cardiovascular magnetic resonance frontiers: tissue characterisation with mapping

The clinical use of cardiovascular magnetic resonance (CMR) imaging has expanded rapidly over the last decade. Its role in cardiac morphological and functional assessment is established, with perfusion and late gadolinium enhancement (LGE) imaging for scar increasingly used in day-to-day clinical decision making. LGE allows a virtual histological assessment of the myocardium, with the pattern of scar suggesting disease aetiology, and the extent of predicting risk. However, even combined, the full range of pathological processes occurring in the myocardium are not interrogated. Mapping is a new frontier where the intrinsic magnetic properties of heart muscle are measured to probe further. T1, T2 and T2* mapping measures the three fundamental tissue relaxation rate constants before contrast, and the extracellular volume (ECV) after contrast. These are displayed in colour, often providing an immediate appreciation of pathology. These parameters are differently sensitive to pathologies. Iron (cardiac siderosis, intramyocardial haemorrhage) makes T1, T2 and T2* fall. T2 also falls with fat infiltration (Fabry disease). T2 increases with oedema (acute infarction, takotsubo cardiomyopathy, myocarditis, rheumatological disease). Native T1 increases with fibrosis, oedema and amyloid. Some of these changes are large (e.g. iron, oedema, amyloid), others more modest (diffuse fibrosis). They can be used to detect early disease, distinguish aetiology and, in some circumstances, guide therapy. In this review, we discuss these processes, illustrating clinical application and future advances.