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Journal Article

  • Kaaouana, T, Bertrand, A, Ouamer, F, Law-Ye, B, Pyatigorskaya, N, Bouyahia, A, Thiery, N, Dufouil, C, Delmaire, C, Dormont, D, de Rochefort, L & Chupin, M 2017, “Improved cerebral microbleeds detection using their magnetic signature on T2*-phase-contrast: A comparison study in a clinical setting”, NeuroImage. Clinical, vol. 15, p. 274-283.
    Résumé : INTRODUCTION/PURPOSE: In vivo detection of cerebral microbleeds (CMBs) from T2* gradient recalled echo (GRE) magnitude image suffers from low specificity, modest inter-rater reproducibility and is biased by its sensitivity to acquisition parameters. New methods were proposed for improving this identification, but they mostly rely on 3D acquisitions, not always feasible in clinical practice. A fast 2D phase processing technique for computing internal field maps (IFM) has been shown to make it possible to characterize CMBs through their magnetic signature in routine clinical setting, based on 2D multi-slice acquisitions. However, its clinical interest for CMBs identification with respect to more common images remained to be assessed. To do so, systematic experiments were undertaken to compare the ratings obtained by trained observers with several image types, T2* magnitude, Susceptibility Weighted Imaging reconstructions (SWI) and IFM built from the same T2*-weighted acquisition. MATERIALS/METHODS: 15 participants from the MEMENTO multi-center cohort were selected: six subjects with numerous CMBs (20 ± 6 CMBs), five subjects with a few CMBs (2 ± 1 CMBs) and four subjects without CMB. 2D multi-slice T2* GRE sequences were acquired on Philips and Siemens 3T systems. After pilot experiments, T2* magnitude, Susceptibility Weighted Imaging (SWI) minimum intensity projection (mIP) on three slices and IFM were considered for the rating experiments. A graphical user interface (GUI) was designed in order to consistently display images in random order. Six raters of various background and expertise independently selected "definite" or "possible" CMBs. Rating results were compared with respect to a specific consensus reference, on both lesion and subject type points of view. RESULTS: IFM yielded increased sensitivity and decreased false positives rate (FPR) for CMBs identification compared to T2* magnitude and SWI-mIP images. Inter-rater variability was decreased with IFM when identifying subjects with numerous lesions, with only a limited increase in rating time. IFM thus appears as an interesting candidate to improve CMBs identification in clinical setting.
    Mots-clés : Magnetic susceptibility, Microbleeds, Phase MRI, SWI.

  • Kee, Y, Liu, Z, Zhou, L, Dimov, A, Cho, J, de Rochefort, L, Seo, JK & Wang, Y 2017, “Quantitative Susceptibility Mapping (QSM) Algorithms: Mathematical Rationale and Computational Implementations”, IEEE transactions on bio-medical engineering, vol. 64, no. 11, p. 2531-2545.
    Résumé : Quantitative susceptibility mapping (QSM) solves the magnetic field-to-magnetization (tissue susceptibility) inverse problem under conditions of noisy and incomplete field data acquired using magnetic resonance imaging. Therefore, sophisticated algorithms are necessary to treat the ill-posed nature of the problem and are reviewed here. The forward problem is typically presented as an integral form, where the field is the convolution of the dipole kernel and tissue susceptibility distribution. This integral form can be equivalently written as a partial differential equation (PDE). Algorithmic challenges are to reduce streaking and shadow artifacts characterized by the fundamental solution of the PDE. Bayesian maximum a posteriori estimation can be employed to solve the inverse problem, where morphological and relevant biomedical knowledge (specific to the imaging situation) are used as priors. As the cost functions in Bayesian QSM framework are typically convex, solutions can be robustly computed using a gradient-based optimization algorithm. Moreover, one can not only accelerate Bayesian QSM, but also increase its effectiveness at reducing shadows using prior knowledge based preconditioners. Improving the efficiency of QSM is under active development, and a rigorous analysis of preconditioning needs to be carried out for further investigation.Quantitative susceptibility mapping (QSM) solves the magnetic field-to-magnetization (tissue susceptibility) inverse problem under conditions of noisy and incomplete field data acquired using magnetic resonance imaging. Therefore, sophisticated algorithms are necessary to treat the ill-posed nature of the problem and are reviewed here. The forward problem is typically presented as an integral form, where the field is the convolution of the dipole kernel and tissue susceptibility distribution. This integral form can be equivalently written as a partial differential equation (PDE). Algorithmic challenges are to reduce streaking and shadow artifacts characterized by the fundamental solution of the PDE. Bayesian maximum a posteriori estimation can be employed to solve the inverse problem, where morphological and relevant biomedical knowledge (specific to the imaging situation) are used as priors. As the cost functions in Bayesian QSM framework are typically convex, solutions can be robustly computed using a gradient-based optimization algorithm. Moreover, one can not only accelerate Bayesian QSM, but also increase its effectiveness at reducing shadows using prior knowledge based preconditioners. Improving the efficiency of QSM is under active development, and a rigorous analysis of preconditioning needs to be carried out for further investigation.
    Mots-clés : Bayes methods, Inverse problems, Kernel, Magnetic Resonance Imaging, Magnetic susceptibility, Partial differential equations, Resource description framework, snc.

  • Leguerney, I, de Rochefort, L, Poirier-Quinot, M, Ingels, A, Violas, X, Robin, S, Opolon, P, Dubuisson, R-M, Pitre-Champagnat, S, Robert, P & Lassau, N 2017, “Molecular Imaging to Predict Response to Targeted Therapies in Renal Cell Carcinoma”, Contrast Media & Molecular Imaging, vol. 2017, p. 7498538.
    Résumé : Molecular magnetic resonance imaging targeted to an endothelial integrin involved in neoangiogenesis was compared to DCE-US and immunochemistry to assess the early response of three different therapeutic agents in renal cell carcinoma. Human A498 renal cells carcinoma was subcutaneously inoculated into 24 nude mice. Mice received either phosphate-buffered saline solution, sunitinib, everolimus, or bevacizumab during 4 days. DCE-US and molecular MRI targeting αvβ3 were performed at baseline and 4 days after treatment initiation. PI, AUC, relaxation rate variations ΔR2(⁎), and percentage of vessels area quantified on CD31-stained microvessels were compared. Significant decreases were observed for PI and AUC parameters measured by DCE-US for bevacizumab group as early as 4 days, whereas molecular αvβ3-targeted MRI was able to detect significant changes in both bevacizumab and everolimus groups. Percentage of CD31-stained microvessels was significantly correlated with DCE-US parameters, PI (R = 0.87, p = 0.0003) and AUC (R = 0.81, p = 0.0013). The percentage of vessel tissue area was significantly reduced (p < 0.01) in both sunitinib and bevacizumab groups. We report an early detection of neoangiogenesis modification after induction of targeted therapies, using DCE-US or αvβ3-targeted MRI. We consider these outcomes should encourage clinical trial developments to further evaluate the potential of this molecular MRI technique.

  • Schweser, F, Robinson, SD, de Rochefort, L, Li, W & Bredies, K 2017, “An illustrated comparison of processing methods for phase MRI and QSM: removal of background field contributions from sources outside the region of interest”, NMR in biomedicine, vol. 30, no. 4.
    Résumé : The elimination of so-called background fields is an essential step in phase MRI and quantitative susceptibility mapping (QSM). Background fields, which are caused by sources outside the region of interest (ROI), are often one to two orders of magnitude stronger than tissue-related field variations from within the ROI, hampering quantitative interpretation of field maps. This paper reviews the current literature on background elimination algorithms for QSM and provides insights into similarities and differences between the many algorithms proposed. We discuss the basic theoretical foundations and derive fundamental limitations of background field elimination. Copyright © 2016 John Wiley & Sons, Ltd.
    Mots-clés : background field removal, phase imaging, phase processing, QSM, review, snc, susceptibility mapping.


Journal Article

  • Wang, H, Sebrié, C, Ruaud, J-P, Guillot, G, Bouazizi-Verdier, K, Willoquet, G, Maître, X, Darrasse, L & de Rochefort, L 2016, “Aerosol deposition in the lungs of spontaneously breathing rats using Gd-DOTA-based contrast agents and ultra-short echo time MRI at 1.5 Tesla”, Magnetic Resonance in Medicine, vol. 75, no. 2, p. 594-605.
    Résumé : PURPOSE: Aerosol toxicology and drug delivery through the lungs, which depend on various parameters, require methods to quantify particle deposition. Intrapulmonary-administered MRI contrast agent combined with lung-specific imaging sequences has been proposed as a high performance technique for aerosol research. Here, aerosol deposition is assessed using ultra-short echo (UTE) sequences. METHODS: Before and after administration of Gd-DOTA-based aerosol delivered nose-only in free-breathing healthy rats, a T1 -weighted 3D UTE sequence was applied in a clinical 1.5 Tesla scanner. Administration lasted 14 min, and the experiment was performed on six rats. A contrast-enhanced quantitative analysis was done. RESULTS: Fifty percent signal enhancement was obtained in the lung parenchyma. Lung clearance of the contrast agent was evaluated to be 14% per h (corresponding to a characteristic clearance time of 3.6 h) and aerosol deposition was shown to be homogeneous throughout the lung in healthy rats. The total deposited dose was estimated to be 1.05 µmol/kg body weight, and the concentration precision was 0.02 mM. CONCLUSION: The UTE protocol with nebulized Gd-DOTA is replicable to significantly enhance the lung parenchyma and to map aerosol deposition. This functional strategy, applied in a clinical system with a clinical nebulization setup and a low inhaled dose, suggests a feasible translation to human.
    Mots-clés : Administration, Inhalation, Administration, Intranasal, aerosol deposition, Aerosols, Animals, contrast enhancement, Contrast Media, Feasibility Studies, Gd-DOTA, Heterocyclic Compounds, Image Processing, Computer-Assisted, Lung, lung MRI, Magnetic Resonance Imaging, Cine, Male, nebulization, Organometallic Compounds, Rats, Rats, Wistar, ultra-short echo.


Journal Article

  • Abi-Abdallah Rodriguez, D, Durand, E, de Rochefort, L, Boudjemline, Y & Mousseaux, E 2015, “Simultaneous pressure-volume measurements using optical sensors and MRI for left ventricle function assessment during animal experiment”, Medical Engineering & Physics, vol. 37, no. 1, p. 100-108.
    Résumé : Simultaneous pressure and volume measurements enable the extraction of valuable parameters for left ventricle function assessment. Cardiac MR has proven to be the most accurate method for volume estimation. Nonetheless, measuring pressure simultaneously during MRI acquisitions remains a challenge given the magnetic nature of the widely used pressure transducers. In this study we show the feasibility of simultaneous in vivo pressure-volume acquisitions with MRI using optical pressure sensors. Pressure-volume loops were calculated while inducing three inotropic states in a sheep and functional indices were extracted, using single beat loops, to characterize systolic and diastolic performance. Functional indices evolved as expected in response to positive inotropic stimuli. The end-systolic elastance, representing the contractility index, the diastolic myocardium compliance, and the cardiac work efficiency all increased when inducing inotropic state enhancement. The association of MRI and optical pressure sensors within the left ventricle successfully enabled pressure-volume loop analysis after having respective data simultaneously recorded during the experimentation without the need to move the animal between each inotropic state.
    Mots-clés : Animals, Blood pressure, Cardiac Imaging Techniques, Cardiovascular magnetic resonance, Contractility index, Diastolic compliance, Effective arterial elastance, Feasibility Studies, Female, Heart Ventricles, Magnetic Resonance Imaging, Myocardial Contraction, Optical Imaging, Optical pressure measurements, Organ Size, Pressure, PV loop functional parameters, Sheep, Ventricular Function.

  • Kaaouana, T, de Rochefort, L, Samaille, T, Thiery, N, Dufouil, C, Delmaire, C, Dormont, D & Chupin, M 2015, “2D harmonic filtering of MR phase images in multicenter clinical setting: toward a magnetic signature of cerebral microbleeds”, NeuroImage, vol. 104, p. 287-300.
    Résumé : Cerebral microbleeds (CMBs) have emerged as a new imaging marker of small vessel disease. Composed of hemosiderin, CMBs are paramagnetic and can be detected with MRI sequences sensitive to magnetic susceptibility (typically, gradient recalled echo T2* weighted images). Nevertheless, their identification remains challenging on T2* magnitude images because of confounding structures and lesions. In this context, T2* phase image may play a key role in better characterizing CMBs because of its direct relationship with local magnetic field variations due to magnetic susceptibility difference. To address this issue, susceptibility-based imaging techniques were proposed, such as Susceptibility Weighted Imaging (SWI) and Quantitative Susceptibility Mapping (QSM). But these techniques have not yet been validated for 2D clinical data in multicenter settings. Here, we introduce 2DHF, a fast 2D phase processing technique embedding both unwrapping and harmonic filtering designed for data acquired in 2D, even with slice-to-slice inconsistencies. This method results in internal field maps which reveal local field details due to magnetic inhomogeneity within the region of interest only. This technique is based on the physical properties of the induced magnetic field and should yield consistent results. A synthetic phantom was created for numerical simulations. It simulates paramagnetic and diamagnetic lesions within a 'brain-like' tissue, within a background. The method was evaluated on both this synthetic phantom and multicenter 2D datasets acquired in standardized clinical setting, and compared with two state-of-the-art methods. It proved to yield consistent results on synthetic images and to be applicable and robust on patient data. As a proof-of-concept, we finally illustrate that it is possible to find a magnetic signature of CMBs and CMCs on internal field maps generated with 2DHF on 2D clinical datasets that give consistent results with CT-scans in a subsample of 10 subjects acquired with both modalities.
    Mots-clés : 2D multislice, Algorithms, Calcification, Cerebral Hemorrhage, Databases, Factual, Harmonic filter, Humans, Image Interpretation, Computer-Assisted, Magnetic Resonance Imaging, Microbleeds, phase processing, Susceptibility.


Journal Article

  • Chenoune, M, De Rochefort, L, Bruneval, P, Lidouren, F, Kohlhauer, M, Seemann, A, Ghaleh, B, Korn, M, Dubuisson, R-M, Ben Yahmed, A, Maître, X, Isabey, D, Ricard, J-D, Kerber, RE, Darrasse, L, Berdeaux, A & Tissier, R 2014, “Evaluation of lung recovery after static administration of three different perfluorocarbons in pigs”, BMC pharmacology & toxicology, vol. 15, p. 53.
    Résumé : BACKGROUND: The respiratory properties of perfluorocarbons (PFC) have been widely studied for liquid ventilation in humans and animals. Several PFC were tested but their tolerance may depend on the species. Here, the effects of a single administration of liquid PFC into pig lungs were assessed and compared. Three different PFC having distinct evaporative and spreading coefficient properties were evaluated (Perfluorooctyl bromide [PFOB], perfluorodecalin [PFD] and perfluoro-N-octane [PFOC]). METHODS: Pigs were anesthetized and submitted to mechanical ventilation. They randomly received an intra-tracheal administration of 15 ml/kg of either PFOB, PFD or PFOC with 12 h of mechanical ventilation before awakening and weaning from ventilation. A Control group was submitted to mechanical ventilation with no PFC administration. All animals were followed during 4 days after the initial PFC administration to investigate gas exchanges and clinical recovery. They were ultimately euthanized for histological analyses and assessment of PFC residual concentrations within the lungs using dual nuclei fluorine and hydrogen Magnetic Resonance Imaging (MRI). Sixteen animals were included (4/group). RESULTS: In the PFD group, animals tended to be hypoxemic after awakening. In PFOB and PFOC groups, blood gases were not significantly different from the Control group after awakening. The poor tolerance of PFD was likely related to a large amount of residual PFC, as observed using MRI in all lung samples (≈10% of lung volume). This percentage was lower in the PFOB group (≈1%) but remained significantly greater than in the Control group. In the PFOC group, the percentage of residual PFC was not significantly different from that of the Control group (≈0.1%). Histologically, the most striking feature was an alveolar infiltration with foam macrophages, especially in the groups treated by PFD or PFOB. CONCLUSIONS: Of the three tested perfluorocarbons, PFOC offered the best tolerance in terms of lung function, gas exchanges and residuum in the lung. PFOC was rapidly cleared from the lungs and virtually disappeared after 4 days whereas PFOB persisted at significant levels and led to foam macrophage infiltration. PFOC could be relevant for short term total liquid ventilation with a rapid weaning.
    Mots-clés : Animals, Fluorocarbons, Lung, Magnetic Resonance Imaging, Respiration, Artificial, Swine.

  • Khalifé, M, Decoene, A, Caetano, F, de Rochefort, L, Durand, E & Rodríguez, D 2014, “Estimating absolute aortic pressure using MRI and a one-dimensional model”, Journal of Biomechanics, vol. 47, no. 13, p. 3390-3399.
    Résumé : Aortic blood pressure is a strong indicator to cardiovascular diseases and morbidity. Clinically, pressure measurements are done by inserting a catheter in the aorta. However, imaging techniques have been used to avoid the invasive procedure of catheterization. In this paper, we combined MRI measurements to a one-dimensional model in order to simulate blood flow in an aortic segment. Absolute pressure was estimated in the aorta by using MRI measured flow as boundary conditions and MRI measured compliance as a pressure law for solving the model. Model computed pressure was compared to catheter measured pressure in an aortic phantom. Furthermore, aortic pressure was estimated in vivo in three healthy volunteers.
    Mots-clés : Adult, Aorta, Arterial Pressure, Blood Flow Velocity, Blood pressure, Cardiovascular Diseases, Cardiovascular imaging, Catheterization, Compliance, Hemodynamics, Humans, Magnetic Resonance Imaging, Male, Models, Cardiovascular, MRI, Non-invasive, One-dimensional model, Phantoms, Imaging.

  • Martin, L, Maître, X, de Rochefort, L, Sarracanie, M, Friese, M, Hagot, P & Durand, E 2014, “Phase-contrast velocity mapping for highly diffusive fluids: optimal bipolar gradient pulse parameters for hyperpolarized helium-3”, Magnetic Resonance in Medicine, vol. 72, no. 4, p. 1072-1078.
    Résumé : PURPOSE: In MR-velocity phase-contrast measurements, increasing the encoding bipolar gradient, i.e., decreasing the field of speed, usually improves measurement precision. However, in gases, fast diffusion during the bipolar gradient pulses may dramatically decrease the signal-to-noise ratio, thus degrading measurement precision. These two effects are contradictory. This work aims at determining the optimal sequence parameters to improve the velocity measurement precision. THEORY AND METHODS: This work presents the theoretical optimization of bipolar gradient parameters (duration and amplitude) to improve velocity measurement precision. An analytical approximation is given as well as a numerical optimization. It is shown that the solution depends on the diffusion coefficient and T2 *. Experimental validation using hyperpolarized (3) He diluted in various buffer gases ((4) He, N2 , and SF6 ) is presented at 1.5 Tesla (T) in a straight pipe. RESULTS: Excellent agreement was found with the theoretical results for prediction of optimal field of speed and good agreement was found for the precision in measured velocity, but for SF6 buffered gas. CONCLUSION: The theoretical predictions were validated, providing a way to optimize velocity mapping in gases.
    Mots-clés : airflow velocity, Algorithms, Contrast Media, diffusion, flow, gas, Helium, hyperpolarized helium-3, Image Interpretation, Computer-Assisted, Isotopes, Magnetic Resonance Imaging, phase-contrast, phase-contrast MRI, Radiopharmaceuticals, Reproducibility of Results, Rheology, Sensitivity and Specificity.


Journal Article

  • Khalife, M, Rodriguez, D, de Rochefort, L & Durand, E 2012, “In vitro validation of non-invasive aortic compliance measurements using MRI”, Computer Methods in Biomechanics and Biomedical Engineering, vol. 15 Suppl 1, p. 83-84.
    Mots-clés : Aorta, Compliance, Humans, In Vitro Techniques, Magnetic Resonance Imaging.

  • Liu, J, Liu, T, de Rochefort, L, Ledoux, J, Khalidov, I, Chen, W, Tsiouris, AJ, Wisnieff, C, Spincemaille, P, Prince, MR & Wang, Y 2012, “Morphology enabled dipole inversion for quantitative susceptibility mapping using structural consistency between the magnitude image and the susceptibility map”, NeuroImage, vol. 59, no. 3, p. 2560-2568.
    Résumé : The magnetic susceptibility of tissue can be determined in gradient echo MRI by deconvolving the local magnetic field with the magnetic field generated by a unit dipole. This Quantitative Susceptibility Mapping (QSM) problem is unfortunately ill-posed. By transforming the problem to the Fourier domain, the susceptibility appears to be undersampled only at points where the dipole kernel is zero, suggesting that a modest amount of additional information may be sufficient for uniquely resolving susceptibility. A Morphology Enabled Dipole Inversion (MEDI) approach is developed that exploits the structural consistency between the susceptibility map and the magnitude image reconstructed from the same gradient echo MRI. Specifically, voxels that are part of edges in the susceptibility map but not in the edges of the magnitude image are considered to be sparse. In this approach an L1 norm minimization is used to express this sparsity property. Numerical simulations and phantom experiments are performed to demonstrate the superiority of this L1 minimization approach over the previous L2 minimization method. Preliminary brain imaging results in healthy subjects and in patients with intracerebral hemorrhages illustrate that QSM is feasible in practice.
    Mots-clés : Adult, Algorithms, Brain Injuries, Brain Mapping, Computer Simulation, Echo-Planar Imaging, Electromagnetic Fields, Fourier Analysis, Humans, Image Interpretation, Computer-Assisted, Image Processing, Computer-Assisted, Intracranial Hemorrhages, Phantoms, Imaging, Reproducibility of Results.


Journal Article

  • Liu, T, Khalidov, I, de Rochefort, L, Spincemaille, P, Liu, J, Tsiouris, AJ & Wang, Y 2011, “A novel background field removal method for MRI using projection onto dipole fields (PDF)”, NMR in biomedicine, vol. 24, no. 9, p. 1129-1136.
    Résumé : For optimal image quality in susceptibility-weighted imaging and accurate quantification of susceptibility, it is necessary to isolate the local field generated by local magnetic sources (such as iron) from the background field that arises from imperfect shimming and variations in magnetic susceptibility of surrounding tissues (including air). Previous background removal techniques have limited effectiveness depending on the accuracy of model assumptions or information input. In this article, we report an observation that the magnetic field for a dipole outside a given region of interest (ROI) is approximately orthogonal to the magnetic field of a dipole inside the ROI. Accordingly, we propose a nonparametric background field removal technique based on projection onto dipole fields (PDF). In this PDF technique, the background field inside an ROI is decomposed into a field originating from dipoles outside the ROI using the projection theorem in Hilbert space. This novel PDF background removal technique was validated on a numerical simulation and a phantom experiment and was applied in human brain imaging, demonstrating substantial improvement in background field removal compared with the commonly used high-pass filtering method.
    Mots-clés : Algorithms, Brain, Computer Simulation, Humans, Magnetic Fields, Magnetic Resonance Imaging, Phantoms, Imaging, Reproducibility of Results.

  • Liu, T, Liu, J, de Rochefort, L, Spincemaille, P, Khalidov, I, Ledoux, JR & Wang, Y 2011, “Morphology enabled dipole inversion (MEDI) from a single-angle acquisition: comparison with COSMOS in human brain imaging”, Magnetic Resonance in Medicine, vol. 66, no. 3, p. 777-783.
    Résumé : Magnetic susceptibility varies among brain structures and provides insights into the chemical and molecular composition of brain tissues. However, the determination of an arbitrary susceptibility distribution from the measured MR signal phase is a challenging, ill-conditioned inverse problem. Although a previous method named calculation of susceptibility through multiple orientation sampling (COSMOS) has solved this inverse problem both theoretically and experimentally using multiple angle acquisitions, it is often impractical to carry out on human subjects. Recently, the feasibility of calculating the brain susceptibility distribution from a single-angle acquisition was demonstrated using morphology enabled dipole inversion (MEDI). In this study, we further improved the original MEDI method by sparsifying the edges in the quantitative susceptibility map that do not have a corresponding edge in the magnitude image. Quantitative susceptibility maps generated by the improved MEDI were compared qualitatively and quantitatively with those generated by calculation of susceptibility through multiple orientation sampling. The results show a high degree of agreement between MEDI and calculation of susceptibility through multiple orientation sampling, and the practicality of MEDI allows many potential clinical applications.
    Mots-clés : Adult, Algorithms, Brain Mapping, Female, Humans, Image Enhancement, Image Processing, Computer-Assisted, Imaging, Three-Dimensional, Linear Models, Magnetic Resonance Imaging, Male, Reproducibility of Results, Sensitivity and Specificity.


Journal Article

  • de Rochefort, L, Liu, T, Kressler, B, Liu, J, Spincemaille, P, Lebon, V, Wu, J & Wang, Y 2010, “Quantitative susceptibility map reconstruction from MR phase data using bayesian regularization: validation and application to brain imaging”, Magnetic Resonance in Medicine, vol. 63, no. 1, p. 194-206.
    Résumé : The diagnosis of many neurologic diseases benefits from the ability to quantitatively assess iron in the brain. Paramagnetic iron modifies the magnetic susceptibility causing magnetic field inhomogeneity in MRI. The local field can be mapped using the MR signal phase, which is discarded in a typical image reconstruction. The calculation of the susceptibility from the measured magnetic field is an ill-posed inverse problem. In this work, a bayesian regularization approach that adds spatial priors from the MR magnitude image is formulated for susceptibility imaging. Priors include background regions of known zero susceptibility and edge information from the magnitude image. Simulation and phantom validation experiments demonstrated accurate susceptibility maps free of artifacts. The ability to characterize iron content in brain hemorrhage was demonstrated on patients with cavernous hemangioma. Additionally, multiple structures within the brain can be clearly visualized and characterized. The technique introduces a new quantitative contrast in MRI that is directly linked to iron in the brain.
    Mots-clés : Algorithms, Artificial Intelligence, Bayes Theorem, Brain, Cerebral Hemorrhage, Humans, Image Enhancement, Image Interpretation, Computer-Assisted, Magnetic Resonance Imaging, Pattern Recognition, Automated, Phantoms, Imaging, Reproducibility of Results, Sensitivity and Specificity.

  • Kressler, B, de Rochefort, L, Liu, T, Spincemaille, P, Jiang, Q & Wang, Y 2010, “Nonlinear regularization for per voxel estimation of magnetic susceptibility distributions from MRI field maps”, IEEE transactions on medical imaging, vol. 29, no. 2, p. 273-281.
    Résumé : Magnetic susceptibility is an important physical property of tissues, and can be used as a contrast mechanism in magnetic resonance imaging (MRI). Recently, targeting contrast agents by conjugation with signaling molecules and labeling stem cells with contrast agents have become feasible. These contrast agents are strongly paramagnetic, and the ability to quantify magnetic susceptibility could allow accurate measurement of signaling and cell localization. Presented here is a technique to estimate arbitrary magnetic susceptibility distributions by solving an ill-posed inversion problem from field maps obtained in an MRI scanner. Two regularization strategies are considered: conventional Tikhonov regularization and a sparsity promoting nonlinear regularization using the l(1) norm. Proof of concept is demonstrated using numerical simulations, phantoms, and in a stroke model in a rat. Initial experience indicates that the nonlinear regularization better suppresses noise and streaking artifacts common in susceptibility estimation.
    Mots-clés : Algorithms, Animals, Brain, Computer Simulation, Contrast Media, Image Processing, Computer-Assisted, Magnetic Resonance Imaging, Male, Models, Theoretical, Nonlinear Dynamics, Phantoms, Imaging, Rats, Rats, Wistar, Stroke.

  • Liu, T, Spincemaille, P, de Rochefort, L, Wong, R, Prince, M & Wang, Y 2010, “Unambiguous identification of superparamagnetic iron oxide particles through quantitative susceptibility mapping of the nonlinear response to magnetic fields”, Magnetic Resonance Imaging, vol. 28, no. 9, p. 1383-1389.
    Résumé : Superparamagnetic iron oxide (SPIO) particles generate signal void regions on gradient echo images due to their strong magnetization. In practice, the signal void region might be indistinguishable from that generated by air. However, the response of SPIO to an externally applied magnetic field is nonlinear. Magnetization of SPIO saturates at around 1 T while magnetization of water and air increase linearly with field strength. Phantom experiment and mice experiments demonstrated the feasibility of a nonambiguous identification of superparamagnetic contrast agents.
    Mots-clés : Air, Animals, Computer Simulation, Contrast Media, Ferric Compounds, Magnetics, Mice, Models, Theoretical, Phantoms, Imaging, Water.


Journal Article

  • Liu, T, Spincemaille, P, de Rochefort, L, Kressler, B & Wang, Y 2009, “Calculation of susceptibility through multiple orientation sampling (COSMOS): a method for conditioning the inverse problem from measured magnetic field map to susceptibility source image in MRI”, Magnetic Resonance in Medicine, vol. 61, no. 1, p. 196-204.
    Résumé : Magnetic susceptibility differs among tissues based on their contents of iron, calcium, contrast agent, and other molecular compositions. Susceptibility modifies the magnetic field detected in the MR signal phase. The determination of an arbitrary susceptibility distribution from the induced field shifts is a challenging, ill-posed inverse problem. A method called "calculation of susceptibility through multiple orientation sampling" (COSMOS) is proposed to stabilize this inverse problem. The field created by the susceptibility distribution is sampled at multiple orientations with respect to the polarization field, B(0), and the susceptibility map is reconstructed by weighted linear least squares to account for field noise and the signal void region. Numerical simulations and phantom and in vitro imaging validations demonstrated that COSMOS is a stable and precise approach to quantify a susceptibility distribution using MRI.
    Mots-clés : Algorithms, Computer Simulation, Humans, Image Enhancement, Image Interpretation, Computer-Assisted, Magnetic Resonance Imaging, Magnetics, Models, Biological, Phantoms, Imaging, Reproducibility of Results, Sensitivity and Specificity.

  • Wang, Y, de Rochefort, L, Liu, T & Kressler, B 2009, “Magnetic source MRI: a new quantitative imaging of magnetic biomarkers”, Conference proceedings: .. Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Annual Conference, vol. 2009, p. 53-56.
    Résumé : A new approach to generating MRI contrast by solving the magnetic field to susceptibility source inverse problem is presented to address the quantification difficulties associated with traditional T1/T2 relaxation and susceptibility weighted T2* methods. The forward problem from source to field is reviewed. Its inverse field to source problem is ill posed. Accurate solutions are found by conditioning the data acquisition or regularizing the solution. Preclinical and clinical applications using this magnetic source MRI are discussed for quantitative mapping magnetic biomarkers such as contrast agents in molecular MRI and iron deposits in diseases.
    Mots-clés : Algorithms, Biomarkers, Contrast Media, Diagnosis, Computer-Assisted, Humans, Magnetic Resonance Imaging, Magnetic Resonance Spectroscopy, Male, Reproducibility of Results, Sensitivity and Specificity.


Journal Article

  • de Rochefort, L, Brown, R, Prince, MR & Wang, Y 2008, “Quantitative MR susceptibility mapping using piece-wise constant regularized inversion of the magnetic field”, Magnetic Resonance in Medicine, vol. 60, no. 4, p. 1003-1009.
    Résumé : Magnetic properties characterized by susceptibility and chemical shift linearly modify the local magnetic field experienced by spins. A piece-wise constant solution using magnetic resonance imaging is found to the challenging inversion problem from field to magnetic properties. The magnetic field shifts were estimated from MR phase images. The MR magnitude images were segmented into many regions of uniform magnetic properties. Standard linear regression using the calculated magnetic field from each region allowed accurate susceptibility quantification. The technique was experimentally validated on a variety of samples including water, vegetable oil, air, Gadolinium, and superparamagnetic iron oxides. Susceptibility was measured with a precision better than 0.1 ppm, in a range of 10 ppm. In vivo feasibility was shown on the forearm for which soft-tissue, cortical bone, and bone marrow susceptibility, and chemical shift values in good agreement with literature data were obtained.
    Mots-clés : Algorithms, Computer Simulation, Electromagnetic Fields, Magnetic Resonance Spectroscopy, Models, Chemical, Reproducibility of Results, Sensitivity and Specificity.

  • de Rochefort, L, Nguyen, T, Brown, R, Spincemaille, P, Choi, G, Weinsaft, J, Prince, MR & Wang, Y 2008, “In vivo quantification of contrast agent concentration using the induced magnetic field for time-resolved arterial input function measurement with MRI”, Medical Physics, vol. 35, no. 12, p. 5328-5339.
    Résumé : For pharmacokinetic modeling of tissue physiology, there is great interest in measuring the arterial input function (AIF) from dynamic contrast-enhanced (DCE) magnetic resonance imaging (MRI) using paramagnetic contrast agents. Due to relaxation effects, the measured signal is a nonlinear function of the injected contrast agent concentration and depends on sequence parameters, system calibration, and time-of-flight effects, making it difficult to accurately measure the AIF during the first pass. Paramagnetic contrast agents also affect susceptibility and modify the magnetic field in proportion to their concentration. This information is contained in the MR signal phase which is discarded in a typical image reconstruction. However, quantifying AIF through contrast agent susceptibility induced phase changes is made difficult by the fact that the induced magnetic field is nonlocal and depends upon the contrast agent spatial distribution and thus on organ and vessel shapes. In this article, the contrast agent susceptibility was quantified through inversion of magnetic field shifts using a piece-wise constant model. Its feasibility is demonstrated by a determination of the AIF from the susceptibility-induced field changes of an intravenous bolus. After in vitro validation, a time-resolved two-dimensional (2D) gradient echo scan, triggered to diastole, was performed in vivo on the aortic arch during a bolus injection of 0.1 mmol/kg Gd-DTPA. An approximate geometrical model of the aortic arch constructed from the magnitude images was used to calculate the spatial variation of the field associated with the bolus. In 14 subjects, Gd concentration curves were measured dynamically (one measurement per heart beat) and indirectly validated by independent 2D cine phase contrast flow rate measurements. Flow rate measurements using indicator conservation with this novel quantitative susceptibility imaging technique were found to be in good agreement with those obtained from the cine phase contrast measurements in all subjects. Contrary to techniques that rely on intensity, the accuracy of this signal phase based method is insensitive to factors influencing signal intensity such as flip angle, coil sensitivity, relaxation changes, and time-of-flight effects extending the range of pulse sequences and contrast doses for which quantitative DCE-MRI can be applied.
    Mots-clés : Adolescent, Adult, Aged, Angiography, Arteries, Contrast Media, Electromagnetic Fields, Female, Gadolinium, Humans, Magnetic Resonance Imaging, Male, Middle Aged, Models, Theoretical, Signal Processing, Computer-Assisted.

  • Nguyen, TD, de Rochefort, L, Spincemaille, P, Cham, MD, Weinsaft, JW, Prince, MR & Wang, Y 2008, “Effective motion-sensitizing magnetization preparation for black blood magnetic resonance imaging of the heart”, Journal of magnetic resonance imaging: JMRI, vol. 28, no. 5, p. 1092-1100.
    Résumé : PURPOSE: To investigate the effectiveness of flow signal suppression of a motion-sensitizing magnetization preparation (MSPREP) sequence and to optimize a 2D MSPREP steady-state free precession (SSFP) sequence for black blood imaging of the heart. MATERIALS AND METHODS: Using a flow phantom, the effect of varying field of speed (FOS), b-value, voxel size, and flow pattern on the flow suppression was investigated. In seven healthy volunteers, black blood images of the heart were obtained at 1.5T with MSPREP-SSFP and double inversion recovery fast spin echo (DIR-FSE) techniques. Myocardium and blood signal-to-noise ratio (SNR) and myocardium-to-blood contrast-to-noise ratio (CNR) were measured. The optimal FOS that maximized the CNR for MSPREP-SSFP was determined. RESULTS: Phantom data demonstrated that the flow suppression was induced primarily by the velocity encoding effect. In humans, FOS=10-20 cm/s was found to maximize the CNR for short-axis (SA) and four-chamber (4C) views. Compared to DIR-FSE, MSPREP-SSFP provided similar blood SNR efficiency in the SA basal and mid-views and significantly lower blood SNR efficiency in the SA apical (P=0.02) and 4C (P=0.01) views, indicating similar or better blood suppression. CONCLUSION: Velocity encoding is the primary flow suppression mechanism of the MSPREP sequence and 2D MSPREP-SSFP black blood imaging of the heart is feasible in healthy subjects.
    Mots-clés : Adult, Algorithms, Female, Heart, Humans, Image Enhancement, Image Interpretation, Computer-Assisted, Magnetic Resonance Imaging, Male, Middle Aged, Motion, Phantoms, Imaging, Reproducibility of Results, Sensitivity and Specificity, Signal Processing, Computer-Assisted.


Journal Article

  • de Rochefort, L, Vial, L, Fodil, R, Maître, X, Louis, B, Isabey, D, Caillibotte, G, Thiriet, M, Bittoun, J, Durand, E & Sbirlea-Apiou, G 2007, “In vitro validation of computational fluid dynamic simulation in human proximal airways with hyperpolarized 3He magnetic resonance phase-contrast velocimetry”, Journal of Applied Physiology (Bethesda, Md.: 1985), vol. 102, no. 5, p. 2012-2023.
    Résumé : Computational fluid dynamics (CFD) and magnetic resonance (MR) gas velocimetry were concurrently performed to study airflow in the same model of human proximal airways. Realistic in vivo-based human airway geometry was segmented from thoracic computed tomography. The three-dimensional numerical description of the airways was used for both generation of a physical airway model using rapid prototyping and mesh generation for CFD simulations. Steady laminar inspiratory experiments (Reynolds number Re = 770) were performed and velocity maps down to the fourth airway generation were extracted from a new velocity mapping technique based on MR velocimetry using hyperpolarized (3)He gas. Full two-dimensional maps of the velocity vector were measured within a few seconds. Numerical simulations were carried out with the experimental flow conditions, and the two sets of data were compared between the two modalities. Flow distributions agreed within 3%. Main and secondary flow velocity intensities were similar, as were velocity convective patterns. This work demonstrates that experimental and numerical gas velocity data can be obtained and compared in the same complex airway geometry. Experiments validated the simulation platform that integrates patient-specific airway reconstruction process from in vivo thoracic scans and velocity field calculation with CFD, hence allowing the results of this numerical tool to be used with confidence in potential clinical applications for lung characterization. Finally, this combined numerical and experimental approach of flow assessment in realistic in vivo-based human airway geometries confirmed the strong dependence of airway flow patterns on local and global geometrical factors, which could contribute to gas mixing.
    Mots-clés : Bronchi, Bronchography, Computer Simulation, Helium, Humans, Image Processing, Computer-Assisted, Isotopes, Magnetic Resonance Imaging, Male, Middle Aged, Models, Anatomic, Models, Biological, Reproducibility of Results, Respiration, Respiratory Mechanics, Rheology, Tomography, X-Ray Computed, Trachea.


Journal Article

  • de Rochefort, L, Maître, X, Bittoun, J & Durand, E 2006, “Velocity-selective RF pulses in MRI”, Magnetic Resonance in Medicine, vol. 55, no. 1, p. 171-176.
    Résumé : A family of velocity-selective pulses consisting of a series of RF hard pulses followed by bipolar gradients was designed. The succession of required pulses was deduced using a k-space approach within a small tip-angle approximation. Fourier transform of the desired velocity excitation determined the flip-angle series, and the corresponding position in the generalized k-space identified the bipolar-gradient first moments. Spins from any velocity class can be selected. To illustrate this approach we designed and experimentally tested a velocity-slice selection that is analogous to standard spatial-slice selection but involves excitation of spins moving at a chosen velocity (velocity-slice center) and within a given interval (velocity-slice thickness). The assumed approximation does not limit the design to small angles, because velocity selection still holds for angles up to 90 degrees. Velocity slices were experimentally selected, centered on velocities ranging from -1 m s(-1) to 1 m s(-1) with a velocity-slice thickness of 0.4 m s(-1). The experimental velocity-slice profile was assessed and the flow was quantified.
    Mots-clés : Fourier Analysis, Magnetic Resonance Imaging, Phantoms, Imaging, Radio Waves.

  • de Rochefort, L, Maître, X, Fodil, R, Vial, L, Louis, B, Isabey, D, Croce, C, Darrasse, L, Apiou, G, Caillibotte, G, Bittoun, J & Durand, E 2006, “Phase-contrast velocimetry with hyperpolarized 3He for in vitro and in vivo characterization of airflow”, Magnetic Resonance in Medicine, vol. 55, no. 6, p. 1318-1325.
    Résumé : This paper describes a technique that combines radial MRI and phase contrast (PC) to map the velocities of hyperpolarized gases ((3)He) in respiratory airways. The method was evaluated on well known geometries (straight and U-shaped pipes) before it was applied in vivo. Dynamic 2D maps of the three velocity components were obtained from a 10-mm slice with an in-plane spatial resolution of 1.6 mm within 1 s. Integration of the in vitro through-plane velocity over the slice matched the input flow within a relative precision of 6.4%. As expected for the given Reynolds number, a parabolic velocity profile was obtained in the straight pipe. In the U-shaped pipe the three velocity components were measured and compared to a fluid-dynamics simulation so the precision was evaluated as fine as 0.025 m s(-1). The technique also demonstrated its ability to visualize vortices and localize characteristic points, such as the maximum velocity and vortex-center positions. Finally, in vivo feasibility was demonstrated in the human trachea during inhalation.
    Mots-clés : Contrast Media, Helium, Humans, Image Enhancement, Image Interpretation, Computer-Assisted, Isotopes, Lung, Magnetic Resonance Imaging, Phantoms, Imaging, Pulmonary Ventilation, Rheology.


Journal Article

  • Vignaud, A, Maître, X, Guillot, G, Durand, E, de Rochefort, L, Robert, P, Vivès, V, Santus, R & Darrasse, L 2005, “Magnetic susceptibility matching at the air-tissue interface in rat lung by using a superparamagnetic intravascular contrast agent: influence on transverse relaxation time of hyperpolarized helium-3”, Magnetic Resonance in Medicine, vol. 54, no. 1, p. 28-33.
    Résumé : Transverse relaxation of hyperpolarized helium-3 magnetization in respiratory airways highly depends on local magnetic field gradients induced by the magnetic susceptibility difference between gas and pulmonary tissue. Fast transverse relaxation is known to be an important feature that yields information about lung microstructure and function, but it is also an essential limitation in designing efficient strategies for lung imaging. Using intravascular injections of a superparamagnetic contrast agent in rats, it was possible to increase the overall susceptibility of the perfused lung tissues and hence to match it with the gas susceptibility. The transverse decay time constant of inhaled hyperpolarized helium-3 was measured in multiple-spin-echo experiments at 1.5 T as a function of the superparamagnetic contrast agent concentration in the animal blood. The time constant was increased by a factor of 3 when an optimal concentration was reached as predicted for susceptibility matching by combining intrinsic susceptibilities of tissue, blood, and gas.
    Mots-clés : Administration, Inhalation, Air, Algorithms, Animals, Contrast Media, Dextrans, Dose-Response Relationship, Drug, Ferrosoferric Oxide, Helium, Image Enhancement, Injections, Intravenous, Iron, Isotopes, Lung, Magnetic Resonance Imaging, Magnetite Nanoparticles, Male, Oxides, Rats, Rats, Sprague-Dawley, Reproducibility of Results, Sensitivity and Specificity, Surface Properties.
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