Assessment of collateral blood flow in the brain using magnetic resonance imaging

Collateral blood flow is the compensatory flow of blood to the tissue through secondary channels when the primary channel is compromised. It is of vital importance in cerebrovascular disease where collateral flow can maintain large regions of brain tissue which would otherwise have suffered ischaemic damage. Traditional x-ray based techniques for visualising collateral flow are invasive and carry risks to the patient. In this thesis novel magnetic resonance imaging techniques for performing vessel-selective labelling of brain feeding arteries are explored and developed to reveal the source and extent of collateral flow in the brain non-invasively and without the use of contrast agents. Vessel-encoded pseudo-continuous arterial spin labelling (VEPCASL) allows the selective labelling of blood water in different combinations of brain feeding arteries that can be combined in post-processing to yield vascular territory maps. The mechanism of VEPCASL was elucidated and optimised through simulations of the Bloch equations and phantom experiments, including its sensitivity to sequence parameters, blood velocity and off-resonance effects. An implementation of the VEPCASL pulse sequence using an echo-planar imaging (EPI) readout was applied in healthy volunteers to enable optimisation of the post-labelling delay and choice of labelling plane position. Improvements to the signal-to-noise ratio (SNR) and motion-sensitivity were made through the addition of background suppression pulses and a partial-Fourier scheme. Experiments using a three-dimensional gradient and spin echo (3D-GRASE) readout were somewhat compromised by significant blurring in the slice direction, but showed potential for future work with a high SNR and reduced dropout artefacts. The VEPCASL preparation was also applied to a dynamic 2D angiographic readout, allowing direct visualisation of collateral blood flow in the brain as well as a morphological and functional assessment of the major cerebral arteries. The application of a balanced steady-state free precession (bSSFP) readout significantly increased the acquisition efficiency, allowing the generation of dynamic 3D vessel-selective angiograms. A theoretical model of the dynamic angiographic signal was also derived, allowing quantification of blood flow through specified vessels, providing a significant advantage over qualitative x-ray based methods. Finally, these methods were applied to a number of patient groups, including those with vertebro-basilar disease, carotid stenosis and arteriovenous malformation. These preliminary studies demonstrate that useful clinical information regarding collateral blood flow can be obtained with these techniques.EThOS - Electronic Theses Online ServiceGBUnited Kingdo