Cerebral autoregulation evidence by synchronized low frequency oscillations in blood pressure and resing-state fMRI

Resting-state functional magnetic resonance imaging (rs-fMRI) is a widely used technique for mapping the brain’s functional architecture, so delineating the main sources of variance comprising the signal is crucial. Low frequency oscillations (LFO) that are not of neural origin, but which are driven by mechanisms related to cerebral autoregulation (CA), are present in the blood-oxygenation-level-dependent (BOLD) signal within the rs-fMRI frequency band. In this study we use a MR compatible device (Caretaker, Biopac) to obtain a non-invasive estimate of beat-to-beat mean arterial pressure (MAP) fluctuations concurrently with rs-fMRI at 3T. Healthy adult subjects (n=9; 5 male) completed two 20-minute rs-fMRI scans. MAP fluctuations were decomposed into different frequency scales using a discrete wavelet transform, and oscillations at approximately 0.1Hz show a high degree of spatially structured correlations with matched frequency fMRI fluctuations. On average across subjects, MAP fluctuations at this scale of the wavelet decomposition explain ~ 2.2% of matched frequency fMRI signal variance. Additionally, a simultaneous multi-slice multi-echo acquisition was used to collect 10-minute rs-fMRI at three echo times at 7T in a separate group of healthy adults (n=5; 5 male). Multiple echo times were used to estimate the R2* decay at every time point, and MAP was shown to strongly correlate with this signal, which suggests a purely BOLD (i.e. blood flow related) origin. This study demonstrates that there is a significant component of the BOLD signal that has a systemic physiological origin, and highlights the fact that not all localized BOLD signal changes necessarily reflect blood flow supporting local neural activity. Instead, these data show that a proportion of BOLD signal fluctuations in rs-fMRI are due to localized control of blood flow that is independent of local neural activity, most likely reflecting more general systemic autoregulatory processes. Thus, fMRI is a promising tool for studying flow changes associated with cerebral autoregulation with high spatial resolution

Optimized simultaneous ASL and BOLD functional imaging of the whole brain

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/106990/1/jmri24273.pd

Developing advanced MR imaging to assess spinal cord function and tract integrity.

The overall purpose of this thesis is to develop a way to match diffusion and functional acquisition techniques in the spinal cord (SC) in order to offer a comprehensive assessment of factors responsible for functional and structural integrity. I began by optimising a pipeline to acquire and process spinal functional data and I finished by matching the functional information with that derived from diffusion imaging (DI) performed during the same scan session as fM RI. In order to characterize the interactions between local structural connections (derived from DI) and functional activation of the SC it has been necessary to develop an imaging protocol that acquires transverse SC images with both modalities, matching their spatial and geometrical characteristics. This is because transverse cord images possess the relevant anatomical information in terms of grey-white matter structure and allow better localisation of the functional response and structural properties within the spinal cord. My main contribution to the field has been: 1. To demonstrate that it is possible to use the “ZOOM” sequence for spinal fM RI 2. To characterize the signal obtained and the comparison of different image analysis approaches 3. To propose a final pipeline for acquisition and analysis of spinal fM RI 4. To demonstrate that there is a dependency of pathological functional and structural changes The same ZOOM-EPI sequence has been applied for all the functional studies reported in this thesis. The outcome of the optimisation for spinal fMRI has been matched by a DI protocol, using standard DI parameters for spinal microstructural characterization and constitutes the final MR protocol used in a pilot study including a group of healthy controls and a group of patients affected by multiple sclerosis (MS). Based on the gathered experience and results from data acquired and analysed over the years I have concluded with some recommendations for future studies and development strategies for structural and functional MRI of the spinal cor