The measurement of diffusion and perfusion in biological systems using magnetic resonance imaging

The aim of this review is to describe two recent developments in the use of magnetic resonance imaging (MRI) in the study of biological systems: diffu sion and perfusion MRI. Diffusion MRI measures the molecular mobility of wa ter in tissue, while perfusion MRI measures the rate at which blood is deli vered to tissue. Therefore, both these techniques measure quantities which have direct physiological relevance. It is shown that diffusion in biologic al systems is a complex phenomenon, influenced directly by tissue microstru cture, and that its measurement can provide a large amount of information a bout the organization of this structure in normal and diseased tissue. Perf usion reflects the delivery of essential nutrients to tissue, and so is dir ectly related to its status. The concepts behind the techniques are explain ed, and the theoretical models that are used to convert MRI data to quantit ative physical parameters are outlined. Examples of current applications of diffusion and perfusion MRI are given. In particular, the use of the techn iques to study the pathophysiology of cerebral ischaemia/stroke is describe d. II is hoped that the biophysical insights provided by this approach will help to define the mechanisms of cell damage and allow evaluation of thera pies aimed at reducing this damage.