Mapping techniques

Abstract

Parametric mapping provides quantitative information on changes in magnetic properties of the heart, which reflect alterations in myocardial tissue composition and their environment. These magnetic properties include T1 (spin-lattice), T2 (spin–spin), and T2* relaxation times. The basic principle of mapping techniques is that each tissue type has a normal range for these parameters, deviation from which may indicate disease or a change in physiology. The two principal outputs derived from T1 mapping are native T1 values and extracellular volume fraction (ECV). Native T1 represents a composite signal from the intra- and extracellular components of the myocardium. ECV quantification is derived from native and post-contrast T1 values of the myocardium and blood, using an extracellular contrast agent, and the haematocrit. It permits assessment of the interstitial space as a surrogate marker for diffuse myocardial fibrosis or other aetiologies of ECV expansion. T2 and native T1 are increased by the accumulation of free water content in the myocardium. T2*-mapping is used to detect myocardial iron loading, as T2* (but also T1) is shortened by paramagnetic molecules such as iron and oxygen. Parametric mapping may detect subclinical or diffuse myocardial processes not accessible to conventional CMR methods, and has relevant applications in a wide range of cardiac diseases. With increasing evidence of clinical utility, parametric mapping techniques are expected to become important diagnostic and prognostic tools in myocardial tissue characterization.