Probing brain microstructure with multidimensional diffusion MRI: Encoding, interpretation, and the role of exchange
Author
Summary, in English
In Paper I, we optimized an acquisition protocol for filter exchange imaging (FEXI). We found slow rates of diffusional exchange in normal brain tissue. In patients with gliomas and meningiomas, faster exchange was tentatively associated with higher tumor grade.
In Paper II, we used tensor-valued diffusion encoding to test the NODDI microstructure model. The NODDI assumptions were contradicted by independent data and parameter estimates were found to be biased in normal brain and in gliomas. The CODIVIDE model combined data acquired with different b-tensor shapes to remove NODDI assumptions and reduce the susceptibility to bias.
In Paper III, we used tensor-valued diffusion encoding with multiple echo times to investigate challenges in estimating neurite density. We found that microscopic anisotropy in the brain reflected axons but not dendrites. We could not separate the densities and T2 values of a two-component model in normal brain, but we did detect different component T2 values in white matter lesions. Microstructure models ranked regions from normal brain and white matter lesions inconsistently with respect to neurite density.
In Paper IV, we optimized an acquisition protocol for tensor-valued diffusion encoding with multiple echo times. The data allowed removing all assumptions on diffusion and T2 relaxation from a two-component model. This increased the measurable parameters from two to six and reduced their susceptibility to bias. Data from the normal brain showed different component T2 values and contradicted common model assumptions.
In Paper V, we used tensor-valued diffusion encoding in malformations of cortical development. Lesions that appeared gray matter-like in T1- and T2-weighted contrasts featured white matter-like regions with high microscopic diffusion anisotropy. We interpreted these regions as myelin-poor white matter with a high axonal content. By primarily reflecting axons and not dendrites or myelin, microscopic anisotropy may differentiate tissue where alterations to myelin confound conventional MRI contrasts.
In Paper VI, we used SDE with multiple diffusion times in patients with acute ischemic stroke. Subacute lesions exhibited elevated diffusional exchange that predicted later infarction. MD reduction was partially reversible and did not predict infarction. Diffusional exchange may improve definition of ischemic core and identify additional patients for late revascularization.
Department/s
Publishing year
2021-10-04
Language
English
Full text
- Available as PDF - 43 MB
- Download statistics
Document type
Dissertation
Publisher
Lund University, Faculty of Science
Topic
- Radiology, Nuclear Medicine and Medical Imaging
Keywords
- MRI
- Diffusion
- Human
- Brain
- Microstructure
- Fysicumarkivet A:2021:Lampinen
Status
Published
Research group
- Multidimensional microstructure imaging
- MR Physics
ISBN/ISSN/Other
- ISBN: 978-91-8039-038-5
- ISBN: 978-91-8039-037-8
Defence date
28 October 2021
Defence time
13:00
Defence place
Föreläsningssal 2, Centralblocket, Entrégatan 7, Skånes Universitetssjukhus, Lund. Join via zoom: https://www.msf.lu.se/evenemang/disputation-bjorn-lampinen
Opponent
- Susie Yi Huang (Assistant professor)