Perfusion and diffusion magnetic resonance imaging studies of cerebral ischaemia

Magnetic resonance imaging (MRI) has opened up new avenues of anatomical and physiological research. Its non-invasive nature and the ability to obtain images with a high degree of spatial resolution, have been exploited in this research as well as in routine clinical investigations. However, the use of standard MRI techniques in the investigation of the acute phases of cerebrovascular diseases such as stroke, has been hampered by impaired sensitivity to detect areas of ischaemic damage. The relatively recent development of the MRI techniques of diffusion and perfusion imaging, described in this thesis, have the capability to fill this gap. The diffusive mobility of water is a sensitive indicator of tissue energy status, and perfusion imaging provides quantitative measurements of cerebral blood flow (CBF). This thesis describes the implementation of these two techniques in experimental studies of cerebral ischaemia. Special emphasis is placed upon the improvement of the temporal resolution of these methods in order that time course studies of the rapidly evolving pathophysiology can be carried out. In particular, an innovative modification of the spin-labelling perfusion technique of flow-sensitive alternating inversion recovery (FAIR), has been developed that allows rapid, mapping of CBF. This method was implemented in an important study of reperfusion injury. High time-resolution sequences for rapid, quantitative diffusion imaging were implemented on both high-field and low-field MRI systems. The high-field environment sensitises the images to various artefacts and a novel enhancement of a standard rapid imaging technique was required in order to obtain accurate measurements. On the low-field system, fast imaging of the trace of the diffusion tensor (trace(D)) was optimised and employed during a study of repeated ischaemic episodes that is a model of transient ischaemic attacks. The combination of these two MRI techniques has provided unique insights into experimental cerebral ischaemia.