Isotropic Diffusion Weighting Provides Insight on Diffusion Compartments in Human Brain White Matter In vivo
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1
University Medical Center Freiburg, Department of Radiology, Germany
Modeling the origin of diffusion-weighted signal is the engine of tissue microstructure investigation using diffusion MRI. Models aim at predicting the signal given properties of cell species that form the tissue of interest. Beyond shape parameters of cells and their volume fractions, the relevant cell properties include such biophysical characteristics as diffusion coefficients inside cellular compartments. These compartmental diffusivities are difficult to access experimentally. While all
compartments contribute to the signal, interpretation of measurements in terms of individual compartments is an ill-posed problem. The uncertainty in the cellular parameters has been recently scrutinized for a typical measurement in brain white matter. Results indicated a marked ambiguity. On the top level, there are two essentially different solutions that describe experimental data equally well. They correspond to two cases in which the diffusivity in the direction parallel to the axons is higher in either extraaxonal space or in intraaxonal space. This study aims at resolving the ambiguity. Another aim is to inspect the presence of fully restricted water in the tissue. Also known as “still water” or “dot compartment", it is often used in microstructural models to explain slow signal decay at large b-values.
The method is based on application of isotropic diffusion weighting in healthy subjects. The isotropic weighted signal is sensitive to the trace of diffusion tensor, which changes the balance of intra- and extraaxonal contributions in the signal and can distinguish isotropically restricted water from a partial restriction in e.g. elongated compartments. All measurements were performed in a 3T scanner with the gradient system capable to develop 80 mT/m (Siemens Prisma). The maximal b-value was b = 5.4 ms/μm2 when related to the mean diffusivity.
Measurement results reveal a nearly monoexponential decay until the remaining signal at the maximal b-value is around 2.7%. This rules out the isotropically restricted compartment with the volume fraction above a few percent. Further, the low isotropic kurtosis implies close values of the traces of diffusion tensors in intra- and extraaxonal space. Since the extraaxonal tensor has non-zero transverse components, its longitudinal component should be smaller than the longitudinal intraaxonal
diffusivity.
It is currently unclear what are the biophysical reasons for the absence of isotropically restricted water in the diffusion-weighted signal. We hope that our report is able to initiate a deep discussion of this and other, more technical issues.
Keywords:
Diffusion,
MRI,
dMRI,
multidimensional,
diffusion encoding
Conference:
New dimensions in diffusion encoding, Fjälkinge, Sweden, 11 Jan - 14 Jan, 2016.
Presentation Type:
Oral presentation
Topic:
New Dimensions in Diffusion Encoding
Citation:
Dhital
B,
Kellner
E,
Kiselev
VG and
Reisert
M
(2016). Isotropic Diffusion Weighting Provides Insight on Diffusion Compartments in Human Brain White Matter In vivo.
Front. Phys.
Conference Abstract:
New dimensions in diffusion encoding.
doi: 10.3389/conf.FPHY.2016.01.00011
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Received:
07 Jul 2016;
Published Online:
07 Jul 2016.
*
Correspondence:
Dr. Marco Reisert, University Medical Center Freiburg, Department of Radiology, Freiburg, 79106, Germany, marco.reisert@uniklinik-freiburg.de