses on two such areas: 1) Adaption of the methodology to photon-counting detectors, and 2) violation of the shift-invariance and stationarity assumptions in non-cylindrical geometries. We have developed a Fourier-based framework to study the image quality in a photon-counting tomosynthesis system, assuming locally linear, stationary, and shift-invariant system response. The framework includes a cascaded-systems model to propagate the modulation-transfer function (MTF) and noise-power spectrum (NPS) through the system. The model was validated by measurements of the MTF and NPS. High degrees of non-shift invariance and non-stationarity were observed, in particular for the depth resolution as the angle of incidence relative the reconstruction plane varied throughout the imaging volume. The largest effects on image quality in a given point in space were caused by interpolation from the inherent coordinate system of the x-rays to the coordinate system that was used for reconstruction. This study is part of our efforts to fully characterize the spectral tomosynthesis system, we intend to extend the model further to include the detective-quantum efficiency, observer modelling, and spectral effects.

  • 8.
    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.

  • 9.
    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.

  • 10.
    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.

  • 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, 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.

  • 13.
    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.

  • 14.
    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.

  • 15.
    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.

  • 16.
    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.

  • 17.
    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.

  • 18.
    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.

  • 19. Cederström, B.
    et al.
    Fredenberg, E.
    Berggren, Karl
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging. Mammography Solutions, Philips, Sweden.
    Erhard, K.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Wallis, M.
    Lesion characterization in spectral photon-counting tomosynthesis2017In: Medical Imaging 2017: Physics of Medical Imaging, SPIE - International Society for Optical Engineering, 2017, Vol. 10132, article id 1013205Conference paper (Refereed)
    Abstract [en]

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

  • 20.
    Cederström, Björn
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Fredenberg, Erik
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Lundqvist, Mats
    Sectra Mamea AB, Solna, Sweden.
    Ericson, Tove
    Sectra Mamea AB, Solna, Sweden.
    Åslund, Magnus
    Sectra Mamea AB, Solna, Sweden.
    Observer-model optimization of X-ray system in photon-counting breast imaging2011In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 648, no Supplement 1, p. S54-S57Article in journal (Refereed)
    Abstract [en]

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

  • 21.
    Cederström, Björn
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Streubuehr, Ursula
    Comparison of photon-counting to storage phosphor plate mammography using contrast-detail phantom analysis2007In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 580, no 2, p. 1101-1104Article in journal (Refereed)
    Abstract [en]

    Two digital mammography systems, one based on scanning photon counting silicon detectors and the other on storage phosphor plates (CR), have been compared in terms of image quality and dose. Sets of images of a contrast-detail phantom (CDMAM3.4) were acquired for each system and dose level. The images were evaluated in the disc diameter range 0.16-1 mm using a computer program (CDcom) and the results were fitted to a psychometric curve for each disc diameter. The contrast-detail curve was summarized into one single figure of merit, the image quality index, and the dose efficiency was calculated. The errors of the calculated parameters were assessed using statistical analysis. It was found that the scanning photon-counting system can achieve the same image quality as the storage phosphor plate (CR) system at 30-38% of the average glandular dose.

  • 22.
    Chen, Han
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Characterization and Optimization of Silicon-strip Detectors for Mammography and Computed Tomography2016Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The goal in medical x-ray imaging is to obtain the image quality requiredfor a given detection task, while ensuring that the patient dose is kept as lowas reasonably achievable. The two most common strategies for dose reductionare: optimizing incident x-ray beams and utilizing energy informationof transmitted beams with new detector techniques (spectral imaging). Inthis thesis, dose optimization schemes were investigated in two x-ray imagingsystems: digital mammography and computed tomography (CT).

    In digital mammography, the usefulness of anti-scatter grids was investigatedas a function of breast thickness with varying geometries and experimentalconditions. The general conclusion is that keeping the grid is optimalfor breasts thicker than 5 cm, whereas the dose can be reduced without a gridfor thinner breasts.

    A photon-counting silicon-strip detector developed for spectral mammographywas characterized using synchrotron radiation. Energy resolution, ΔE/Ein, was measured to vary between 0.11-0.23 in the energy range 15-40 keV, which is better than the energy resolution of 0.12-0.35 measured inthe state-of-the-art photon-counting mammography system. Pulse pileup hasshown little effect on energy resolution.

    In CT, the performance of a segmented silicon-strip detector developedfor spectral CT was evaluated and a theoretical comparison was made withthe state-of-the-art CT detector for some clinically relevant imaging tasks.The results indicate that the proposed photon-counting silicon CT detector issuperior to the state-of-the-art CT detector, especially for high-contrast andhigh-resolution imaging tasks.

    The beam quality was optimized for the proposed photon-counting spectralCT detector in two head imaging cases: non-enhanced imaging and Kedgeimaging. For non-enhanced imaging, a 120-kVp spectrum filtered by 2half value layer (HVL) copper (Z = 29) provides the best performance. Wheniodine is used in K-edge imaging, the optimal filter is 2 HVL iodine (Z = 53)and the optimal kVps are 60-75 kVp. In the case of gadolinium imaging, theradiation dose can be minimized at 120 kVp filtered by 2 HVL thulium (Z =69).

  • 23.
    Chen, Han
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Xu, Cheng
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Persson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Karlsson, Staffan
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    A photon-counting silicon-strip detector for digital mammography with an ultrafast 0.18-mu m CMOS ASIC2014In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 749, p. 1-6Article in journal (Refereed)
    Abstract [en]

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

  • 24.
    Chen, Han
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    On imaging with or without grid in digital mammography2014In: Proceedings of SPIE, the International Society for Optical Engineering, ISSN 0277-786X, E-ISSN 1996-756X, Vol. 9033, p. 903346-Article in journal (Refereed)
    Abstract [en]

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

  • 25.
    Chen, Han
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Xu, Cheng
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Size-dependent scanning parameters (kVp and mAs) for photon-counting spectral CT system in pediatric imaging: simulation study2016In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 61, no 11Article in journal (Refereed)
    Abstract [en]

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

  • 26.
    Chen, Han
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Xu, Cheng
    KTH, School of Engineering Sciences (SCI), Physics.
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Physics.
    On image quality metrics and the usefulness of grids in digital mammography2015In: Journal of medical imaging (Bellingham, Wash.), ISSN 2329-4302, Vol. 2, no 1, p. 013501-013501Article in journal (Refereed)
    Abstract [en]

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

  • 27.
    Chen, Han
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Xu, Cheng
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Persson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Optimization Of Beam Quality For Photon-Counting Spectral Computed Tomography In Head Imaging: Simulation Study2015In: Journal of Medical Imaging, ISSN 2329-4302, E-ISSN 2329-4310, Vol. 2, no 4, p. 043504-1-043504-16, article id 043504Article in journal (Refereed)
    Abstract [en]

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

  • 28.
    Dahlman, Nils
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Evaluation of Photon-Counting Spectral Breast Tomosynthesis2011Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    The superposition of anatomical structures often greatly impedes detectability in conventional mammography. Spectral imaging and tomosynthesis are two promising methods used for suppression of the anatomical background. The aim of this thesis is to compare and evaluate the benefits of tomosynthesis and spectral imaging, both in combination and separately. A computer model for signal and noise transfer in tomosynthesis was developed and combined with an existing model for spectral imaging. Measurements were performed to validate the models. An ideal-observer detectability index incorporating anatomical noise was used as a figure of merit to compare the different modalities. For detection of a contrast-enhanced tumor in a breast with high anatomical background, the optimum performance for spectral tomosynthesis was found at a tomo-angle of 10 degrees. The improvement was in the order of a factor 10 compared to non-energy-resolved tomosynthesis with the same angular extent. This was supported by clinical results.

  • 29.
    Dahlman, Nils
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Fredenberg, Erik
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Aslund, Magnus
    Lundqvist, Mats
    Diekmann, Felix
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Evaluation of photon-counting spectral breast tomosynthesis2011In: MEDICAL IMAGING 2011: PHYSICS OF MEDICAL IMAGING / [ed] Pelc, NJ; Samei, E; Nishikawa, RM, 2011, Vol. 7961Conference paper (Refereed)
    Abstract [en]

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

  • 30.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Challenges and Opportunities with Photon Counting CT2012In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 39, no 6, p. 3989-3989Article in journal (Other academic)
  • 31.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Collimator element2001Patent (Other (popular science, discussion, etc.))
    Abstract [en]

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

  • 32.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    MAMMOGRAPHY SHIFTS TOWARD SPECTRAL IMAGING-Photon counting is an intuitive way to detect x-rays, which by nature are digital and have a color spectrum2009In: Diagnostic Imaging, Vol. 25, no 7Article in journal (Refereed)
  • 33.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    MO‐D‐210A‐01: Photon Counting Detectors for Mammography2009In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 36, no 6, p. 2699-2699Article in journal (Refereed)
    Abstract [en]

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

  • 34.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Philips Microdose Mammography - the Technology and Physics Behind the First FDA Approved Photon Counting X-Ray Imaging System2012In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 39, no 6, p. 4017-4017Article in journal (Other academic)
    Abstract [en]

    Purpose: To validate the use of 4D‐Computed Tomography (4D‐CT) for pre‐treatment evaluation of fractional regional ventilation in patients with lung cancer by benchmarking its performance against scintigraphy V/Q imaging, the current gold‐standard. The second aim is to further corroborate the results of 4D‐CT estimation of lung aeration against the results of Pulmonary Function Testing. Methods: Scintigraphy V/Q and 4D‐CT studies were acquired in four lung cancer patients prior to treatment with radiation therapy. PFTs were acquired in 3 out of the 4 patients. 4D‐CT images were used to create 3D fractional regional ventilation maps by applying a ‘mass correction’ and subtracting the spatially matched end‐exhale and end‐inhale images. Ventilation maps were then collapsed in the anterior‐posterior dimension to create a coronal 2D projection image consistent with the scintigraphy V/Q images. The left and right lung fields were isolated on the projection image and divided into 3 sections of equal height. Summation of the signal intensity in each of the sections was carried out on the maps analogous to the analysis performed on V/Q scans and statistically compared using the Kendall's tau rank correlation. Results: The non‐parametric Kendall's tau estimate ranged between 0.87–0.95 for N=4, with corresponding p‐values ranging between 0.005–0.0002. Mean functional residual capacities (FRC) from the PFTs (N=3) versus calculated FRCs was 2.7 +/− 0.6 L and 2.4 +/− 0.7 L, and the null hypothesis could not be rejected (p = 0.61). The mean fractional regional ventilation versus the ratio of tidal‐volume/FRC was 0.24 +/− 0.11 and 0.22 +/− 0.08, and the null hypothesis could not be rejected (p=0.73). Conclusions: There was a strong correlation between 4D‐CT and scintigraphy V/Q. The similarity between the calculated and measured FRCs further validates the utility of 4D‐CT and supports its use in evaluating lung ventilation in patients with pulmonary neoplasms.

  • 35.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    System and method for imaging using radio-labeled substances, especially suitable for studying of biological processes2006Patent (Other (popular science, discussion, etc.))
    Abstract [en]

    An imaging system is based on in-line x-ray optics arranged in combination with an X-ray detector to detect radiation from radio-labeled substances within an object to be imaged. This arrangement will provide a nuclear imaging device with potentially orders of magnitude higher resolution and efficiency and it will moreover be relatively easy to align and to produce and assemble in large quantities.

  • 36.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    TH‐A‐217BCD‐01: Challenges and Opportunities with Photon Counting CT2012In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 39, no 6, p. 3989-3989Article in journal (Refereed)
    Abstract [en]

    There is currently a large interest in photon counting CT detector research in both academia and industry. There are several detector systems and strategies to handle major challenges such as the very high count‐rate, while the energy information for each photon is retained. Another challenge is cross talk, which may compromise the energy estimation for the photons and can cause double counts, which gets worse with smaller pixel size. If implemented in the clinic, photon counting CT will likely enable a dose reduction when this is important, as for example in pediatric CT. Photon counting CT will also make possible quantitative measurements, energy weighting and/or tissue decomposition techniques that can be of great importance for a number of imaging tasks.

  • 37.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    TH‐E‐217A‐01: Philips Microdose Mammography ‐ the Technology and Physics Behind the First FDA Approved Photon Counting X‐Ray Imaging System2012In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 39, no 6, p. 4017-4017Article in journal (Refereed)
    Abstract [en]

    Purpose: To validate the use of 4D‐Computed Tomography (4D‐CT) for pre‐treatment evaluation of fractional regional ventilation in patients with lung cancer by benchmarking its performance against scintigraphy V/Q imaging, the current gold‐standard. The second aim is to further corroborate the results of 4D‐CT estimation of lung aeration against the results of Pulmonary Function Testing. Methods: Scintigraphy V/Q and 4D‐CT studies were acquired in four lung cancer patients prior to treatment with radiation therapy. PFTs were acquired in 3 out of the 4 patients. 4D‐CT images were used to create 3D fractional regional ventilation maps by applying a ‘mass correction’ and subtracting the spatially matched end‐exhale and end‐inhale images. Ventilation maps were then collapsed in the anterior‐posterior dimension to create a coronal 2D projection image consistent with the scintigraphy V/Q images. The left and right lung fields were isolated on the projection image and divided into 3 sections of equal height. Summation of the signal intensity in each of the sections was carried out on the maps analogous to the analysis performed on V/Q scans and statistically compared using the Kendall's tau rank correlation. Results: The non‐parametric Kendall's tau estimate ranged between 0.87–0.95 for N=4, with corresponding p‐values ranging between 0.005–0.0002. Mean functional residual capacities (FRC) from the PFTs (N=3) versus calculated FRCs was 2.7 +/− 0.6 L and 2.4 +/− 0.7 L, and the null hypothesis could not be rejected (p = 0.61). The mean fractional regional ventilation versus the ratio of tidal‐volume/FRC was 0.24 +/− 0.11 and 0.22 +/− 0.08, and the null hypothesis could not be rejected (p=0.73). Conclusions: There was a strong correlation between 4D‐CT and scintigraphy V/Q. The similarity between the calculated and measured FRCs further validates the utility of 4D‐CT and supports its use in evaluating lung ventilation in patients with pulmonary neoplasms.

  • 38.
    Danielsson, Mats
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Cederwall, Bo
    KTH, School of Engineering Sciences (SCI), Physics, Nuclear Physics.
    Bohm, Christian
    3rd International Conference on Imaging Techniques in Subatomic Physics, Astrophysics, Medicine, Biology and Industry - Stockholm, Sweden, June 27-30, 2006: Preface2007In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 580, no 2, p. V-VArticle in journal (Other academic)
  • 39.
    Fang, Yuan
    et al.
    US FDA, Div Radiol Hlth, Off In Vitro Diagnost & Radiol Hlth, Ctr Devices & Radiol Hlth, 10903 New Hampshire Ave, Silver Spring, MD 20993 USA..
    Xu, Cheng
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Yao, Yuan
    Stanford Univ, Dept Bioengn, Stanford, CA 94305 USA.;Stanford Univ, Dept Radiol, Stanford, CA 94305 USA..
    Pelc, Norbert
    Stanford Univ, Dept Bioengn, Stanford, CA 94305 USA.;Stanford Univ, Dept Radiol, Stanford, CA 94305 USA.;Stanford Univ, Dept Elect Engn, Stanford, CA 94305 USA..
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Badano, Aldo
    US FDA, Div Imaging Diagnost & Software Reliabil, Off Sci & Engn Labs, Ctr Devices & Radiol Hlth, 10903 New Hampshire Ave, Silver Spring, MD 20993 USA..
    Modeling charge transport in photon-counting detectors2018In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 899, p. 115-121Article in journal (Refereed)
    Abstract [en]

    The purpose of this study is to review and compare simulation methods for describing the transport of charge clouds in silicon based semiconductor detectors and investigate the effects on energy spectrum for silicon based photon-counting strip detectors. Charge clouds and detailed carrier transport are simulated and compared using two different approaches including analytical and Monte Carlo schema. The results of the simulations are evaluated using pulse-height spectra (PHS) for a silicon strip detector with edge on geometry at two energies (25 and 75 keV) at various X-ray absorption locations relative to the pixel boundary and detector depth. The findings confirm carrier diffusion plays a large role in the charge sharing effect in photon counting detectors, in particular when the photon is absorbed near the pixel boundary far away from the pixel electrode. The results are further compared in terms of the double-counting probability for X-ray photons absorbed near the pixel boundary as a function of the threshold energy. Monte Carlo and analytical models show reasonable agreement (2% relative error in swank factor) for charge sharing effects for a silicon strip detector with edge-on geometry. For 25 keV mono-energetic photons absorbed at 5 mu m from the pixel boundary, the theoretical threshold energy at 10% double-counting probability based on charge sharing is 5.5, 8.5 and 9.2 keV for absorption depths of 50, 250 and 450 mu m from the electrode, respectively. The transport of charge clouds affects the spectral characteristics of photon counting detectors and the double-counting probability results show the theoretical threshold energy to avoid double-counting as a function of X-ray energy and X-ray interaction locations for silicon and can be considered for future studies of charge sharing effects.

  • 40. Fredenberg, E.
    et al.
    Berggren, Karl
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging. Philips Health Systems, Sweden.
    Bartels, M.
    Erhard, K.
    Volumetric breast-density measurement using spectral photon-counting tomosynthesis: First clinical results2016In: 13th International Workshop on Breast Imaging, IWDM 2016, Springer, 2016, p. 576-584Conference paper (Refereed)
    Abstract [en]

    Measurements of breast density have the potential to improve the efficiency and reduce the cost of screening mammography through personalized screening. Breast density has traditionally been evaluated from the dense area in a mammogram, but volumetric assessment methods, which measure the volumetric fraction of fibro-glandular tissue in the breast, are potentially more consistent and physically sound. The purpose of the present study is to evaluate a method for measuring the volumetric breast density using photon-counting spectral tomosynthesis. The performance of the method was evaluated using phantom measurements and clinical data from a small population (n = 18). The precision was determined to be 2.4 percentage points (pp) of volumetric breast density. Strong correlations were observed between contralateral (R2 = 0.95) and ipsilateral (R2 = 0.96) breast-density measurements. The measured breast density was anticorrelated to breast thickness, as expected, and exhibited a skewed distribution in the range [3.7%, 55%] and with a median of 18%. We conclude that the method yields promising results that are consistent with expectations. The relatively high precision of the method may enable novel applications such as treatment monitoring.

  • 41.
    Fredenberg, Erik
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging.
    Modern teknik kan förbättra mammografin2010In: Läkartidningen, ISSN 0023-7205, E-ISSN 1652-7518, Vol. 107, no 8, p. 499-Article in journal (Refereed)
  • 42.
    Fredenberg, Erik
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Spectral Mammography with X-Ray Optics and a Photon-Counting Detector2009Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Early detection is vital to successfully treating breast cancer, and mammography screening is the most efficient and wide-spread method to reach this goal. Imaging low-contrast targets, while minimizing the radiation exposure to a large population is, however, a major challenge. Optimizing the image quality per unit radiation dose is therefore essential. In this thesis, two optimization schemes with respect to x-ray photon energy have been investigated: filtering the incident spectrum with refractive x-ray optics (spectral shaping), and utilizing the transmitted spectrum with energy-resolved photon-counting detectors (spectral imaging).

    Two types of x-ray lenses were experimentally characterized, and modeled using ray tracing, field propagation, and geometrical optics. Spectral shaping reduced dose approximately 20% compared to an absorption-filtered reference system with the same signal-to-noise ratio, scan time, and spatial resolution. In addition, a focusing pre-object collimator based on the same type of optics reduced divergence of the radiation and improved photon economy by about 50%.

    A photon-counting silicon detector was investigated in terms of energy resolution and its feasibility for spectral imaging. Contrast-enhanced tumor imaging with a system based on the detector was characterized and optimized with a model that took anatomical noise into account. Improvement in an ideal-observer detectability index by a factor of 2 to 8 over that obtained by conventional absorption imaging was found for different levels of anatomical noise and breast density. Increased conspicuity was confirmed by experiment. Further, the model was extended to include imaging of unenhanced lesions. Detectability of microcalcifications increased no more than a few percent, whereas the ability to detect large tumors might improve on the order of 50% despite the low attenuation difference between glandular and cancerous tissue. It is clear that inclusion of anatomical noise and imaging task in spectral optimization may yield completely different results than an analysis based solely on quantum noise.

  • 43. Fredenberg, Erik
    et al.
    Berggren, Karl
    KTH, School of Engineering Sciences (SCI), Physics, Physics of Medical Imaging. Philips Health Systems, Sweden.
    Bartels, Matthias
    Erhard, Klaus
    Volumetric Breast-Density Measurement Using Spectral Photon-Counting Tomosynthesis: First Clinical Results2016In: Breast Imaging, IWDM 2016, Springer, 2016, p. 576-584Conference paper (Refereed)
    Abstract [en]

    Measurements of breast density have the potential to improve the efficiency and reduce the cost of screening mammography through personalized screening. Breast density has traditionally been evaluated from the dense area in a mammogram, but volumetric assessment methods, which measure the volumetric fraction of fibro-glandular tissue in the breast, are potentially more consistent and physically sound. The purpose of the present study is to evaluate a method for measuring the volumetric breast density using photon-counting spectral tomo-synthesis. The performance of the method was evaluated using phantom measurements and clinical data from a small population (n = 18). The precision was determined to be 2.4 percentage points (pp) of volumetric breast density. Strong correlations were observed between contralateral (R-2 = 0.95) and ipsilateral (R-2 = 0.96) breast-density measurements. The measured breast density was anti-correlated to breast thickness, as expected, and exhibited a skewed distribution in the range [3.7 %, 55 %] and with a median of 18 %. We conclude that the method yields promising results that are consistent with expectations. The relatively high precision of the method may enable novel applications such as treatment monitoring.

  • 44.
    Fredenberg, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Energy filtering with x-ray lenses: Optimization for photon-counting mammography2010In: Radiation Protection Dosimetry, ISSN 0144-8420, E-ISSN 1742-3406, Vol. 139, p. 339-342Article in journal (Refereed)
    Abstract [en]

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

  • 45.
    Fredenberg, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Lundqvist, Mats
    Ribbing, Carolina
    Åslund, Magnus
    Diekmann, Felix
    Nishikawa, Robert
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Contrast-enhanced dual-energy subtraction imaging using electronic spectrum-splitting and multi-prism x-ray lenses2008In: Medical Imaging 2008 - Physics of Medical Imaging: PTS 1-3 / [ed] Hsieh, J; Samei, E, 2008, Vol. 6913, p. 91310-91310Conference paper (Refereed)
    Abstract [en]

    Dual-energy subtraction imaging (DES) is a method to improve the detectability of contrast agents over a lumpy background. Two images, acquired at x-ray energies above and below an absorption edge of the agent material, are logarithmically subtracted, resulting in suppression of the signal from the tissue background and a relative enhancement of the signal from the agent. Although promising, DES is still not widely used in clinical practice. One reason may be the need for two distinctly separated x-ray spectra that are still close to the absorption edge, realized through dual exposures which may introduce motion unsharpness. In this study, electronic spectrum-splitting with a silicon-strip detector is theoretically and experimentally investigated for a mammography model with iodinated contrast agent. Comparisons are made to absorption imaging and a near-ideal detector using a signal-to-noise ratio that includes both statistical and structural noise. Similar to previous studies, heavy absorption filtration was needed to narrow the spectra at the expense of a large reduction in x-ray flux. Therefore, potential improvements using a chromatic multi-prism x-ray lens (MPL) for filtering were evaluated theoretically. The MPL offers a narrow tunable spectrum, and we show that the image quality can be improved compared to conventional filtering methods.

  • 46.
    Fredenberg, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Nillius, Peter
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Ribbing, Carolina
    Uppsala Univ..
    Karlsson, Staffan
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    A low-absorption x-ray energy filter for small-scale applications2009In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 17, no 14, p. 11388-11398Article in journal (Refereed)
    Abstract [en]

    We present an experimental and theoretical evaluation of an x-ray energy filter based on the chromatic properties of a prism-array lens (PAL). It is intended for small-scale applications such as medical imaging. The PAL approximates a Fresnel lens and allows for high efficiency compared to filters based on ordinary refractive lenses, however at the cost of a lower energy resolution. Geometrical optics was found to provide a good approximation for the performance of a flawless lens, but a field-propagation model was used for quantitative predictions. The model predicted a 0.29 ΔE/E energy resolution and an intensity gain of 6.5 for a silicon PAL at 23.5 keV. Measurements with an x-ray tube showed good agreement with the model in energy resolution and peak energy, but a blurred focal line contributed to a 29% gain reduction. We believe the blurring to be caused mainly by lens imperfections, in particular at the periphery of the lens.

  • 47.
    Fredenberg, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Applied Physics, Biomedical and X-ray Physics.
    Ribbing, Carolina
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Prism-array lenses for energy filtering in medical X-ray imaging: Physics of Medical Imaging, Pts 1-32007In: Medical Imaging 2007: Physics of Medical Imaging, Pts 1-3 / [ed] Hsieh, J; Flynn, MJ, BELLINGHAM: SPIE-INT SOC OPTICAL ENGINEERING , 2007, Vol. 6510, p. U270-U281Conference paper (Refereed)
    Abstract [en]

    Conventional energy filters for x-ray imaging are based on absorbing materials which attenuate low energy photons, sometimes combined with an absorption edge, thus also discriminating towards 'photons of higher energies. These filters are fairly inefficient, in particular for photons of higher energies, and other methods for achieving a narrower bandwidth have been proposed. Such methods include various types of monochromators, based on for instance mosaic crystals or refractive multi-prism x-ray lenses (MPL's). Prism-array lenses (PAL's) are similar to MPL's, but are shorter, have larger apertures, and higher transmission. A PAL consists of a number of small prisms arranged in columns perpendicular to the optical axis. The column height decreases along the optical axis so that the projection of lens material is approximately linear with a Resnel phase-plate pattern superimposed on it. The focusing effect is one dimensional, and the lens is chromatic. Hence, unwanted energies can be blocked by placing a slit in the image plane of a desired energy. We present the first experimental and theoretical results on an energy filter based on a silicon PAL. The study includes an evaluation of the spectral shaping properties of the filter as well as a quantification of the achievable increase in dose efficiency compared to standard methods. Previously, PAL's have been investigated with synchrotron radiation, but in this study a medical imaging setup, based on a regular x-ray tube, is considered.

  • 48.
    Fredenberg, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Åslund, Magnus
    KTH, School of Engineering Sciences (SCI), Physics.
    Nillius, Peter
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    An efficient pre-object collimator based on an x-ray lens2009In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 36, no 2, p. 626-633Article in journal (Refereed)
    Abstract [en]

    A multiprism lens (MPL) is a refractive x-ray lens with one-dimensional focusing properties. If used as a pre-object collimator in a scanning system for medical x-ray imaging, it reduces the divergence of the radiation and improves on photon economy compared to a slit collimator. Potential advantages include shorter acquisition times, a reduced tube loading, or improved resolution. We present the first images acquired with a MPL in a prototype for a scanning mammography system. The lens showed a gain of flux of 1.32 compared to a slit collimator at equal resolution, or a gain in resolution of 1.31–1.44 at equal flux. We expect the gain of flux in a clinical setup with an optimized MPL and a custom-made absorption filter to reach 1.67, or 1.45–1.54 gain in resolution.

  • 49.
    Fredenberg, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Åslund, Magnus
    Nillius, Peter
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Lundqvist, Mats
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Imaging with multi-prism x-ray lenses2008In: Medical Imaging 2008 - Physics of Medical Imaging: PTS 1-3 / [ed] Hsieh, J; Samei, E, 2008, Vol. 6913, p. 91308-91308Conference paper (Refereed)
    Abstract [en]

    The multi-prism lens (MPL) is a refractive x-ray lens consisting of two rows of prisms facing each other at an angle. Rays entering the lens at the periphery will encounter a larger number of prisms than will central ones, hence experiencing a greater refraction. The focusing effect of the MPL can be used to gather radiation from a large aperture onto a smaller detector, and accordingly to make better use of the available x-ray flux in medical x-ray imaging. Potential advantages of a better photon economy include shorter acquisition times, a reduced tube loading, or an improved resolution. Since the focusing effect is one-dimensional it matches the design of scanning systems. In this study we present the first images acquired with an MPL instead of the pre-breast slit collimator in a scanning mammography system. According to the measurements, the MPL is able to increase the flux 32% at equal resolution compared to the slit collimator, or to improve the resolution 2.4 mm(-1) at equal flux. If used with a custom-made absorption filter in a clinical set-up, the gain of flux of the MPL is expected to be at least 45%, and the corresponding improvement in resolution to be 3 mm(-1).

  • 50.
    Fredenberg, Erik
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Cederström, Björn
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    Åslund, Magnus
    Ribbing, Carolina
    Uppsala universitet.
    Danielsson, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Medical Imaging.
    A Tunable Energy Filter for Medical X-Ray Imaging2008In: X-Ray Optics and Instrumentation, ISSN 1687-7632, Vol. 2008Article in journal (Refereed)
    Abstract [en]

    A multiprism lens (MPL) is a refractive X-ray lens, and its chromatic properties can be employed in an energy filtering setup to obtain a narrow tunable X-ray spectrum. We present the first evaluation of such a filter for medical X-ray imaging. The experimental setup yields a 6.6 gain of flux at 20 keV, and we demonstrate tunability by altering the energy spectrum to center also around 17 and 23 keV. All measurements are found to agree well with ray-tracing and a proposed geometrical model. Compared to a model mammography system with absorption filtering, the experimental MPL filter reduces dose 13–25% for 3–7 cm breasts if the spectrum is centered around the optimal energy. Additionally, the resolution is improved 2.5 times for a 5 cm breast. The scan time is increased 3 times but can be reduced with a slightly decreased energy filtering and resolution.

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