Publications by authors named "Matthias Weigel"

38 Publications

Imaging multiple sclerosis pathology at 160 μm isotropic resolution by human whole-brain ex vivo magnetic resonance imaging at 3 T.

Sci Rep 2021 Jul 29;11(1):15491. Epub 2021 Jul 29.

Translational Imaging in Neurology (ThINk) Basel, Department of Biomedical Engineering, University Hospital Basel and University of Basel, Gewerbestrasse 14, 4123, Allschwil, Switzerland.

Postmortem magnetic resonance imaging (MRI) of the fixed healthy and diseased human brain facilitates spatial resolutions and image quality that is not achievable with in vivo MRI scans. Though challenging-and almost exclusively performed at 7 T field strength-depicting the tissue architecture of the entire brain in fine detail is invaluable since it enables the study of neuroanatomy and uncovers important pathological features in neurological disorders. The objectives of the present work were (1) to develop a 3D isotropic ultra-high-resolution imaging approach for human whole-brain ex vivo acquisitions working on a standard clinical 3 T MRI system; and (2) to explore the sensitivity and specificity of this concept for specific pathoanatomical features of multiple sclerosis. The reconstructed images demonstrate unprecedented resolution and soft tissue contrast of the diseased human brain at 3 T, thus allowing visualization of sub-millimetric lesions in the different cortical layers and in the cerebellar cortex, as well as unique cortical lesion characteristics such as the presence of incomplete/complete iron rims, and patterns of iron accumulation. Further details such as the subpial molecular layer, the line of Gennari, and some intrathalamic nuclei are also well distinguishable.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1038/s41598-021-94891-1DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8322069PMC
July 2021

Microstructure-Weighted Connectomics in Multiple Sclerosis.

Brain Connect 2021 Sep 8. Epub 2021 Sep 8.

Neurology Clinic and Policlinic, Departments of Medicine, Clinical Research and Biomedical Engineering, University Hospital Basel and University of Basel, Basel, Switzerland.

Graph theory has been applied to study the pathophysiology of multiple sclerosis (MS) since it provides global and focal measures of brain network properties that are affected by MS. Typically, the connection strength and, consequently, the network properties are computed by counting the number of streamlines (NOS) connecting couples of gray matter regions. However, recent studies have shown that this method is not quantitative. We evaluated diffusion-based microstructural measures extracted from three different models to assess the network properties in a group of 66 MS patients and 64 healthy subjects. Besides, we assessed their correlation with patients' disability and with a biological measure of neuroaxonal damage. Graph metrics extracted from connectomes weighted by intra-axonal microstructural components were the most sensitive to MS pathology and the most related to clinical disability. In contrast, measures of network segregation extracted from the connectomes weighted by maps describing extracellular diffusivity were the most related to serum concentration of neurofilament light chain. Network properties assessed with NOS were neither sensitive to MS pathology nor correlated with clinical and pathological measures of disease impact in MS patients. Using tractometry-derived graph measures in MS patients, we identified a set of metrics based on microstructural components that are highly sensitive to the disease and that provide sensitive correlates of clinical and biological deterioration in MS patients. Impact statement Graph theory has been widely used to study the alterations in the structural connectivity of multiple sclerosis (MS) patients. Usually, brain graphs used for the extraction of network metrics are created by counting the number of streamlines connecting gray matter regions, however, this method is not quantitative. In this study, we used tractometry to average the values of diffusion-based microstructural maps along the reconstructed streamlines. Our results show that network metrics extracted from the connectomes weighted on microstructural maps provide sensitive information to MS pathology, which correlate with clinical and biological measures of disease impact.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1089/brain.2021.0047DOI Listing
September 2021

Chronic White Matter Inflammation and Serum Neurofilament Levels in Multiple Sclerosis.

Neurology 2021 08 4;97(6):e543-e553. Epub 2021 Jun 4.

From the Department of Neurology (P.M., V.v.P.), Cliniques Universitaires Saint-Luc, Université Catholique de Louvain, Brussels, Belgium; Departments of Neurology (P.M., A.M., M.T., C.P., R.D.P.) and Radiology (J.K., M.W., R.G., P.-J.L., R.R., D.L., L.K., C.G.), Lausanne University Hospital and Lausanne University; Departments of Medicine, Clinical Research, and Biomedical Engineering (J.K., M.W., R.G., P.-J.L., R.R., D.L., L.K., C.G.) and Translational Imaging in Neurology (ThINk), Department of Biomedical Engineering Basel (M.W., R.G., R.G., P.-J.L., R.R., C.G.), Neurologic Clinic and Policlinic, MS Center and Research Center for Clinical Neuroimmunology and Neuroscience Basel (RC2NB), and Clinical Trial Unit, Department of Clinical Research (S.S., P.B.), University Hospital Basel and University of Basel, Switzerland; Institute of Neuropathology (F.v.d.M., C.S.), University Medical Center Göttingen, Germany; Radiological Physics, Department of Radiology (M.W.), University Hospital Basel; Signal Processing Laboratory (LTS5) (F.L.R., M.B.C.), Ecole Polytechnique Fédérale de Lausanne; CIBM Center for Biomedical Imaging (F.L.R., M.B.C.), Lausanne, Switzerland; Department of Neurology (P.S.), Cedars-Sinai Medical Center, Los Angeles, CA; Translational Neuroradiology Section (P.S., D.S.R., M.A.), National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda; and Department of Neurology (D.S.R., M.A.), Johns Hopkins University, Baltimore, MD.

Objective: To assess whether chronic white matter inflammation in patients with multiple sclerosis (MS) as detected in vivo by paramagnetic rim MRI lesions (PRLs) is associated with higher serum neurofilament light chain (sNfL) levels, a marker of neuroaxonal damage.

Methods: In 118 patients with MS with no gadolinium-enhancing lesions or recent relapses, we analyzed 3D-submillimeter phase MRI and sNfL levels. Histopathologic evaluation was performed in 25 MS lesions from 20 additional autopsy MS cases.

Results: In univariable analyses, participants with ≥2 PRLs (n = 43) compared to those with ≤1 PRL (n = 75) had higher age-adjusted sNfL percentiles (median, 91 and 68; < 0.001) and higher Multiple Sclerosis Severity Scale scores (MSSS median, 4.3 and 2.4; = 0.003). In multivariable analyses, sNfL percentile levels were higher in PRLs ≥2 cases (β, 16.3; 95% confidence interval [CI], 4.6-28.0; < 0.01), whereas disease-modifying treatment (DMT), Expanded Disability Status Scale (EDSS) score, and T2 lesion load did not affect sNfL. In a similar model, sNfL percentile levels were highest in cases with ≥4 PRLs (n = 30; β, 30.4; 95% CI, 15.6-45.2; < 0.01). Subsequent multivariable analysis revealed that PRLs ≥2 cases also had higher MSSS (β, 1.1; 95% CI, 0.3-1.9; < 0.01), whereas MSSS was not affected by DMT or T2 lesion load. On histopathology, both chronic active and smoldering lesions exhibited more severe acute axonal damage at the lesion edge than in the lesion center (edge vs center: = 0.004 and = 0.0002, respectively).

Conclusion: Chronic white matter inflammation was associated with increased levels of sNfL and disease severity in nonacute MS, suggesting that PRL contribute to clinically relevant, inflammation-driven neurodegeneration.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1212/WNL.0000000000012326DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8424501PMC
August 2021

GAMER-MRI in Multiple Sclerosis Identifies the Diffusion-Based Microstructural Measures That Are Most Sensitive to Focal Damage: A Deep-Learning-Based Analysis and Clinico-Biological Validation.

Front Neurosci 2021 6;15:647535. Epub 2021 Apr 6.

Translational Imaging in Neurology (ThINk) Basel, Department of Biomedical Engineering, University Hospital Basel and University of Basel, Basel, Switzerland.

Conventional magnetic resonance imaging (cMRI) in multiple sclerosis (MS) patients provides measures of focal brain damage and activity, which are fundamental for disease diagnosis, prognosis, and the evaluation of response to therapy. However, cMRI is insensitive to the damage to the microenvironment of the brain tissue and the heterogeneity of MS lesions. In contrast, the damaged tissue can be characterized by mathematical models on multishell diffusion imaging data, which measure different compartmental water diffusion. In this work, we obtained 12 diffusion measures from eight diffusion models, and we applied a deep-learning attention-based convolutional neural network (CNN) (GAMER-MRI) to select the most discriminating measures in the classification of MS lesions and the perilesional tissue by attention weights. Furthermore, we provided clinical and biological validation of the chosen metrics-and of their most discriminative combinations-by correlating their respective mean values in MS patients with the corresponding Expanded Disability Status Scale (EDSS) and the serum level of neurofilament light chain (sNfL), which are measures of disability and neuroaxonal damage. Our results show that the neurite density index from neurite orientation and dispersion density imaging (NODDI), the measures of the intra-axonal and isotropic compartments from microstructural Bayesian approach, and the measure of the intra-axonal compartment from the spherical mean technique NODDI were the most discriminating (respective attention weights were 0.12, 0.12, 0.15, and 0.13). In addition, the combination of the neurite density index from NODDI and the measures for the intra-axonal and isotropic compartments from the microstructural Bayesian approach exhibited a stronger correlation with EDSS and sNfL than the individual measures. This work demonstrates that the proposed method might be useful to select the microstructural measures that are most discriminative of focal tissue damage and that may also be combined to a unique contrast to achieve stronger correlations to clinical disability and neuroaxonal damage.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.3389/fnins.2021.647535DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8055933PMC
April 2021

Normalization of Spinal Cord Total Cross-Sectional and Gray Matter Areas as Quantified With Radially Sampled Averaged Magnetization Inversion Recovery Acquisitions.

Front Neurol 2021 25;12:637198. Epub 2021 Mar 25.

Neurologic Clinic and Policlinic, University Hospital Basel, Basel, Switzerland.

MR imaging of the spinal cord (SC) gray matter (GM) at the cervical and lumbar enlargements' level may be particularly informative in lower motor neuron disorders, e. g., spinal muscular atrophy, but also in other neurodegenerative or autoimmune diseases affecting the SC. Radially sampled averaged magnetization inversion recovery acquisition (rAMIRA) is a novel approach to perform SC imaging in clinical settings with favorable contrast and is well-suited for SC GM quantitation. However, before applying rAMIRA in clinical studies, it is important to understand (i) the sources of inter-subject variability of total SC cross-sectional areas (TCA) and GM area (GMA) measurements in healthy subjects and (ii) their relation to age and sex to facilitate the detection of pathology-associated changes. In this study, we aimed to develop normalization strategies for rAMIRA-derived SC metrics using skull and spine-based metrics to reduce anatomical variability. Sixty-one healthy subjects (age range 11-93 years, 37.7% women) were investigated with axial two-dimensional rAMIRA imaging at 3T MRI. Cervical and thoracic levels including the level of the cervical (C4/C5) and lumbar enlargements (T) were examined. SC T2-weighted sagittal images and high-resolution 3D whole-brain T1-weighted images were acquired. TCA and GMAs were quantified. Anatomical variables with associations of || > 0.30 in univariate association with SC areas, and age and sex were used to construct normalization models using backward selection with TCA as outcome. The effect of the normalization was assessed by % relative standard deviation (RSD) reductions. Mean inter-individual variability and the SD of the SC area metrics were considerable: TCA: 8.1%/9.0; TCA: 8.9%/6.5; GMA: 8.6%/2.2; GMA: 12.2%/3.8. Normalization based on sex, brain WM volume, and spinal canal area resulted in RSD reductions of 23.7% for TCAs and 12.0% for GM areas at C4/C5. Normalizations based on the area of spinal canal alone resulted in RSD reductions of 10.2% for TCAs and 9.6% for GM areas at C4/C5, respectively. Anatomic inter-individual variability of SC areas is substantial. This study identified effective normalization models for inter-subject variability reduction in TCA and SC GMA in healthy subjects based on rAMIRA imaging.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.3389/fneur.2021.637198DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8027254PMC
March 2021

Fast Open-Source Toolkit for Water T2 Mapping in the Presence of Fat From Multi-Echo Spin-Echo Acquisitions for Muscle MRI.

Front Neurol 2021 26;12:630387. Epub 2021 Feb 26.

Department of Neurology, Buffalo Neuroimaging Analysis Center, Jacobs School of Medicine and Biomedical Sciences, University at Buffalo, The State University of New York, Buffalo, NY, United States.

Imaging has become a valuable tool in the assessment of neuromuscular diseases, and, specifically, quantitative MR imaging provides robust biomarkers for the monitoring of disease progression. Quantitative evaluation of fat infiltration and quantification of the T2 values of the muscular tissue's water component (wT2) are two of the most essential indicators currently used. As each voxel of the image can contain both water and fat, a two-component model for the estimation of wT2 must be used. In this work, we present a fast method for reconstructing wT2 maps obtained from conventional multi-echo spin-echo (MESE) acquisitions and released as Free Open Source Software. The proposed software is capable of fast reconstruction thanks to extended phase graphs (EPG) simulations and dictionary matching implemented on a general-purpose graphic processing unit. The program can also perform more conventional biexponential least-squares fitting of the data and incorporate information from an external water-fat acquisition to increase the accuracy of the results. The method was applied to the scans of four healthy volunteers and five subjects suffering from facioscapulohumeral muscular dystrophy (FSHD). Conventional multi-slice MESE acquisitions were performed with 17 echoes, and additionally, a 6-echo multi-echo gradient-echo (MEGE) sequence was used for an independent fat fraction calculation. The proposed reconstruction software was applied on the full datasets, and additionally to reduced number of echoes, respectively, to 8, 5, and 3, using EPG and biexponential least-squares fitting, with and without incorporating information from the MEGE acquisition. The incorporation of external fat fraction maps increased the robustness of the fitting with a reduced number of echoes per datasets, whereas with unconstrained fitting, the total of 17 echoes was necessary to retain an independence of wT2 from the level of fat infiltration. In conclusion, the proposed software can successfully be used to calculate wT2 maps from conventional MESE acquisition, allowing the usage of an optimized protocol with similar precision and accuracy as a 17-echo acquisition. As it is freely released to the community, it can be used as a reference for more extensive cohort studies.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.3389/fneur.2021.630387DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7952742PMC
February 2021

Myelin and axon pathology in multiple sclerosis assessed by myelin water and multi-shell diffusion imaging.

Brain 2021 07;144(6):1684-1696

Department of Medicine and Biomedical Engineering, Translational Imaging in Neurology Basel, University Hospital Basel and University of Basel, Basel, Switzerland.

Damage to the myelin sheath and the neuroaxonal unit is a cardinal feature of multiple sclerosis; however, a detailed characterization of the interaction between myelin and axon damage in vivo remains challenging. We applied myelin water and multi-shell diffusion imaging to quantify the relative damage to myelin and axons (i) among different lesion types; (ii) in normal-appearing tissue; and (iii) across multiple sclerosis clinical subtypes and healthy controls. We also assessed the relation of focal myelin/axon damage with disability and serum neurofilament light chain as a global biological measure of neuroaxonal damage. Ninety-one multiple sclerosis patients (62 relapsing-remitting, 29 progressive) and 72 healthy controls were enrolled in the study. Differences in myelin water fraction and neurite density index were substantial when lesions were compared to healthy control subjects and normal-appearing multiple sclerosis tissue: both white matter and cortical lesions exhibited a decreased myelin water fraction and neurite density index compared with healthy (P < 0.0001) and peri-plaque white matter (P < 0.0001). Periventricular lesions showed decreased myelin water fraction and neurite density index compared with lesions in the juxtacortical region (P < 0.0001 and P < 0.05). Similarly, lesions with paramagnetic rims showed decreased myelin water fraction and neurite density index relative to lesions without a rim (P < 0.0001). Also, in 75% of white matter lesions, the reduction in neurite density index was higher than the reduction in the myelin water fraction. Besides, normal-appearing white and grey matter revealed diffuse reduction of myelin water fraction and neurite density index in multiple sclerosis compared to healthy controls (P < 0.01). Further, a more extensive reduction in myelin water fraction and neurite density index in normal-appearing cortex was observed in progressive versus relapsing-remitting participants. Neurite density index in white matter lesions correlated with disability in patients with clinical deficits (P < 0.01, beta = -10.00); and neurite density index and myelin water fraction in white matter lesions were associated to serum neurofilament light chain in the entire patient cohort (P < 0.01, beta = -3.60 and P < 0.01, beta = 0.13, respectively). These findings suggest that (i) myelin and axon pathology in multiple sclerosis is extensive in both lesions and normal-appearing tissue; (ii) particular types of lesions exhibit more damage to myelin and axons than others; (iii) progressive patients differ from relapsing-remitting patients because of more extensive axon/myelin damage in the cortex; and (iv) myelin and axon pathology in lesions is related to disability in patients with clinical deficits and global measures of neuroaxonal damage.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1093/brain/awab088DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8374972PMC
July 2021

Model-informed machine learning for multi-component T relaxometry.

Med Image Anal 2021 04 17;69:101940. Epub 2020 Dec 17.

Signal Processing Lab 5 (LTS5), École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland; Department of Radiology, Lausanne University Hospital and University of Lausanne, Switzerland.

Recovering the T distribution from multi-echo T magnetic resonance (MR) signals is challenging but has high potential as it provides biomarkers characterizing the tissue micro-structure, such as the myelin water fraction (MWF). In this work, we propose to combine machine learning and aspects of parametric (fitting from the MRI signal using biophysical models) and non-parametric (model-free fitting of the T distribution from the signal) approaches to T relaxometry in brain tissue by using a multi-layer perceptron (MLP) for the distribution reconstruction. For training our network, we construct an extensive synthetic dataset derived from biophysical models in order to constrain the outputs with a priori knowledge of in vivo distributions. The proposed approach, called Model-Informed Machine Learning (MIML), takes as input the MR signal and directly outputs the associated T distribution. We evaluate MIML in comparison to a Gaussian Mixture Fitting (parametric) and Regularized Non-Negative Least Squares algorithms (non-parametric) on synthetic data, an ex vivo scan, and high-resolution scans of healthy subjects and a subject with Multiple Sclerosis. In synthetic data, MIML provides more accurate and noise-robust distributions. In real data, MWF maps derived from MIML exhibit the greatest conformity to anatomical scans, have the highest correlation to a histological map of myelin volume, and the best unambiguous lesion visualization and localization, with superior contrast between lesions and normal appearing tissue. In whole-brain analysis, MIML is 22 to 4980 times faster than the non-parametric and parametric methods, respectively.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.media.2020.101940DOI Listing
April 2021

GAMER MRI: Gated-attention mechanism ranking of multi-contrast MRI in brain pathology.

Neuroimage Clin 2021 3;29:102522. Epub 2020 Dec 3.

Translational Imaging in Neurology (ThINk) Basel, Department of Medicine and Biomedical Engineering, University Hospital Basel and University of Basel, Basel, Switzerland; Neurologic Clinic and Policlinic, Departments of Medicine, Clinical Research and Biomedical Engineering, University Hospital Basel and University of Basel, Basel, Switzerland; Research Center for Clinical Neuroimmunology and Neuroscience Basel, University Hospital Basel and University of Basel, Basel, Switzerland.

Introduction: During the last decade, a multitude of novel quantitative and semiquantitative MRI techniques have provided new information about the pathophysiology of neurological diseases. Yet, selection of the most relevant contrasts for a given pathology remains challenging. In this work, we developed and validated a method, Gated-Attention MEchanism Ranking of multi-contrast MRI in brain pathology (GAMER MRI), to rank the relative importance of MR measures in the classification of well understood ischemic stroke lesions. Subsequently, we applied this method to the classification of multiple sclerosis (MS) lesions, where the relative importance of MR measures is less understood.

Methods: GAMER MRI was developed based on the gated attention mechanism, which computes attention weights (AWs) as proxies of importance of hidden features in the classification. In the first two experiments, we used Trace-weighted (Trace), apparent diffusion coefficient (ADC), Fluid-Attenuated Inversion Recovery (FLAIR), and T1-weighted (T1w) images acquired in 904 acute/subacute ischemic stroke patients and in 6,230 healthy controls and patients with other brain pathologies to assess if GAMER MRI could produce clinically meaningful importance orders in two different classification scenarios. In the first experiment, GAMER MRI with a pretrained convolutional neural network (CNN) was used in conjunction with Trace, ADC, and FLAIR to distinguish patients with ischemic stroke from those with other pathologies and healthy controls. In the second experiment, GAMER MRI with a patch-based CNN used Trace, ADC and T1w to differentiate acute ischemic stroke lesions from healthy tissue. The last experiment explored the performance of patch-based CNN with GAMER MRI in ranking the importance of quantitative MRI measures to distinguish two groups of lesions with different pathological characteristics and unknown quantitative MR features. Specifically, GAMER MRI was applied to assess the relative importance of the myelin water fraction (MWF), quantitative susceptibility mapping (QSM), T1 relaxometry map (qT1), and neurite density index (NDI) in distinguishing 750 juxtacortical lesions from 242 periventricular lesions in 47 MS patients. Pair-wise permutation t-tests were used to evaluate the differences between the AWs obtained for each quantitative measure.

Results: In the first experiment, we achieved a mean test AUC of 0.881 and the obtained AWs of FLAIR and the sum of AWs of Trace and ADC were 0.11 and 0.89, respectively, as expected based on previous knowledge. In the second experiment, we achieved a mean test F1 score of 0.895 and a mean AW of Trace = 0.49, of ADC = 0.28, and of T1w = 0.23, thereby confirming the findings of the first experiment. In the third experiment, MS lesion classification achieved test balanced accuracy = 0.777, sensitivity = 0.739, and specificity = 0.814. The mean AWs of T1map, MWF, NDI, and QSM were 0.29, 0.26, 0.24, and 0.22 (p < 0.001), respectively.

Conclusions: This work demonstrates that the proposed GAMER MRI might be a useful method to assess the relative importance of MRI measures in neurological diseases with focal pathology. Moreover, the obtained AWs may in fact help to choose the best combination of MR contrasts for a specific classification problem.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.nicl.2020.102522DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7773673PMC
June 2021

RimNet: A deep 3D multimodal MRI architecture for paramagnetic rim lesion assessment in multiple sclerosis.

Neuroimage Clin 2020 4;28:102412. Epub 2020 Sep 4.

Signal Processing Laboratory (LTS5), Ecole Polytechnique Fédérale de Lausanne, Switzerland; Medical Image Analysis Laboratory (MIAL), Center for Biomedical Imaging (CIBM), University of Lausanne, Switzerland; Department of Radiology, Lausanne University Hospital and University of Lausanne, Switzerland. Electronic address:

Objectives: In multiple sclerosis (MS), the presence of a paramagnetic rim at the edge of non-gadolinium-enhancing lesions indicates perilesional chronic inflammation. Patients featuring a higher paramagnetic rim lesion burden tend to have more aggressive disease. The objective of this study was to develop and evaluate a convolutional neural network (CNN) architecture (RimNet) for automated detection of paramagnetic rim lesions in MS employing multiple magnetic resonance (MR) imaging contrasts.

Materials And Methods: Imaging data were acquired at 3 Tesla on three different scanners from two different centers, totaling 124 MS patients, and studied retrospectively. Paramagnetic rim lesion detection was independently assessed by two expert raters on T2*-phase images, yielding 462 rim-positive (rim+) and 4857 rim-negative (rim-) lesions. RimNet was designed using 3D patches centered on candidate lesions in 3D-EPI phase and 3D FLAIR as input to two network branches. The interconnection of branches at both the first network blocks and the last fully connected layers favors the extraction of low and high-level multimodal features, respectively. RimNet's performance was quantitatively evaluated against experts' evaluation from both lesion-wise and patient-wise perspectives. For the latter, patients were categorized based on a clinically relevant threshold of 4 rim+ lesions per patient. The individual prediction capabilities of the images were also explored and compared (DeLong test) by testing a CNN trained with one image as input (unimodal).

Results: The unimodal exploration showed the superior performance of 3D-EPI phase and 3D-EPI magnitude images in the rim+/- classification task (AUC = 0.913 and 0.901), compared to the 3D FLAIR (AUC = 0.855, Ps < 0.0001). The proposed multimodal RimNet prototype clearly outperformed the best unimodal approach (AUC = 0.943, P < 0.0001). The sensitivity and specificity achieved by RimNet (70.6% and 94.9%, respectively) are comparable to those of experts at the lesion level. In the patient-wise analysis, RimNet performed with an accuracy of 89.5% and a Dice coefficient (or F1 score) of 83.5%.

Conclusions: The proposed prototype showed promising performance, supporting the usage of RimNet for speeding up and standardizing the paramagnetic rim lesions analysis in MS.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.nicl.2020.102412DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7509077PMC
June 2021

Imaging of the thoracic spinal cord using radially sampled averaged magnetization inversion recovery acquisitions.

J Neurosci Methods 2020 09 21;343:108825. Epub 2020 Jun 21.

Division of Radiological Physics, Department of Radiology, University Hospital Basel, Petersgraben 4, 4031 Basel, Switzerland; Department of Biomedical Engineering, University of Basel, Gewerbestrasse 14, 4123 Allschwil, Switzerland.

Background: Spinal cord (SC) gray and white matter atrophy quantification by advanced morphometric MRI can help to better characterize the course of neurodegenerative diseases in vivo, such as e.g. lower motor neuron disorders. Imaging the lower thoracic cord - containing those motor neurons that control leg function - could be particularly informative, however, is challenging due to tissue composition, physiological motion and large field of views.

New Method: An "averaged magnetization inversion recovery acquisitions" (AMIRA) approach with a radial k-space acquisition scheme was developed. The method is designed for morphometric SC imaging with a focus on the thoracic SC.

Results: In a typical setting, radial AMIRA acquires transverse slices with a high 0.50 × 0.50mm in-plane resolution and a pronounced positive contrast between thoracic gray and white matter, within typically 2:39 min. Additional proof-of-concept measurements in patients demonstrate that such contrast and resolving capability is indeed necessary to assess potential atrophy of the anterior horns.

Comparison With Existing Method(s): Radial AMIRA utilizes two benefits of radial MRI techniques: being generally less prone to motion effects and that fold over artifacts can manifest less intrusively. These benefits are united with the original AMIRA approach which allows the contrast to be 'tuned' and improved based on the combination of five simultaneously acquired images of different tissue contrast.

Conclusions: Radial AMIRA is a promising approach for in vivo SC gray and white matter atrophy visualization and quantification in lower motor neuron diseases and other autoimmune or genetic diseases involving the entire (not only cervical) spinal cord.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.jneumeth.2020.108825DOI Listing
September 2020

On the application of balanced steady-state free precession to MR microscopy.

MAGMA 2019 Aug 16;32(4):437-447. Epub 2019 Jan 16.

Department of Radiology, Medical Physics, Faculty of Medicine, Medical Center - University of Freiburg, Killianstrasse 5a, 79106, Freiburg, Germany.

Objective: The applicability of the balanced steady-state free precession (bSSFP) sequence to the field of MR microscopy was investigated, since the potentially high SNR makes bSSFP attractive. However, particularly at ultra-high magnetic fields, a number of constraints emerge: the frequency sensitivity of the bSSFP signal, the duty cycle of the imaging gradients, and the intrinsic diffusion attenuation of the steady state due to the imaging gradients.

Materials And Methods: Optimization of the bSSFP sequence was performed on three imaging systems (7 T and 9.4 T) suited for MR microscopy. Since biological samples are often imaged in the very proximity of materials from sample containers/holder or devices such as electrodes, several microscopy phantoms representing such circumstances were fabricated and examined with 3D bSSFP.

Results: Artifact-free microscopic bSSFP images could be obtained with voxel sizes down to 16 µm × 16 µm × 78 µm and with an SNR gain of 25% over standard gradient echo images.

Conclusion: With appropriate choice of phantom materials, optimization of the flip angle to the diffusion-attenuated steady state and protocols considering duty-cycle limitations, bSSFP can be a valuable tool in MR microscopy.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1007/s10334-019-00736-4DOI Listing
August 2019

Spinal cord imaging using averaged magnetization inversion recovery acquisitions.

Magn Reson Med 2018 04 16;79(4):1870-1881. Epub 2017 Jul 16.

Department of Radiology, Division of Radiological Physics, University of Basel Hospital, Basel, Switzerland.

Purpose: To establish a novel approach for fast high-resolution spinal cord (SC) imaging using averaged magnetization inversion recovery acquisitions (AMIRA).

Methods: The AMIRA concept is based on an inversion recovery (IR) prepared, segmented, and time-limited cine balanced steady state free precession sequence. Typically, for the fastest SC imaging without any signal averaging, eight consecutive images in time with an in-plane resolution of 0.67 × 0.67 mm and 6 mm to 8 mm slice thickness are acquired in 51 s. AMIRA does not require parallel acquisition techniques.

Results: AMIRA measures eight images of remarkable tissue contrast variation between spinal cord gray (GM) and white matter (WM) and cerebrospinal fluid (CSF). Following the AMIRA concept, averaging the first IR contrast images not only improves the signal-to-noise ratio but also offers a surprising enhancement of the contrast-to-noise ratio between GM and WM, whereas averaging the last images considerably improves the contrast-to-noise ratio between WM and CSF. These observations are supported by quantitative data.

Conclusion: The AMIRA concept provides 2D spinal cord imaging with multiple tissue contrasts and enhanced contrast-to-noise ratios with a typical 0.67 × 0.67 mm in-plane resolution and a slice thickness between 4 mm and 8 mm acquired in only 1 to 2 min per slice. Magn Reson Med 79:1870-1881, 2018. © 2017 International Society for Magnetic Resonance in Medicine.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/mrm.26833DOI Listing
April 2018

Introduction to Magnetic Resonance Imaging for Neurologists.

Continuum (Minneap Minn) 2016 Oct;22(5, Neuroimaging):1379-1398

Purpose Of Review: In neuroradiology, highly sophisticated methods such as MRI are implemented to investigate different entities of the central nervous system and to acquire miscellaneous images where tissues display varying degrees of characteristic signal intensity or brightness. Compared to x-ray, CT, and ultrasound, MRI produces clearer images of tissues, body fluids, and fat. The basics of MRI may be unknown to neurologists; this article introduces MRI physics, techniques, and interpretation guidelines.

Recent Findings: This article discusses the basics of MRI to provide clinicians with the scientific underpinning of MRI technology and to help them better understand image features and improve their diagnosis and differential diagnosis by combining MRI characteristics with their knowledge of pathology and neurology.

Summary: This article will help neurologists deepen their knowledge and understanding of MRI by introducing the basics of MRI physics, technology, image acquisition, protocols, and image interpretation.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1212/CON.0000000000000391DOI Listing
October 2016

Intrinsic diffusion sensitivity of the balanced steady-state free precession (bSSFP) imaging sequence.

NMR Biomed 2015 Nov 7;28(11):1383-92. Epub 2015 Sep 7.

University Medical Center Freiburg, Department of Radiology, Medical Physics, Freiburg, Germany.

The purpose of this work was to analyze the intrinsic diffusion sensitivity of the balanced steady-state free precession (bSSFP) imaging sequence, meaning the observation of diffusion-induced attenuation of the bSSFP steady-state signal due to the imaging gradients. Although these diffusion effects are usually neglected for most clinical gradient systems, such strong gradient systems are employed for high resolution imaging of small animals or MR Microscopy. The impact on the bSSFP signal of the imaging gradients characterized by their b-values was analyzed with simulations and experiments at a 7T animal scanner using a gradient system with maximum gradient amplitude of approx. 700 mT/m. It was found that the readout gradients have a stronger impact on the attenuation than the phase encoding gradients. Also, as the PE gradients are varying with each repetition interval, the diffusion effects induce strong modulations of the bSSFP signal over the sequence repetition cycles depending on the phase encoding gradient table. It is shown that a signal gain can be obtained through a change of flip angle as a new optimal flip angle maximizing the signal can be defined. The dependency of the diffusion effects on relaxation times and b-values were explored with simulations. The attenuation increases with T2. In conclusion, diffusion attenuation of the bSSFP signal becomes significant for high resolution imaging voxel size (roughly < 100 μm) of long T2 substances.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/nbm.3380DOI Listing
November 2015

Extended phase graphs: dephasing, RF pulses, and echoes - pure and simple.

Authors:
Matthias Weigel

J Magn Reson Imaging 2015 Feb 16;41(2):266-95. Epub 2014 Apr 16.

Department of Radiology, Medical Physics, University Medical Center Freiburg, Freiburg, Germany; Radiological Physics, University of Basel Hospital, Basel, Switzerland.

The extended phase graph (EPG) concept represents a powerful tool for depicting and understanding the magnetization response of a broad variety of MR sequences. EPGs focus on echo generation as well as on classification and use a Fourier based magnetization description in terms of "configurations states". The effect of gradients, radiofrequency (RF) pulses, relaxation, and motion phenomena during the MR sequence is characterized as the action of a few matrix operations on these configuration states. Thus, the EPG method allows for fast and precise quantitation of echo intensities even if several gradients and RF pulses are applied. EPG diagrams aid in the comprehension of different types of echoes and their corresponding echo time. Despite its several benefits in regard to a large number of problems and issues, researchers and users still often refrain from applying EPGs. It seems that "phase graphing" is still seen as a kind of "magic." The present review investigates the foundation of EPGs and sheds light on prerequisites for adding more advanced phenomena such as diffusion. The links between diagrams and calculations are discussed. A further focus is on limitations and simplifications as well recent extensions within the EPG concept. To make the review complete, representative software for EPG coding is provided.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/jmri.24619DOI Listing
February 2015

Reproducibility and accuracy of optic nerve sheath diameter assessment using ultrasound compared to magnetic resonance imaging.

BMC Neurol 2013 Dec 1;13:187. Epub 2013 Dec 1.

Department of Neurology, University Medical Center Freiburg, Breisacher Str, 64, 79106 Freiburg, Germany.

Background: Quantification of the optic nerve sheath diameter (ONSD) by transbulbar sonography is a promising non-invasive technique for the detection of altered intracranial pressure. In order to establish this method as follow-up tool in diseases with intracranial hyper- or hypotension scan-rescan reproducibility and accuracy need to be systematically investigated.

Methods: The right ONSD of 15 healthy volunteers (mean age 24.5 ± 0.8 years) were measured by both transbulbar sonography (9 - 3 MHz) and 3 Tesla MRI (half-Fourier acquisition single-shot turbo spin-echo sequences, HASTE) 3 and 5 mm behind papilla. All volunteers underwent repeated ultrasound and MRI examinations in order to assess scan-rescan reproducibility and accuracy. Moreover, inter- and intra-observer variabilities were calculated for both techniques.

Results: Scan-rescan reproducibility was robust for ONSD quantification by sonography and MRI at both depths (r > 0.75, p ≤ 0.001, mean differences < 2%). Comparing ultrasound- and MRI-derived ONSD values, we found acceptable agreement between both methods for measurements at a depth of 3 mm (r = 0.72, p = 0.002, mean difference < 5%). Further analyses revealed good inter- and intra-observer reliability for sonographic measurements 3 mm behind the papilla and for MRI at 3 and 5 mm (r > 0.82, p < 0.001, mean differences < 5%).

Conclusions: Sonographic ONSD quantification 3 mm behind the papilla can be performed with good reproducibility, measurement accuracy and observer agreement. Thus, our findings emphasize the feasibility of this technique as a non-invasive bedside tool for longitudinal ONSD measurements.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1186/1471-2377-13-187DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4219451PMC
December 2013

Accurate semiautomatic assessment of ligament length variations from MRI data.

Med Phys 2013 Sep;40(9):092301

Medical Physics, Department of Radiology, University Medical Center Freiburg, 79106 Freiburg, Germany.

Purpose: A semiautomatic method for the assessment of ligament length variations during different joint positions based on MRI data is proposed.

Methods: Ligament lengths are represented as distances between points marking characteristic locations in the ligament insertion regions on the bones. These points are defined manually for one single reference joint position and for all other joint positions they are automatically mapped with high accuracy to the correct locations using image registration methods. The methodology is validated using data from 16 volunteers depicting the coracoclavicular ligaments in the left shoulder during different arm abductions.

Results: The method yielded a superior reproducibility of the point locations over different joint positions compared to manual point marking. Significant ligament length variations were found for different abductions which was not possible with manual measurements. Acquisition related geometric distortions and inaccuracies during the registration and segmentation process were small.

Conclusions: The proposed method provides superior accuracy for the in vivo analysis of ligament dynamics compared to manual measurements. This permits a better understanding of the ligament behavior during joint motion and offers new possibilities for presurgical planning which to date has not been possible with manual data analysis.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1118/1.4818058DOI Listing
September 2013

Influence of knee flexion angle and weight bearing on the Tibial Tuberosity-Trochlear Groove (TTTG) distance for evaluation of patellofemoral alignment.

Knee Surg Sports Traumatol Arthrosc 2014 Nov 29;22(11):2655-61. Epub 2013 May 29.

Department of Orthopedic and Trauma Surgery, University Medical Center Freiburg, Hugstetter Straße 55, 79106, Freiburg, Germany,

Purpose: The aim of the present study was to investigate the influence of knee flexion and weight bearing on the Tibial Tuberosity-Trochlear Groove (TTTG) distance.

Materials And Methods: Magnetic resonance imaging of the knee was carried out in 8 healthy volunteers. An open 0.25 T scanner equipped with a C-shaped permanent tilting magnet allowing examinations in weight-bearing conditions was used for the present investigation. A 3D gradient-echo sequence with axial slice orientation was obtained in a lying and an upright position with the knee straight and at 30° of knee flexion. The medial, central and lateral trochlear heights as well as the TTTG were determined.

Results: The mean medial trochlear height was 76.2 ± 4%, the central trochlear height was 72.2 ± 3%, and lateral trochlear height was 82.9 ± 3 %. The mean TTTG distance was 11.6 ± 4.4 mm in lying position at 0° knee flexion and 7.3 ± 2.9 mm (n.s.) at 30° knee flexion. Under weight bearing, the mean TTTG was significantly smaller at both 0° knee flexion 6.3 ± 3.2 mm (p = 0.040) and 30° knee flexion 4.9 ± 3.9 mm (p = 0.006) compared to the lying position with 0° knee flexion.

Conclusion: Tibial Tuberosity-Trochlear Groove distance depends on both knee flexion angle and weight bearing. The latter only seems to be of relevance in full extension.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1007/s00167-013-2537-5DOI Listing
November 2014

Spin echo magnetic resonance imaging.

J Magn Reson Imaging 2013 Apr;37(4):805-17

Department of Radiology, Medical Physics, University Medical Center, Freiburg, Germany.

The spin echo sequence is a fundamental pulse sequence in MRI. Many of today's applications in routine clinical use are based on this elementary sequence. In this review article, the principles of the spin echo formation are demonstrated on which the generation of the fundamental image contrasts T1, T2, and proton density is based. The basic imaging parameters repetition time (TR) and echo time (TE) and their influence on the image contrast are explained. Important properties such as the behavior in multi-slice imaging or in the presence of flow are depicted and the basic differences with gradient echo imaging are illustrated. The characteristics of the spin echo sequence for different magnetic field strengths with respect to clinical applications are discussed.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/jmri.24068DOI Listing
April 2013

Tendon graft fixation sites at the coracoid process for reconstruction of the coracoclavicular ligaments: a kinematic evaluation of three different surgical techniques.

Arthroscopy 2013 Feb 27;29(2):317-24. Epub 2012 Dec 27.

Department of Orthopaedic and Trauma Surgery, University Hospital Freiburg, Freiburg, Germany.

Purpose: The virtual graft length kinematics of 3 operative techniques were investigated and compared with kinematics of the native coracoclavicular ligaments.

Methods: Thirteen healthy volunteers underwent magnetic resonance imaging (MRI) of the shoulder in 30° increments of abduction (0° to 120°). A 3-dimensional model of the coracoid process (CP) and the clavicle (CL) was created. Footprints of the conoid and the trapezoid ligament were identified. At the CP the potential fixation sites of 3 techniques for reconstruction of the coracoclavicular ligaments (CCLs) were marked. The techniques investigated were (1) horizontal transcoracoid drilling (TH), (2) transclavicular-transcoracoid drilling (TT), and (3) tendon graft passage underneath (PU) the coracoid process. Distances between the clavicular and coracoidal footprints of the coracoclavicular ligaments and to the virtual footprints on the coracoid process were determined for each abduction increment.

Results: All methods investigated resulted in a significantly longer virtual trapezoidal graft (P = .001). In PU, in addition, the virtual conoidal graft was significantly longer. TT resulted in a virtual conoidal graft and conoid ligament of equal length. TH showed identical length and distance regulation of the virtual conoidal graft and the conoid ligament, but significant shortening of the virtual trapezoidal graft during abduction. PU showed isometry of the virtual trapezoidal and conoidal grafts.

Conclusions: None of the described procedures for graft fixation restores the kinematics of the native coracoclavicular ligaments. Graft fixation techniques should be chosen with respect to the preoperative type of instability. Persisting isolated vertical instability might benefit from fixation of the conoidal grafts at the native clavicular footprint. For horizontal clavicular instabilities, techniques more preserving of trapezoid ligament kinematics might be favorable.

Clinical Relevance: The data suggest that the technique of fixation in conoid and trapezoid ligament reconstruction should depend on the underlying type of instability.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.arthro.2012.08.026DOI Listing
February 2013

Diffusion sensitivity of turbo spin echo sequences.

Magn Reson Med 2012 Jun 24;67(6):1528-37. Epub 2012 Apr 24.

Department of Radiology, Medical Physics, University Medical Center Freiburg, Freiburg, Germany.

This article introduces an effective b-factor b(TSE) for turbo spin echo (TSE) sequences to quantify their inherent diffusion sensitivity. b(TSE) is investigated for a broad variety of two-dimensional- and three-dimensional-TSE sequences using constant and varying flip angles (transitions between pseudo steady states, SPACE, VISTA, Cube, etc.). The inherent TSE diffusion sensitivity becomes important for high-resolution protocols, which can lead to subtle contrast modifications or even fluid suppressions in a clinical setting or animal imaging regime. The b(TSE) values obtained considerably depend on the relaxation times and diffusion coefficient and, thus, on the tissue under observation. The fractional b(TSE) contributions per TSE imaging encoding axis are highly anisotropic. Further noteworthy effects such as decreasing b-factors along a TSE train are pointed out and explained. The results are also discussed in combination with recent findings regarding contrast properties and possible diffusion sensitivity of TSE sequences. Identical but well more pronounced b(TSE) effects are observed in the animal imaging regime due to smaller field of view and higher resolutions.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/mrm.24286DOI Listing
June 2012

Imaging with positive T1-contrast using superstimulated echoes.

Magn Reson Med 2012 Oct 21;68(4):1157-65. Epub 2011 Dec 21.

Department of Radiology, Medical Physics, University Medical Center Freiburg, Freiburg, Germany.

This article presents the basic principles of the superstimulated echo mechanism and shows preliminary results of its application to T1-weighted imaging with positive T1-contrast. A superstimulated echo scheme uses a preparation of square-wave modulated, periodically inverted z-magnetization, which after signal evolution during the mixing time TM is fully converted into transverse magnetization. This avoids the 50% signal loss of a conventional stimulated echo. Furthermore, its implementation as a preparation module for standard turbo spin echo (TSE) imaging allows producing images with positive T1-contrast.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/mrm.24111DOI Listing
October 2012

Prospective motion correction with continuous gradient updates in diffusion weighted imaging.

Magn Reson Med 2012 Feb 9;67(2):326-38. Epub 2011 Dec 9.

Department of Radiology, Medical Physics, University Medical Center Freiburg, Freiburg, Germany.

Despite the existence of numerous motion correction methods, head motion during MRI continues to be a major source of artifacts and can greatly reduce image quality. This applies particularly to diffusion weighted imaging, where strong gradients are applied during long encoding periods. These are necessary to encode microscopic movements. However, they also make the technique highly sensitive to bulk motion. In this work, we present a prospective motion correction method where all applied gradients are adjusted continuously to compensate for changes of the object position and ensure the desired phase evolution in the image coordinate frame. Additionally, in phantom experiments this new technique is used to reproduce motion artifacts with high accuracy by changing the position of the imaging frame relative to the measured object. In vivo measurements demonstrate the validity of the new correction method.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/mrm.23230DOI Listing
February 2012

In vivo analysis of coracoclavicular ligament kinematics during shoulder abduction.

Am J Sports Med 2012 Jan 3;40(1):185-92. Epub 2011 Oct 3.

Department of Orthopaedic and Trauma Surgery, University Hospital Freiburg, Freiburg, Germany.

Background: Anatomic reconstruction of the coracoclavicular ligaments for the treatment of acromioclavicular joint separations provides superior biomechanical stability compared with other procedures. Clavicular and coracoidal footprints of the conoid ligament (CL) and the trapezoid ligament (TL) are well described. So far, little is known about their kinematics and the changes of the coracoclavicular distance during shoulder abduction.

Hypothesis: The coracoclavicular distance along the coracoclavicular ligaments changes significantly with shoulder abduction and weightbearing.

Study Design: Descriptive laboratory study.

Methods: With use of an open magnetic resonance imaging scanner, the shoulders of 13 healthy volunteers were examined in supine and sitting positions. Three-dimensional magnetic resonance images of the shoulders were obtained in 30° increments of abduction (0°-120°). A manual segmentation of the scapula, the clavicle, and the coracoclavicular ligaments was performed. The insertion points of the coracoclavicular ligaments were identified, and automated measures along the ligamentous course were carried out.

Results: During transfer from the lying to sitting position, the coracoclavicular distance showed significant lengthening of 3 mm along the center of the CL, which significantly increased another 3 mm during shoulder abduction to a total lengthening of 6 mm. In the supine position, the coracoclavicular distance along the TL did not elongate significantly. In the sitting position, the distance along the medial portion of the TL shortened significantly, whereas the distance along the center portion did not elongate significantly during shoulder abduction.

Conclusion: The distances between the coracoclavicular insertion points depend on both patient and shoulder positioning. To prevent overconstraining of the graft, the CL should be fixated during 90° to 120° of shoulder abduction in a sitting position. Isometric reconstruction of the TL can be achieved if precise fixation of the graft at the centers of the conoidal and clavicular footprints is performed.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1177/0363546511423015DOI Listing
January 2012

Whole-body diffusion-weighted imaging with a continuously moving table acquisition method: preliminary results.

Magn Reson Med 2011 Jun 22;65(6):1557-63. Epub 2011 Mar 22.

Department of Electrical Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Republic of Korea.

In this study, a method for whole-body diffusion-weighted imaging (wbDWI) during continuous table motion has been developed and implemented on a clinical scanner based on a short-Tau inversion recovery echo-planar DWI sequence. Unlike currently available multistation wbDWI, which has disadvantages such as long scanning times, poor image quality, and troublesome data realignment, continuously moving table wbDWI can overcome these technical problems while extending the longitudinal field of view in MRI systems. In continuously moving table wbDWI, images are acquired consecutively at the isocenter of the magnet, having less geometric distortions and various possibilities of spatial and temporal coverage of an extended field of view. The acquired images, together with an apparent diffusion coefficient analysis, show that continuously moving table wbDWI can be used by appropriately adapting the table velocity, scan range, radiofrequency coils, slice resolutions, and spatio-temporal acquisition schemes according to various clinical demands.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/mrm.22833DOI Listing
June 2011

Global fiber reconstruction becomes practical.

Neuroimage 2011 Jan 18;54(2):955-62. Epub 2010 Sep 18.

Department of Radiology, Medical Physics, University Hospital Freiburg, Freiburg, Germany.

Global fiber reconstruction aims at providing a consistent view of the fiber architecture in the whole volume of cerebral white matter on the basis of diffusion-sensitized magnetic resonance imaging. A new realization of this principle is presented. The method utilizes data acquired with high angular resolution diffusion imaging (HARDI), a measurement method that fulfills clinical requirements. For the first time among global reconstruction methods, the computation time is acceptable for a broad class of practical applications. The method does not involve any boundary conditions that prescribe the location of the ends of reconstructed fibers. This helps to minimize necessary user interaction and operator dependence. Results obtained in a physical phantom demonstrate a high reconstruction quality. In vivo results have been obtained in several volunteers. The algorithm found a number of prominent fascicles including those in the limbic system, which had been problematic for a previously published version of global tracking.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.neuroimage.2010.09.016DOI Listing
January 2011

Multiplex RARE: a simultaneous multislice spin-echo sequence that fulfils CPMG conditions.

Magn Reson Med 2010 Jul;64(1):299-305

Department of Diagnostic Radiology, Medical Physics, University Hospital, Freiburg, Germany.

This work presents a new imaging sequence in which multiple slices are simultaneously excited and refocused in a spin-echo train. The multiple spin-echo trains are interleaved in such a manner that (i) the Carr-Purcell-Meiboom-Gill conditions are fulfilled at all times, and (ii) the signals from slices can be separated, preventing aliasing. This paper also demonstrates how the sequence may be used in a novel fat-water Dixon method that enables fast volume coverage. The technique is demonstrated in phantoms and in vivo.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/mrm.22341DOI Listing
July 2010

Investigation and modeling of magnetization transfer effects in two-dimensional multislice turbo spin echo sequences with low constant or variable flip angles at 3 T.

Magn Reson Med 2010 Jan;63(1):230-4

Department of Diagnostic Radiology, Medical Physics, University Hospital Freiburg, Freiburg, Germany.

Magnetization transfer effects represent a major source of contrast in multislice turbo spin echo sequences (TSE)/fast spin echo sequences. Generally, low refocusing flip angles have become common in such MRI sequences, especially to mitigate specific absorption rate problems. Since the strength of magnetization transfer effects is related to the radiofrequency power and therefore specific absorption rate applied, magnetization transfer induced signal attenuations are investigated for a variety of TSE sequences with low constant and variable flip angles. Noticeable differences between the sequences have been observed. In particular, fewer signal attenuations are observed for TSE with low flip angles such as hyperecho-TSE and smooth transitions between pseudo steady states-TSE, leading to contrast that is less dependent on the number of slices. It is shown that the strength of the magnetization transfer-induced signal attenuations can be understood and described by a physical framework, which is based on the mean square flip angle of a given TSE sequence.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/mrm.22145DOI Listing
January 2010

Retrobulbar optic nerve diameter measured by high-speed magnetic resonance imaging as a biomarker for axonal loss in glaucomatous optic atrophy.

Invest Ophthalmol Vis Sci 2009 Sep 30;50(9):4223-8. Epub 2009 Apr 30.

Department of Ophthalmology, University Hospital Freiburg, Freiburg, Germany.

Purpose: To assess a novel magnetic resonance imaging (MRI) protocol for quantifying the optic nerve diameter (OND) as a measure of axonal loss in the optic nerve.

Methods: Included in the study was one eye each from 47 subjects, of whom 9 had no eye disease, 16 had preperimetric glaucoma, 11 had a glaucomatous mean visual field defect of <10 dB and 11 of >10 dB. Each subject underwent automated perimetry, scanning laser polarimetry, optic coherence tomography, scanning laser tomography, and ultrafast high-resolution MRI at 3 T. OND was determined 5, 10, and 15 mm behind the eye with a half Fourier-acquired single-shot turbo spin-echo (HASTE)-sequence requiring 1.5 seconds of data acquisition time per slice and providing a spatial resolution of 0.11 mm. A multiple linear regression model was applied to determine correlations (r) among the different techniques.

Results: The correlation (r) was <0.37 for OND measurements taken 5 mm behind the eye. At 10 mm behind the eye, r increased to 0.57 and was statistically significant in four out six instances. In the orbital apex 15 mm behind the eye, r reached a maximum of 0.80 and was statistically significant in all instances. OND correlated best with the retinal nerve fiber layer thickness measured by optic coherence tomography.

Conclusions: Retina- or optic nerve head-related surrogate markers for axonal content correlated closely with the OND, although only when it was measured in the orbital apex. High-resolution MRI using an ultrafast HASTE-sequence at 3 T proved useful for OND quantification and may be a valuable asset in future neuroprotection trials.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1167/iovs.08-2683DOI Listing
September 2009
-->