-
Magnetic Resonance Imaging May 2022We propose a method that can provide information about the anisotropy and orientation of diffusion in the brain from only 3 orthogonal gradient directions without...
We propose a method that can provide information about the anisotropy and orientation of diffusion in the brain from only 3 orthogonal gradient directions without imposing additional assumptions. The method is based on the Diffusion Anisotropy (DiA) that measures the distance from a diffusion signal to its isotropic equivalent. The original formulation based on a Spherical Harmonics basis allows to go down to only 3 orthogonal directions in order to estimate the measure. In addition, an alternative simplification and a color-coding representation are also proposed. Acquisitions from a publicly available database are used to test the viability of the proposal. The DiA succeeded in providing anisotropy information from the white matter using only 3 diffusion-encoding directions. The price to pay for such reduced acquisition is an increment in the variability of the data and a subestimation of the metric on those tracts not aligned with the acquired directions. Nevertheless, the calculation of anisotropy information from DMRI is feasible using fewer than 6 gradient directions by using DiA. The method is totally compatible with existing acquisition protocols, and it may provide complementary information about orientation in fast diffusion acquisitions.
Topics: Anisotropy; Brain; Diffusion; Diffusion Magnetic Resonance Imaging; White Matter
PubMed: 35122982
DOI: 10.1016/j.mri.2022.01.014 -
Journal of Magnetic Resonance Imaging :... Apr 2023Dynamic diffusion magnetic resonance imaging (ddMRI) metrics can assess transient microstructural alterations in tissue diffusivity but requires additional scan time...
BACKGROUND
Dynamic diffusion magnetic resonance imaging (ddMRI) metrics can assess transient microstructural alterations in tissue diffusivity but requires additional scan time hindering its clinical application.
PURPOSE
To determine whether a diffusion gradient table can simultaneously acquire data to estimate dynamic and diffusion tensor imaging (DTI) metrics.
STUDY TYPE
Prospective.
SUBJECTS
Seven healthy subjects, 39 epilepsy patients (15 female, 31 male, age ± 15).
FIELD STRENGTH/SEQUENCE
Two-dimensional diffusion MRI (b = 1000 s/mm ) at a field strength of 3 T. Sessions in healthy subjects-standard ddMRI (30 directions), standard DTI (15 and 30 directions), and nested cubes scans (15 and 30 directions). Sessions in epilepsy patients-two 30 direction (standard ddMRI, 10 nested cubes) or two 15 direction scans (standard DTI, 5 nested cubes).
ASSESSMENT
Fifteen direction DTI was repeated twice for within-session test-retest measurements in healthy subjects. Bland-Altman analysis computed bias and limits of agreement for DTI metrics using test-retest scans and standard 15 direction vs. 5 nested cubes scans. Intraclass correlation (ICC) analysis compared tensor metrics between 15 direction DTI scans (standard vs. 5 nested cubes) and the coefficients of variation (CoV) of trace and apparent diffusion coefficient (ADC) between 30 direction ddMRI scans (standard vs. 10 nested cubes).
STATISTICAL TESTS
Bland-Altman and ICC analysis using a P-value of 0.05 for statistical significance.
RESULTS
Correlations of mean diffusivity (MD), axial diffusivity (AD), and radial diffusivity (RD) were strong and significant in gray (ICC > 0.95) and white matter (ICC > 0.95) between standard vs. nested cubes DTI acquisitions. Correlation of white matter fractional anisotropy was also strong (ICC > 0.95) and significant. ICCs of the CoV of dynamic ADC measured using repeated cubes and nested cubes acquisitions were modest (ICC >0.60), but significant in gray matter.
CONCLUSION
A nested cubes diffusion gradient table produces tensor-based and dynamic diffusion measurements in a single acquisition.
LEVEL OF EVIDENCE
2 TECHNICAL EFFICACY: Stage 1.
Topics: Humans; Male; Female; Adolescent; Diffusion Tensor Imaging; Prospective Studies; Diffusion Magnetic Resonance Imaging; White Matter; Epilepsy; Anisotropy
PubMed: 36056625
DOI: 10.1002/jmri.28407 -
Journal of Neuroscience Methods Jan 2021Diffusion MRI is a non-invasive technique to study brain microstructure. Differences in the microstructural properties of tissue, including size and anisotropy, can be... (Review)
Review
Diffusion MRI is a non-invasive technique to study brain microstructure. Differences in the microstructural properties of tissue, including size and anisotropy, can be represented in the signal if the appropriate method of acquisition is used. However, to depict the underlying properties, special care must be taken when designing the acquisition protocol as any changes in the procedure might impact on quantitative measurements. This work reviews state-of-the-art methods for studying brain microstructure using diffusion MRI and their sensitivity to microstructural differences and various experimental factors. Microstructural properties of the tissue at a micrometer scale can be linked to the diffusion signal at a millimeter-scale using modeling. In this paper, we first give an introduction to diffusion MRI and different encoding schemes. Then, signal representation-based methods and multi-compartment models are explained briefly. The sensitivity of the diffusion MRI signal to the microstructural components and the effects of curvedness of axonal trajectories on the diffusion signal are reviewed. Factors that impact on the quality (accuracy and precision) of derived metrics are then reviewed, including the impact of random noise, and variations in the acquisition parameters (i.e., number of sampled signals, b-value and number of acquisition shells). Finally, yet importantly, typical approaches to deal with experimental factors are depicted, including unbiased measures and harmonization. We conclude the review with some future directions and recommendations on this topic.
Topics: Anisotropy; Axons; Brain; Diffusion; Diffusion Magnetic Resonance Imaging
PubMed: 33017644
DOI: 10.1016/j.jneumeth.2020.108951 -
NeuroImage. Clinical 2021Misophonia is a condition in which specific ordinary sounds provoke disproportionately strong negative affect and physiological arousal. Evidence for neurobiological...
Misophonia is a condition in which specific ordinary sounds provoke disproportionately strong negative affect and physiological arousal. Evidence for neurobiological abnormalities underlying misophonia is scarce. Since many psychiatric disorders show white matter (WM) abnormalities, we tested for both macro and micro-structural WM differences between misophonia patients and healthy controls. We collected T1-weighted and diffusion-weighted magnetic resonance images from 24 patients and 25 matched controls. We tested for group differences in WM volume using whole-brain voxel-based morphometry and used the significant voxels from this analysis as seeds for probabilistic tractography. After calculation of diffusion tensors, we compared group means for fractional anisotropy, mean diffusivity, and directional diffusivities, and applied tract-based spatial statistics for voxel-wise comparison. Compared to controls, patients had greater left-hemispheric WM volumes in the inferior fronto-occipital fasciculus, anterior thalamic radiation, and body of the corpus callosum connecting bilateral superior frontal gyri. Patients also had lower averaged radial and mean diffusivities and voxel-wise comparison indicated large and widespread clusters of lower mean diffusivity. We found both macro and microstructural WM abnormalities in our misophonia sample, suggesting misophonia symptomatology is associated with WM alterations. These biological alterations may be related to differences in social-emotional processing, particularly recognition of facial affect, and to attention for affective information.
Topics: Anisotropy; Brain; Diffusion Tensor Imaging; Humans; Phobic Disorders; White Matter
PubMed: 34461433
DOI: 10.1016/j.nicl.2021.102787 -
Psychological Medicine May 2023Aberrant microstructure of the uncinate fasciculus (UNC), a white matter (WM) tract implicated in emotion regulation, has been hypothesized as a neurobiological... (Meta-Analysis)
Meta-Analysis Review
Aberrant microstructure of the uncinate fasciculus (UNC), a white matter (WM) tract implicated in emotion regulation, has been hypothesized as a neurobiological mechanism of depression. However, studies testing this hypothesis have yielded inconsistent results. The present meta-analysis consolidates evidence from 44 studies comparing fractional anisotropy (FA) and radial diffusivity (RD), two metrics characterizing WM microstructure, of the UNC in individuals with depression ( = 5016) to healthy individuals ( = 18 425). We conduct meta-regressions to identify demographic and clinical characteristics that contribute to cross-study heterogeneity in UNC findings. UNC FA was reduced in individuals with depression compared to healthy individuals. UNC RD was comparable between individuals with depression and healthy individuals. Comorbid anxiety explained inter-study heterogeneity in UNC findings. Depression is associated with perturbations in UNC microstructure, specifically with respect to UNC FA and not UNC RD. The association between depression and UNC microstructure appears to be moderated by anxiety. Future work should unravel the cellular mechanisms contributing to aberrant UNC microstructure in depression; clarify the relationship between UNC microstructure, depression, and anxiety; and link UNC microstructure to psychological processes, such as emotion regulation.
Topics: Humans; White Matter; Depression; Diffusion Tensor Imaging; Uncinate Fasciculus; Diffusion Magnetic Resonance Imaging; Anisotropy; Brain
PubMed: 37051913
DOI: 10.1017/S0033291723000107 -
Physiological Research 2008The diffusion of neuroactive substances in the extracellular space (ECS) plays an important role in short- and long-distance communication between nerve cells and is the... (Review)
Review
The diffusion of neuroactive substances in the extracellular space (ECS) plays an important role in short- and long-distance communication between nerve cells and is the underlying mechanism of extrasynaptic (volume) transmission. The diffusion properties of the ECS are described by three parameters: 1. ECS volume fraction alpha (alpha=ECS volume/total tissue volume), 2. tortuosity lambda (lambda2=free/apparent diffusion coefficient), reflecting the presence of diffusion barriers represented by, e.g., fine neuronal and glial processes or extracellular matrix molecules and 3. nonspecific uptake k'. These diffusion parameters differ in various brain regions, and diffusion in the CNS is therefore inhomogeneous. Moreover, diffusion barriers may channel the migration of molecules in the ECS, so that diffusion is facilitated in a certain direction, i.e. diffusion in certain brain regions is anisotropic. Changes in the diffusion parameters have been found in many physiological and pathological states in which cell swelling, glial remodeling and extracellular matrix changes are key factors influencing diffusion. Changes in ECS volume, tortuosity and anisotropy significantly affect the accumulation and diffusion of neuroactive substances in the CNS and thus extrasynaptic transmission, neuron-glia communication, transmitter "spillover" and synaptic cross-talk as well as cell migration, drug delivery and treatment.
Topics: Animals; Anisotropy; Cell Movement; Central Nervous System; Diffusion; Drug Delivery Systems; Extracellular Space; Humans; Mice; Mice, Transgenic; Neuroglia; Neurons; Rats; Signal Transduction
PubMed: 18481911
DOI: 10.33549/physiolres.931603 -
Magnetic Resonance in Medicine Jun 2020To demonstrate how triple diffusion encoding (TDE) MRI can be applied to separately estimate the intra-axonal and extra-axonal diffusion tensors in white matter (WM).
PURPOSE
To demonstrate how triple diffusion encoding (TDE) MRI can be applied to separately estimate the intra-axonal and extra-axonal diffusion tensors in white matter (WM).
METHODS
Using a TDE pulse sequence with an axially symmetric b-matrix, diffusion MRI data were acquired at 3T for 3 healthy adults with an axial b-value of 4000 s/mm , a radial b-value of 307 s/mm , and 64 diffusion encoding directions. This acquisition was then repeated with the radial b-value set to 0. A previously proposed theory was applied to these data in order to estimate the intra-axonal diffusivity and axonal water fraction for each WM voxel. Conventional single diffusion encoding data were also obtained with b-values of 1000 and 2000 s/mm , which provided additional information sufficient for determining both the intra-axonal and extra-axonal diffusion tensors.
RESULTS
From the TDE data, the average intra-axonal diffusivity in WM was found to be 2.24 ± 0.18 µm /ms, and the average axonal water fraction was found to be 0.60 ± 0.11. From the 2 diffusion tensors, average WM values were estimated for several compartment-specific diffusion parameters. In particular, the extra-axonal mean diffusivity was 1.09 ± 0.19 µm /ms, the intra-axonal fractional anisotropy was 0.50 ± 0.14, and the extra-axonal fractional anisotropy was 0.23 ± 0.13.
CONCLUSION
By using a simple TDE pulse sequence with an axially symmetric b-matrix, the diffusion tensors for the intra-axonal and extra-axonal spaces can be separately estimated in adult WM. This allows one to determine compartment-specific diffusion properties for these 2 water pools.
Topics: Adult; Anisotropy; Axons; Diffusion; Diffusion Magnetic Resonance Imaging; Diffusion Tensor Imaging; Humans; White Matter
PubMed: 31763730
DOI: 10.1002/mrm.28084 -
Osteoarthritis and Cartilage Mar 2020To date, the pathophysiology of the meniscus has not been fully elucidated. Due to the tissue's limited vascularization, nutrients and other molecular signals spread...
OBJECTIVE
To date, the pathophysiology of the meniscus has not been fully elucidated. Due to the tissue's limited vascularization, nutrients and other molecular signals spread through the extracellular matrix via diffusion or convection (interstitial fluid flow). Understanding transport mechanisms is crucial to elucidating meniscal pathophysiology, and to designing treatments for repair and restoration of the tissue. Similar to other fibrocartilaginous structures, meniscal morphology and composition may affect its diffusive properties. The objective of this study was to investigate the role of solute size, and tissue structure and composition on molecular diffusion in meniscus tissue.
DESIGN
Using a custom FRAP technique developed in our lab, we measured the direction-dependent diffusivity in human meniscus of six different molecular probes of size ranging from ∼300Da to 150,000Da. Diffusivity measurements were related to sample water content. SEM images were used to investigate collagen structure in relation to transport mechanisms.
RESULTS
Diffusivity was anisotropic, being significantly faster in the direction parallel to collagen fibers when compared the orthogonal direction. This was likely due to the unique structural organization of the tissue presenting pores aligned with the fibers, as observed in SEM images. Diffusion coefficients decreased as the molecular size increased, following the Ogston model. No significant correlations were found among diffusion coefficients and water content of the tissue.
CONCLUSIONS
This study provides new knowledge on the mechanisms of molecular transport in meniscal tissue. The reported results can be leveraged to further investigate tissue pathophysiology and to design treatments for tissue restoration or replacement.
Topics: Aged; Anisotropy; Biological Transport; Collagen; Dextrans; Diffusion; Extracellular Fluid; Extracellular Matrix; Female; Fluorescein; Fluorescence Recovery After Photobleaching; Humans; Hydrodynamics; Insulin; Male; Menisci, Tibial; Microscopy, Electron, Scanning; Serum Albumin, Bovine
PubMed: 31917232
DOI: 10.1016/j.joca.2019.12.006 -
Journal of Affective Disorders Jan 2022Bipolar disorder (BD) is a severe mental disorder, characterized by prominent mood swings and emotion regulation (ER) deficits. The uncinate fasciculus (UF), a white... (Meta-Analysis)
Meta-Analysis Review
BACKGROUND
Bipolar disorder (BD) is a severe mental disorder, characterized by prominent mood swings and emotion regulation (ER) deficits. The uncinate fasciculus (UF), a white matter tract connecting the amygdala and the ventral prefrontal cortex, has been implicated in ER. Aberrancies in UF microstructure may be an endophenotype associated with increased risk for BD. However, studies in individuals with BD and their first-degree relatives (REL) have yielded inconsistent findings. This meta-analysis takes a region-of-interest approach to consolidate the available evidence and elucidate the role of the UF in the risk-architecture of BD.
METHODS
Using web-based search engines, we identified diffusion tensor imaging (DTI) studies focusing on the left and right UF and conducted meta-analyses comparing fractional anisotropy (FA) and radial diffusivity (RD) between BD or REL and healthy control participants (HC).
RESULTS
We included 32 studies (n=1186, n=289, n=2315). Compared to HC, individuals with BD showed lower FA in the right (WMD=-0.31, p<0.0001) and left UF (WMD=-0.21, p = 0.010), and higher RD in the right UF (WMD=0.32, p = 0.009). We found no significant differences between REL and HC. In the right but not left UF, REL showed higher FA than BD (p = 0.043).
CONCLUSION
Our findings support aberrant UF microstructure, potentially related to alterations in myelination, as a mechanism, but not as an endophenotype of BD. However, given the limited power in the REL subsample, the latter finding must be considered preliminary. Studies examining the role of the UF in individuals at familial risk for BD are warranted.
Topics: Anisotropy; Bipolar Disorder; Diffusion Tensor Imaging; Humans; Nerve Net; Uncinate Fasciculus; White Matter
PubMed: 34699854
DOI: 10.1016/j.jad.2021.10.045 -
Biomechanics and Modeling in... Dec 2020Fluorescence recovery after photobleaching (FRAP) is a widely used technique for studying diffusion in biological tissues. Most of the existing approaches for the...
Fluorescence recovery after photobleaching (FRAP) is a widely used technique for studying diffusion in biological tissues. Most of the existing approaches for the analysis of FRAP experiments assume isotropic diffusion, while only a few account for anisotropic diffusion. In fibrous tissues, such as articular cartilage, tendons and ligaments, diffusion, the main mechanism for molecular transport, is anisotropic and depends on the fibre alignment. In this work, we solve the general diffusion equation governing a FRAP test, assuming an anisotropic diffusivity tensor and using a general initial condition for the case of an elliptical (thereby including the case of a circular) bleaching profile. We introduce a closed-form solution in the spatial coordinates, which can be applied directly to FRAP tests to extract the diffusivity tensor. We validate the approach by measuring the diffusivity tensor of [Formula: see text] FITC-Dextran in porcine medial collateral ligaments. The measured diffusion anisotropy was [Formula: see text] (SE), which is in agreement with that reported in the literature. The limitations of the approach, such as the size of the bleached region and the intensity of the bleaching, are studied using COMSOL simulations.
Topics: Animals; Anisotropy; Biological Transport; Computer Simulation; Diffusion; Fluorescence Recovery After Photobleaching; Medial Collateral Ligament, Knee; Microscopy, Electron, Scanning; Models, Biological; Models, Theoretical; Swine; Tendons
PubMed: 32562093
DOI: 10.1007/s10237-020-01346-z