The vascular properties of the BOLD signal

Driver, Ian D. (2012) The vascular properties of the BOLD signal. PhD thesis, University of Nottingham.

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Abstract

The work presented in this thesis is intended to contribute towards the understanding of the cerebral vascular behaviour in response to changes in neuronal activation. The blood oxygenation-level dependent (BOLD) functional magnetic resonance imaging (fMRI) signal provides an indirect measure of neuronal activation, arising from a combination of metabolic and vascular (blood flow and blood volume) changes local to the activation. Therefore the BOLD signal is dependent on local vascular properties as well as on the neuronal activation, leading to a variability of the BOLD signal, based on the underlying vascular structure. It has become an important goal to improve understanding of the mechanisms underlying the BOLD signal in order to separate out this vascular variability from the underlying correspondence with the neuronal activation.

The effect of field strength on the temporal characteristics of the BOLD haemodynamic response function is investigated. An earlier BOLD response onset has been measured with increasing static magnetic field strength, consistent with an earlier microvascular (compared with macrovascular) signal response. This result can be used to improve haemodynamic models of the BOLD signal.

Hypercapnia, a vasodilator, has been used both to assess the relationship between transverse relaxation and blood oxygenation at 3 and 7 Tesla and to identify vascular heterogeneity between two equivalent brain regions. A tight, linear relationship was found between the level of hypercapnia and transverse relaxation at both 3 and 7 Tesla, whilst the change in transverse relaxation due to hypercapnia increased 2.1 ± 0.5 fold from 3 to 7 Tesla, indicating an approximately linear relationship across field strength. In a separate experiment, a vascular asymmetry was found between the left and right precentral gyri using hypercapnia. This result highlights the need to account for the vascular contribution to the BOLD signal before using this BOLD signal to make comparisons of neuronal activity across brain regions.

Finally, an improved model for calibrated BOLD has been proposed and implemented, which requires fewer assumptions than existing approaches. This uses the BOLD response to some task, repeated both at normoxia and hyperoxia. To assess the validity of this model, the effects of paramagnetic oxygen molecules are considered, both dissolved in blood plasma and in airspaces adjacent to the brain. These effects were found to be small, except for in the frontal cortex.

Item Type:Thesis (PhD)
Supervisors:Gowland, P.A.
Francis, S.T.
Uncontrolled Keywords:fMRI, BOLD, hypercapnia, hyperoxia
Faculties/Schools:UK Campuses > Faculty of Science > School of Physics and Astronomy
ID Code:2357
Deposited By:Mr Ian D Driver
Deposited On:23 Oct 2012 14:14
Last Modified:23 Oct 2012 14:14

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