Neuroimaging in dementia: an update for the general clinician

Types of imaging Brain imaging techniques currently used in the UK for the clinical assessment of dementia are grouped into three categories: structural, functional and molecular. Structural imaging includes computed tomography (CT) and magnetic resonance imaging (MRI). The images produced by these methods allow one to see the anatomical ‘structure’ of cerebral tissue. They are used to detect areas of visible brain atrophy or ischaemia. Functional imaging includes 99mTc-HMPAO single positron emission CT (HMPAO-SPECT), 18F-FDG positron emission tomography (FDG-PET) and the DaTscan, which use radioactive tracers to give an indication of the functioning of brain tissue. HMPAOSPECT uses a lipophilic tracer that crosses the blood–brain barrier and moves into brain tissue by diffusion, the resulting SPECT image demonstrating the degree of cerebral blood perfusion. FDG-PET uses a glucose analogue to demonstrate the degree of cerebral glucose metabolism. In practice these two imaging modalities give similar information, and are used to identify impaired brain physiology prior to the onset of obvious atrophy. The DaTscan is a type of SPECT used specifically in the assessment of suspected Parkinson’s disease or dementia with Lewy bodies (DLB), and enables the visualisation of dopaminergic activity in the basal ganglia. Finally, molecular imaging using radioactive traces that bind to specific molecules of interest is an emerging field. Amyloid-labelled PET scans, have been used in clinical trials to demonstrate the level of cerebral amyloid plaques in Alzheimer’s disease (AD). Two ligands, 18F-florbetapir and 18F-florbetaben, have recently been licensed for clinical use in the UK.