Introduction

We turn now to some of the more recent developments in MRI, to areas at the cutting edge between research and clinical practice. With the current pace of development, it is likely that some of these techniques will move swiftly forward; however, in this chapter we aim to explore the underlying principles.

The techniques we are exploring are all related to what is sometimes termed ‘micro-contrast’ mechanisms. This term is unfortunate because basic T1 and T2 relaxation depend upon the microscopic (at molecular level) interactions of spins. Nevertheless these new techniques give specific information about aspects of tissue not readily available through conventional contrast mechanisms. These are:

• Diffusion-weighted imaging, which relates to the mobility of water molecules, particularly when mobility is restricted to specific orientations, as in white matter tracts. Diffusion-weighted imaging is the method of choice for the imaging of acute stroke.

• Perfusion-weighted imaging relates to the delivery of blood to tissues, in terms of relative or absolute concentrations, or in terms of rates of delivery or mean transit times. Perfusion-weighted imaging can use exogenous contrast (i.e. a bolus of contrast agent) or endogenous contrast, by the ‘spin labelling’ of arterial blood.

• BOLD (Blood Oxygenation Level Dependent) imaging, sometimes called fMRI (functional MRI – note the small ‘f’), is used to investigate regional brain activation, in lay terms, to observe the brain thinking. The BOLD effect uses the level of blood oxygenation to generate contrast.