Mats Persson

All Publications

• Bias-variance tradeoff in anticorrelated noise reduction for spectral CT. Medical physics Persson, M., Grönberg, F. 2017; 44 (9): e242–e254

  Abstract

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

  View details for DOI 10.1002/mp.12322

  View details for PubMedID 28901607

• Spectral performance and effect of spatial-energetic correlation in PCD with different converter materials Proceedings of The 14th International Meeting on Fully Three-Dimensional Image Reconstruction in Radiology and Nuclear Medicine Li, X., Rajbhandary, P. L., Wang, J., Persson, M., Wu, Z., Pelc, N. J. 2017: 795–800
• Spectral CT reconstruction with anti-correlated noise model and joint prior Proceedings of The 14th International Meeting on Fully Three-Dimensional Image Reconstruction in Radiology and Nuclear Medicine Persson, M., Adler, J. 2017: 580–585
• Optimal sinogram sampling with temporally offset pixels in continuous rotation CT Proc. SPIE 10132, Medical Imaging 2017: Physics of Medical Imaging Sjölin, M., Persson, M. 2017

  View details for DOI 10.1117/12.2254329

• Resolution improvement in x-ray imaging with an energy-resolving detector Proc. SPIE 10132, Medical Imaging 2017: Physics of Medical Imaging, 101321D Persson, M., Danielsson, M. 2017

  View details for DOI 10.1117/12.2254572

• Upper limits of the photon fluence rate on CT detectors: Case study on a commercial scanner MEDICAL PHYSICS Persson, M., Bujila, R., Nowik, P., Andersson, H., Kull, L., Andersson, J., Bornefalk, H., Danielsson, M. 2016; 43 (7): 4398-4411

  Abstract

  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.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.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.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.

  View details for DOI 10.1118/1.4954008

  View details for Web of Science ID 000379171900044

  View details for PubMedID 27370155

• Spatial-frequency-domain study of anticorrelated noise reduction in spectral CT Proc. 4th Intl. Mtg. on image formation in X-ray CT Persson, M., Grönberg, F. 2016: 283–286
• Optimization of beam quality for photon-counting spectral computed tomography in head imaging: simulation study. Journal of medical imaging (Bellingham, Wash.) Chen, H., Xu, C., Persson, M., Danielsson, M. 2015; 2 (4): 043504-?

  Abstract

  Head computed tomography (CT) plays an important role in the comprehensive evaluation of acute stroke. Photon-counting spectral detectors, as promising candidates for use in the next generation of x-ray CT systems, 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-selective images into several basis function images that are inherently free of beam-hardening artifacts, a potential advantage for further improving the diagnosis accuracy. We are developing a photon-counting spectral detector for CT applications. The purpose of this work is to determine the optimal beam quality for material decomposition in two head imaging cases: nonenhanced imaging and K-edge imaging. A cylindrical brain tissue of 16-cm diameter, coated by a 6-mm-thick bone layer and 2-mm-thick skin layer, was used as a head phantom. The imaging target 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 ([Formula: see text]) were studied. Three parameters that affect beam quality were evaluated: kVp settings (50 to 130 kVp), filter materials ([Formula: see text] to 83), and filter thicknesses [0 to 2 half-value layer (HVL)]. The image qualities resulting from the varying x-ray beams were compared in terms of two figures of merit (FOMs): squared signal-difference-to-noise ratio normalized by brain dose ([Formula: see text]) and that normalized by skin dose ([Formula: see text]). For nonenhanced imaging, the results show that the use of the 120-kVp spectrum filtered by 2 HVL copper ([Formula: see text]) provides the best performance in both FOMs. When iodine is used in K-edge imaging, the optimal filter is 2 HVL iodine ([Formula: see text]) and the optimal kVps are 60 kVp in terms of [Formula: see text] and 75 kVp in terms of [Formula: see text]. A tradeoff of 65 kVp was proposed to lower the potential risk of skin injuries if a relatively long exposure time is necessarily performed in the iodinated imaging. In the case of gadolinium imaging, both SD and BD can be minimized at 120 kVp filtered with 2 HVL thulium ([Formula: see text]). The results also indicate that with the same concentration and their respective optimal spectrum, the values of [Formula: see text] and [Formula: see text] in gadolinium imaging are, respectively, around 3 and 10 times larger than those in iodine imaging. However, since gadolinium is used in much lower concentrations than iodine in the clinic, iodine may be a preferable candidate for K-edge imaging.

  View details for DOI 10.1117/1.JMI.2.4.043504

  View details for PubMedID 26835495

• Spectral response model for a multibin photon-counting spectral computed tomography detector and its applications. Journal of medical imaging (Bellingham, Wash.) Liu, X., Persson, M., Bornefalk, H., Karlsson, S., Xu, C., Danielsson, M., Huber, B. 2015; 2 (3): 033502-?

  Abstract

  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.

  View details for DOI 10.1117/1.JMI.2.3.033502

  View details for PubMedID 26839904

• Optimal Frequency-Based Weighting for Spectral X-Ray Projection Imaging IEEE TRANSACTIONS ON MEDICAL IMAGING Yveborg, M., Persson, M., Bornefalk, H. 2015; 34 (3): 779-787

  Abstract

  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.

  View details for DOI 10.1109/TMI.2014.2360932

  View details for Web of Science ID 000350870700010

  View details for PubMedID 25291789

• Allowable Forward Model Misspecification for Accurate Basis Decomposition in a Silicon Detector Based Spectral CT IEEE TRANSACTIONS ON MEDICAL IMAGING Bornefalk, H., Persson, M., Danielsson, M. 2015; 34 (3): 788-795

  Abstract

  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.

  View details for DOI 10.1109/TMI.2014.2361680

  View details for Web of Science ID 000350870700011

  View details for PubMedID 25314697

• Energy Calibration of a Silicon-Strip Detector for Photon-Counting Spectral CT by Direct Usage of the X-ray Tube Spectrum IEEE TRANSACTIONS ON NUCLEAR SCIENCE Liu, X., Chen, H., Bornefalk, H., Danielsson, M., Karlsson, S., Persson, M., Xu, C., Huber, B. 2015; 62 (1): 68-75

  View details for DOI 10.1109/TNS.2014.2373641

  View details for Web of Science ID 000349672700008

• Third material separation in spectral CT with basis decomposition Proceedings of The 13th International Meeting on Fully Three-Dimensional Image Reconstruction in Radiology and Nuclear Medicine Grönberg, F., Persson, M., Bornefalk, H. 2015: 499–501
• Modelling the channel-wise count response of a photon-counting spectral CT detector to a broad x-ray spectrum MEDICAL IMAGING 2015: PHYSICS OF MEDICAL IMAGING Liu, X., Chen, H., Bornefalk, H., Danielsson, M., Karlson, S., Persson, M., Xu, C., Huber, B. 2015; 9412

  View details for DOI 10.1117/12.2081776

  View details for Web of Science ID 000355581700038

• Energy-resolved CT imaging with a photon-counting silicon-strip detector PHYSICS IN MEDICINE AND BIOLOGY Persson, M., Huber, B., Karlsson, S., Liu, X., Chen, H., Xu, C., Yveborg, M., Bornefalk, H., Danielsson, M. 2014; 59 (22): 6709-6727

  Abstract

  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 0.5×0.4 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.

  View details for DOI 10.1088/1361-6560/59/22/6709

  View details for Web of Science ID 000344091000005

  View details for PubMedID 25327497

• A photon-counting silicon-strip detector for digital mammography with an ultrafast 0.18-mu m CMOS ASIC NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT Chen, H., Cederstrom, B., Xu, C., Persson, M., Karlsson, S., Danielsson, M. 2014; 749: 1-6

  View details for DOI 10.1016/j.nima.2014.02.033

  View details for Web of Science ID 000334075000001

• A Silicon-Strip Detector for Photon-Counting Spectral CT: Energy Resolution From 40 keV to 120 keV IEEE TRANSACTIONS ON NUCLEAR SCIENCE Liu, X., Bornefalk, H., Chen, H., Danielsson, M., Karlsson, S., Persson, M., Xu, C., Huber, B. 2014; 61 (3): 1099-1105

  View details for DOI 10.1109/TNS.2014.2300153

  View details for Web of Science ID 000337905600006

• Theoretical Comparison of the Iodine Quantification Accuracy of Two Spectral CT Technologies IEEE TRANSACTIONS ON MEDICAL IMAGING Bornefalk, H., Persson, M. 2014; 33 (2): 556-565

  Abstract

  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 Cramér-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(2), 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.

  View details for DOI 10.1109/TMI.2013.2290198

  View details for Web of Science ID 000331298000027

  View details for PubMedID 24216683

• Necessary forward model specification accuracy for basis material decomposition in spectral CT MEDICAL IMAGING 2014: PHYSICS OF MEDICAL IMAGING Bornefalk, H., Persson, M., Danielsson, M. 2014; 9033

  View details for DOI 10.1117/12.2043702

  View details for Web of Science ID 000338775800086

• Energy-resolved CT imaging with a photon-counting silicon-strip detector MEDICAL IMAGING 2014: PHYSICS OF MEDICAL IMAGING Persson, M., Huber, B., Karlsson, S., Liu, X., Chen, H., Xu, C., Yveborg, M., Bornefalk, H., Danielsson, M. 2014; 9033

  View details for DOI 10.1117/12.2043519

  View details for Web of Science ID 000338775800124

• Characterization of a silicon strip detector for photon-counting spectral CT using monoenergetic photons from 40 keV to 120 keV MEDICAL IMAGING 2014: PHYSICS OF MEDICAL IMAGING Liu, X., Bornefalk, H., Chen, H., Danielsson, M., Karlsson, S., Persson, M., Xu, C., Huber, B. 2014; 9033

  View details for DOI 10.1117/12.2042862

  View details for Web of Science ID 000338775800127

• Eliminated risk of iodine contrast cancellation with multibin spectral CT PHYSICS IN MEDICINE AND BIOLOGY Yveborg, M., Persson, M., Crafoord, J., Danielsson, M., Bornefalk, H. 2013; 58 (14): N201-N209

  Abstract

  This note compares the extent of contrast cancellation induced by iodinated contrast agents in energy integrating and photon counting multibin CT images. The contrast between a hypodense target and soft tissue is modeled for the two systems for a range of iodine concentrations and tube voltages. In energy integrating systems, we show that the contrast vanishes for low concentrations of iodine whereas the same effect is not seen in multibin systems. We conclude that it is the ability of multibin systems to apply weighting schemes post-acquisition that allows the operator to eliminate the risk of contrast cancellation between iodinated targets and the background.

  View details for DOI 10.1088/0031-9155/58/14/N201

  View details for Web of Science ID 000321243600002

  View details for PubMedID 23807652

• Energy resolution of a segmented silicon strip detector for photon-counting spectral CT NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT Xu, C., Chen, H., Persson, M., Karlsson, S., Danielsson, M., Svensson, C., Bornefalk, H. 2013; 715: 11-17

  View details for DOI 10.1016/j.nima.2013.02.030

  View details for Web of Science ID 000319252300002

• Effect of Temperature Variation on the Energy Response of a Photon Counting Silicon CT Detector IEEE TRANSACTIONS ON NUCLEAR SCIENCE Bornefalk, H., Persson, M., Xu, C., Karlsson, S., Svensson, C., Danielsson, M. 2013; 60 (2): 1442-1449

  View details for DOI 10.1109/TNS.2013.2244909

  View details for Web of Science ID 000320856800027

• Evaluation of a Second-Generation Ultra-Fast Energy-Resolved ASIC for Photon-Counting Spectral CT IEEE TRANSACTIONS ON NUCLEAR SCIENCE Xu, C., Persson, M., Chen, H., Karlsson, S., Danielsson, M., Svensson, C., Bornefalk, H. 2013; 60 (1): 437-445

  View details for DOI 10.1109/TNS.2012.2228276

  View details for Web of Science ID 000314973200032

• A Framework for Evaluating Threshold Variation Compensation Methods in Photon Counting Spectral CT IEEE TRANSACTIONS ON MEDICAL IMAGING Persson, M., Bornefalk, H. 2012; 31 (10): 1861-1874

  Abstract

  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.

  View details for DOI 10.1109/TMI.2012.2204274

  View details for Web of Science ID 000310149700003

  View details for PubMedID 22711768

• Quantification of ring artifact visibility in CT MEDICAL IMAGING 2012: PHYSICS OF MEDICAL IMAGING Persson, M., Meyer, B., Bornefalk, H., Danielsson, M. 2012; 8313

  View details for DOI 10.1117/12.910537

  View details for Web of Science ID 000304768000086