Biophysical Investigations and Developments of Novel Functional MRI Techniques
Date Issued
2009
Date
2009
Author(s)
Wu, Changwei W.
Abstract
Since it was introduced in 1992, functional magnetic resonance imaging (fMRI) based on the blood oxygenation level dependent (BOLD) mechanism has become a popular modality for non-invasive mapping of brain functions incorporating sophisticated experimental design. Within one decade, a few novel fMRI techniques, such as vascular-space occupancy (VASO)-based fMRI and resting-state fMRI, were subsequently invented and proven useful for quantifiable features and for reducing the demand of external stimulation, which are what typical fMRI procedure fails to offer. However, the physiological bases of these newly-developed techniques remain to be refined to improve contrast-to-noise ratio. Therefore, resolving physiological and technical issues would undoubtedly benefit functional brain research. he general hypothesis of this dissertation was that VASO-fMRI and resting-state fMRI techniques could be optimized for both clinical and cognitive research. pecific aim 1: To evaluate the influence of water permeability on CBV estimation using VASO-fMRI ASO-fMRI has been recently developed as a promising technique for estimating CBV, which is an important physiological indicator. However, this technique was based on a non-practical water exchange model. In this specific aim, the effects of the permeability model at two extreme cases were evaluated for CBV quantification. Results showed that estimated CBV has slight difference between two permeability models, but such disparity would be enhanced during the transient state of brain activity, leading to a temporal delay on dynamic CBV estimations (Chapter 2)pecific aim 2: To improve the contrast-to-noise ratio (CNR) of VASO-fMRI he key factor affecting the CBV estimation is the CNR of VASO-fMRI technique. In order to improve the CNR, the VASO-fMRI pulse sequence was modified by amending the imaging timing. The resulting CNR was increased by 40% in both block-design and event-related experiments with visual stimuli (Chapter 3). pecific aim 3: To trace the origin of functional connectivity based on resting-state fMRI to metabolic levelsing resting-state fMRI signals, the functional connectivity of brain networks can be retrieved without external stimulations, but the source of such phenomenon remains obscure. To explore its physiological source, the functional connectivity maps of the cerebral metabolic rate of oxygenation (CMRO2) were followed independent of the hemodynamic changes (Chapter 4), implying that the functional connectivity observed by resting-state fMRI signal is originated from spontaneous neuronal oscillations.pecific aim 4: To explore the frequency distribution of the functional connectivity in multiple brain networks using resting-state fMRIunctional connectivity can be observed using low-frequency (<0.1 Hz) fMRI fluctuations. However, the observing frequency has yet been systematically assessed. In this specific aim, the frequency specificity of resting-state fMRI signal was explored through multiple brain networks. Generally, the maximum connectivity strength falls on 0.01-0.06 Hz, but results differ between distinct brain networks (Chapter 5).n summary, the dissertation unveiled in part the underlying physiology of VASO-fMRI and resting-state fMRI techniques and improved their functional contrast. In future directions, two practical studies of absolute CBV quantification and meditation-based functional connectivity were also presented incorporating the two developed techniques (Chapter 6), expressing their potential for clinical and psychological investigations. In conclusion, the physiological exploration provides a better understanding of the fMRI mechanism in relation to neural activity and the proposed improvements profits further studies of brain functions based upon VASO- and resting-state fMRI techniques.
Subjects
functional MRI
blood oxygenation level dependent
perfusion
cerebral blood flow
cerebral blood volume
cerebral metabolic rate of oxygenation
vascular-space occupancy
spontaneous fluctuation
functional connectivity
Type
thesis
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