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New methods for localizing brain activity with Magnetic Resonance Imaging

Vallée, Emilie Claire Yvonne
Doctoral thesis
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URI
http://hdl.handle.net/11250/278888
Date
2014
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  • Institutt for sirkulasjon og bildediagnostikk [1379]
Abstract
Blood-oxygen-level-dependent functional magnetic resonance imaging (BOLD fMRI)

is the most frequently used non-invasive method for localizing brain activity in vivo

with good spatial and fair temporal resolution. It is shown in a number of studies that

BOLD fMRI has clinical potential, such as in neurosurgery, in stroke rehabilitation

or in moderate to severe traumatic brain injury. However, for major clinical benefit

further improvements of the method and technology are necessary. Indeed, BOLD

measures changes in blood oxygenation levels secondary to neuronal activity. BOLD

fMRI provides therefore an indirect measure of brain activity, which implies a poor

specificity to neuronal activity of the depicted functional signal. The main motivation

of this thesis was to improve localization of brain activity using MRI to increase our

knowledge of the brain and its functional organization, as well as to provide better

tools for studying and curing brain diseases in a long-term perspective.

In this thesis, we investigated diffusion functional MRI (DfMRI) as a tool for

depicting brain activity more directly. DfMRI is based on the theory that neuronal

cell swelling during brain activity produces changes in diffusion properties of the space

surrounding the neurons, which can be measured using diffusion weighting functional

MRI using strong diffusion weighting. This method has the potential to measure more

precisely brain activity with excellent specificity to neuronal activity of the signal.

However, we demonstrated that DfMRI only measures SE BOLD related changes and

does not detect brain activity more directly.

Thanks to the recent development of 3D imaging and parallel imaging, it is possible

to improve the BOLD fMRI sensitivity and specificity to neuronal activity. The best

candidate sequences to perform 3D imaging combined with parallel imaging are the

PRESTO and SSFP sequences. In this thesis, we compared these methods and assessed

which was best to perform fMRI studies. We showed that 3D PRESTO combined with

parallel imaging produces activity maps highly sensitive and specific to grey matter

activity, while SSFP depicted mostly the BOLD signal from the veins draining the

activated cortex. The PRESTO sequence was therefore best for fMRI studies.

Our capacity to produce activation maps more specific to brain activity also relies

on our understanding of the BOLD signal and the underlying physiological mechanisms

accompanying it. At the moment, our knowledge of the BOLD effect is incomplete.

However, thanks to the unique properties of the SSFP sequence we could depict

velocity changes induced by neuronal activity in the large arteries feeding the activated

cortex. The similarity between the BOLD signal and the SSFP signal showed that the BOLD signal might be explained by arterial CBF changes. The combination of the

PRESTO and SSFP activity maps allowed a full overview of the BOLD signal from

the large arteries to the capillaries and the draining veins, giving optimum information

for studying neuronal activity.
Publisher
NTNU
Series
Doctoral thesis at NTNU;2014:333

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