View Presentation Video

Keywords: Microstructure, Diffusion/other diffusion imaging techniques

Using an integrative acquisition and processing pipeline that joins concepts from oscillating gradients, tensor-valued encoding, and diffusion-relaxation correlation, we comprehensively explored microstructure and local chemical composition in the human brain. Using both frequency-dependent and tensorial aspects of the encoding spectrum b(ω), we designed an in vivo, whole brain, 40-min 7D D(ω)-R₁-R₂ distribution acquisition protocol at 2mm isotropic resolution. Scanning eleven healthy participants, we demonstrated frequency/time-dependent changes of diffusion-relaxation correlations measures in the human brain. Finally, intra-scan test–retest repeatability of a range of reconstructed parametric maps was investigated.

View Presentation Video

Keywords: Diffusion/other diffusion imaging techniques, Microstructure, diffusion tensor distribution, DTD, gray matter, cortical layers, DTI

We propose a practical new framework for mapping non-parametric diffusion tensor distributions (DTDs). For diffusion MRI data with sufficiently high spatial resolution, we can constrain all microscopic diffusion tensors of the DTD to be diagonalized using a single orthonormal reference frame estimated from the entire mesoscopic voxel. The constrained DTD is determined by the correlation spectrum of the corresponding microscopic principal diffusivities and can be measured very efficiently using Inverse Laplace Transform methods and single diffusion encoded measurements. cDTD spectral components measured in cortical tissue show good sensitivity to cytoarchitectonic domains and reveal lamination patterns observed in corresponding histological images.

View Presentation Video

Keywords: Microstructure, Gray Matter

It is non-trivial to decompose the diffusion MRI signal, as conventionally measured using relatively weak gradients, into contributions from multiple compartments and estimate the cell sizes. Instead, using strong diffusion gradients, a universal compartment-independent localization regime emerges, as they saturate all magnetization except that near cell membranes. The signal sensitivity to the membrane and its surface-to-volume ratio enables the soma size estimation without modeling multiple compartments. Here, we observed the unique functional form of localization regime and estimate the soma size in mouse and pig brains cortical gray matter on a Bruker CryoProbe system.

Erick Jorge Canales-Rodríguez^1,2, Marco Pizzolato^2,3, Feng-Lei Zhou⁴, Muhamed Barakovic^5,6,7, Jean- Philippe Thiran^1,8,9, Derek K. K. Jones¹⁰, Geoffrey J.M. Parker^4,11,12, and Tim B. Dyrby^2,3
1Signal Processing Laboratory (LTS5), École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland, ²Danish Research Centre for Magnetic Resonance, Centre for Functional and Diagnostic Imaging and Research, Copenhagen University Hospital, Amager & Hvidovre, Copenhagen, Denmark, ³Department of Applied Mathematics and Computer Science, Technical University of Denmark, Kongens, Lyngby, Denmark, ⁴Centre for Medical Image Computing, Department of Medical Physics and Biomedical Engineering , University College London, London, United Kingdom, ⁵Translational Imaging in Neurology (ThINk) Basel, Department of Biomedical Engineering, University Hospital Basel and University of Basel, Basel, Switzerland, ⁶MS Center and Research Center for Clinical Neuroimmunology and Neuroscience Basel, University Hospital Basel and University of Basel, Basel, Switzerland, ⁷Roche Pharma Research & Early Development, Neuroscience and Rare Diseases, Roche Innovation Center, Basel, Switzerland, ⁸Radiology Department, Centre Hospitalier Universitaire Vaudois and University of Lausanne, Lausanne, Switzerland, ⁹Centre d’Imagerie Biomédicale (CIBM), EPFL, Lausanne, Switzerland, ¹⁰Cardiff University Brain Research Imaging Centre, Cardiff University, Cardiff, Wales, United Kingdom, ¹¹Department of Neuroinflammation, Queen Square Institute of Neurology, University College London, London, United Kingdom, ¹²Bioxydyn Limited, Manchester, United Kingdom

Keywords: Diffusion/other diffusion imaging techniques, Validation

A new approach for estimating inner axon radii from intra-axonal T₂ relaxation times was recently proposed. However, further validations are required before this technique can be used widely. The main aim of this study is to validate this T₂-based pore size estimation technique in phantoms comprising co-electrospun hollow axon-mimicking fibres designed to have non-circular cross-sections and different radii distributions. For this purpose, a diffusion-relaxation MRI dataset was acquired with a 7T preclinical scanner, from which the intra-fibre T₂ times and pore sizes were estimated. The resulting pore sizes were compared to those measured from Scanning Electron Microscope images.

View Presentation Video

Keywords: Microstructure, Diffusion/other diffusion imaging techniques

Time-dependent DW-MRS can probe the underlying tissue microstructure. However, it has been previously shown that time-dependent apparent diffusivity and apparent kurtosis exhibit different behaviors for water and intracellular metabolite. These differences may be largely explained by exchange between intra- and extracellular spaces occurring for water. The aim of this work is to measure time-dependent diffusion of lactate which, like water, is present in and exchanges between both compartments, but for which the exchange rate is unknown. Comparison with water and intracellular metabolites indicates that lactate exchange is slow (relative to the probed diffusion times up to 500 ms).

Dmitry S Novikov¹, Ricardo Coronado-Leija¹, and Els Fieremans^1
1Radiology, NYU School of Medicine, New York, NY, United States

Keywords: Microstructure, Modelling, exchange, diffusion, structural disorder

Which signatures of the diffusion signal are responsible for non-Gaussian diffusion inside cells or extra-cellular space, and which ones are the hallmark of the exchange between compartments? Answering this unresolved question is vital for mapping tissue microstructure in brain (especially in gray matter) and body. Here we employ the effective medium formalism to extend the multi-site exchange approach onto the case when diffusion in each tissue compartment is arbitrarily complex. We find the time-dependent diffusivity and kurtosis of a general system of exchanging non-Gaussian compartments for all times, validate with Monte Carlo simulations, and discuss practical implications. 

View Presentation Video

Keywords: Microstructure, Modelling

For the first time, we report Neurite Exchange Imaging (NEXI) microstructure model parameters estimates in human cortex in vivo. We also investigate the performance of two extensions of this model, the addition of a dot compartment and a correction for wide pulses. Parameter estimates are consistent with previous findings in the rat cortex in vivo. Importantly, NEXI estimates displayed good scan-rescan reproducibility while retaining sensitivity to inter-subject differences. Future work will focus on improving the precision, in particular of the exchange time estimates, possibly leveraging multi-dimensional diffusion MRI acquisitions.

Sune Nørhøj Jespersen^1,2, Rita Alves³, Jonas Olesen^1,2, Rafael Neto Henriques³, and Noam Shemesh^3
1Department of Physics and Astronomy, Aarhus University, Aarhus, Denmark, ²Center of Functionally Integrative Neuroscience (CFIN) and MINDLab, Department of Clinical Medicine, Aarhus University, Aarhus, Denmark, ³Champalimaud Research, Champalimaud Foundation, Lisboa, Portugal

Keywords: Microstructure, Microstructure

Correlation Tensor MRI (CTI) provided strong evidence for non-vanishing microscopic kurtosis (µK) in neural tissues, both in animals and in humans. However, µK sources remain to be elucidated. Standard Model with Exchange (SMEX) contrasts have been recently proposed by Olesen et al. for mapping exchange properties, especially in gray matter. Here, we derive an expression for µK originating from the SMEX biophysical model due to exchange, and in a model of stroke compare µK derived from SMEX to µK measured with CTI. Our findings suggest that µK(CTI)>µK(SMEX), suggesting a degree of microstructural origin for the CTI contrast.

Arthur Chakwizira¹, Filip Szczepankiewicz¹, and Markus Nilsson^2
1Medical Radiation Physics, Lund, Lund University, Lund, Sweden, ²Clinical Sciences Lund, Lund University, Lund, Sweden

Keywords: Diffusion/other diffusion imaging techniques, Microstructure

Correlation tensor magnetic resonance imaging (CTI) is a recently proposed technique that uses diffusion MRI with double diffusion encoding to disentangle microscopic from isotropic and anisotropic kurtosis. One assumption of the method is that intercompartmental exchange is negligible. Another approach that assumes multi-Gaussian exchange (MGE) can also be applied to data acquired with a CTI protocol, but it interprets the contrast as exchange rather than microscopic kurtosis. Using Monte Carlo simulations in substrates of permeable spheres, cylinders and ellipsoids, we observe that both the CTI parameter μK and the estimated exchange rate from MGE increase with the underlying exchange rate.

View Presentation Video

Keywords: Diffusion/other diffusion imaging techniques, Brain

3D oscillating gradient sequences enable diffusion measurement at short diffusion-time (t_d), but it suffered from low resolution and low SNR in clinical systems, and thus, the t_d-dependency in complex structures, such as cortical gray matter, was not characterized. Here we proposed a twin-navigator-based 3D oscillating gradient diffusion-weighted gradient spin-echo sequence for high-resolution whole-brain t_d–dMRI acquisition. We demonstrated that different cortical regions exhibited distinct td-dependency patterns with different diffusion dispersion exponent (θ) based on the power-law. We found θ was greater than 0.5 with the highest dispersion in the pre- and post-central cortex and lowest value in the frontal region.

View Presentation Video

Keywords: Microstructure, Diffusion/other diffusion imaging techniques

Oscillating gradient spin echo (OGSE) sequence is an effective approach to measure time-dependent diffusion processes at short diffusion times. High-performance, head-only gradient systems open new opportunities for using OGSE to study time-dependent diffusion with both high b-values and oscillating frequencies, such as diffusion kurtosis imaging (DKI). One source of error that can be more problematic for the head-only gradients is gradient nonlinearity (GNL). Here, we measure  the time dependence of diffusion tensor imaging (DTI)/DKI in human brains with OGSE on a head-only MAGNUS gradient and investigate the influence of GNL on the time dependence measures of diffusivity/kurtosis. 

View Presentation Video

Keywords: Diffusion/other diffusion imaging techniques, Microstructure

Diffusion MRI is a widely-used non-invasive imaging method for studying tissue microstructure but often suffers from low signal-to-noise ratios when using high b values. Here, we present a diffusion MRI acquisition technique that can achieve significantly higher SNR efficiency while providing distortion-free high-quality images. It achieves high robustness to phase variations and motion and provides an efficient acquisition technique for microstructural diffusion imaging. Ultra-high b-value and time-dependent experiments were performed to evaluate the improved SNR efficiency of Romer-EPTI.

View Presentation Video

Keywords: Diffusion/other diffusion imaging techniques, Microstructure

In this study, we uncover that the signal fraction of a fully restricted diffusion compartment in fixed white matter tissue, the so-called dot fraction, is dependent on the tissue temperature. In particular, we report a sudden increase in its nominal value of this fraction when the tissue temperature drops below approximately 32°C. The underlying mechanism remains unexplained, but its impact on modeling of ex vivo diffusion MRI cannot be underestimated. The observation corroborates our hypotheses of a temperature-dependent disruption of the multilamellar structure of the myelin sheath, leading to water trapped in vacuoles that are formed in the myelin sheath.

View Presentation Video

Keywords: Validation, Diffusion/other diffusion imaging techniques, Microstructure characterisation, microscopy

The BigMac dataset is an open access resource combining in vivo MRI, postmortem MRI and multi-contrast microscopy data in a single, whole macaque brain. Here we perform data-driven segmentation of the BigMac histology slides to extract quantitative microscopy metrics for myelin, cell density, and cellular morphology (e.g. soma size and packing). Utilising high-quality MRI-microscopy registrations, we work towards building 3D volumes of quantitative microscopy derived metrics obtained at high resolution. This “ground truth” atlas of how cellular distributions vary across the brain is then directly related to co-registered diffusion MRI acquired in the same tissue.

View Presentation Video

Keywords: Diffusion/other diffusion imaging techniques, Brain, Tissue heterogeneity

Non-Gaussian diffusion MRI with a continuous-time random-walk (CTRW) model offers a unique avenue to probing tissue microstructural heterogeneity. The CTRW parameters, α and β, corresponding to temporal and spatial intravoxel diffusion heterogeneities, have empirically been linked to tumor tissue heterogeneity in clinical studies. This study aims at directly establishing the correlation between the CTRW parameters from patients suspected of glioma and tissue microstructural heterogeneity revealed by histology on stereotactic brain biopsies. We developed a practical protocol that integrates quantitative imaging techniques and surgical procedures to perform MR-histology correlation, and demonstrated that lower CTRW parameters correspond to increased tissue microstructural heterogeneity.