New methods for localizing brain activity with Magnetic Resonance Imaging
Doctoral thesis
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http://hdl.handle.net/11250/278888Utgivelsesdato
2014Metadata
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Sammendrag
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.