Neuroimaging

Introduction

Neuroimaging is a standard diagnostic aid in the dementias. Initially this was aimed at excluding non-degenerative pathologies (strokes, tumours etc.) but as clinical assessment of different degenerative syndromes has improved, the role of imaging to enhance positive predictive value has also increased. Imaging is a routine investigation in suspected FTD both for exclusion of alternate pathologies and to improve diagnostic certainty. This section will review neuroimaging findings in FTD with respect to clinical diagnosis, understanding the neural basis for cognitive and behavioural phenomena, and the pathological subtypes. In this chapter, the clinical terms behavioural variant FTD (bv-FTD), semantic dementia (SD) and progressive non-fluent aphasia (PNFA) will be used to describe the subtypes of FTD.

Imaging modalities

The following sections will discuss a variety of imaging methods each with their own strengths and weaknesses. Before discussing specific findings, these methods will be briefly reviewed to help the reader decipher the results of different studies.

T1-weighted magnetic resonance imaging (MRI) is the method of choice for evaluation of structural changes within the brain. In clinical practice, it is routine to inspect visually MRI scans for the presence of regional brain atrophy – the assumption being that the topographical distribution of atrophy can improve diagnostic certainty in specific dementia syndromes. For research purposes two further methods are commonly used to analyse MRI data – region of interest (ROI) analysis and voxel-based analysis as in the technique of voxel-based morphometry (VBM). Region-of-interest methods involve tracing the boundaries of a given brain region on sequential fine-cut slices which can then be stacked to calculate a regional volume. The method is very labour intensive and consequently mostly limited to research studies (although visual-rating scales are sometimes used as a surrogate marker and have greater applicability for clinical practice). As it is not feasible to measure regional atrophy manually across the entire brain, when ROI volumetry is applied to the study of brain–behaviour relationships its utility is critically dependent on the strength of the prior hypothesis. However, the corollary is that ROI methods are of little use where the locus of pathology, or neural substrate for a given behaviour, is uncertain.