Introduction

The advent of neuroimaging techniques which yield physiological in addition to structural information are of particular interest in the scientific and clinical investigations of neurodegenerative and psychiatric disorders. These are groups of conditions where any structural changes which are evident on anatomical imaging sequences generally correlate poorly with clinical diagnostic categories, underlying pathophysiology and disease severity.

T1- and T2-dependent MR sequences which are the mainstay of routine clinical neuroimaging are frequently insensitive to the underlying pathological processes in these diseases. Focal or global atrophy due to associated neuronal loss is also frequently subtle or absent, particularly early in the course of disease.

As a result, clinical brain imaging using standard techniques is frequently normal, or non-specifically abnormal.

Physiological imaging can be considered to have two main purposes in this context: The first is clinical; to provide diagnostic information which augments that available from clinical examination, laboratory tests and conventional structural brain imaging. The aim here is to increase the sensitivity and or specificity of the imaging examination as a whole, and improve diagnostic confidence, which will ultimately guide clinical management. In this context, the technique must provide a surrogate marker of disease, which is of predictive value in diagnosis or prognosis for an individual patient. In practice, this requires a distinctive imaging appearance or, in the case of quantitative techniques, sufficient separation between parameters measured to allow an individual being examined to be placed confidently in a diagnostic or prognostic clinical group. In order to have widespread clinical impact, the technique must improve specificity, sensitivity, safety or cost-efficacy in securing a diagnosis.