T1 Mapping

In cardiac magnetic resonance (CMR) imaging, the T1 relaxation time for the 1H magnetization in myocardial tissue may represent a valuable biomarker for a variety of pathological conditions. This possibility has driven the growing interest in quantifying T1, rather than just relying on its effect on image contrast. The techniques have advanced to where pixellevel myocardial T1 mapping has become a routine component of CMR examinations. Combined with the use of contrast agents, T1 mapping has led an expansive investigation of interstitial remodeling in ischemic and nonischemic heart disease. The purpose of this review was to introduce the reader to the physical principles of T1 mapping, the imaging techniques developed for T1 mapping, the pathophysiological markers accessible by T1 mapping, and its clinical uses. (J Am Coll Cardiol Img 2016;9:67–81) © 2016 by the American College of Cardiology Foundation. O ne of the unique aspects of magnetic resonance imaging (MRI) is the sensitivity of the soft tissue image contrast to tissue composition, which can be a reflection of physiology and pathophysiology. The T1 relaxation time, a measure of how fast the nuclear spin magnetization returns to its equilibrium state after a radiofrequency (RF) pulse in the MRI scanner, is a key source of soft tissue contrast in MRI. It was generally considered sufficient to have the T1 relaxation properties “encoded” in the pixel intensity of images, referred to as T1 weighting (e.g., after contrast administration) to highlight “focal” pathology, such as acute myocardial infarction (MI) and chronic scar tissue, or to detect fatty infiltration in myocardial tissue. The role of diffuse, reactive fibrosis, characterized by a disproportionate accumulation of collagen in the heart, has long been identified as an important factor in the etiologies of diastolic dysfunction, heart failure, and sudden cardiac death. The myocardial interstitium therefore became the subject of intense focus, but the diffuse nature of the structural changes made it almost impossible to detect interstitial fibrosis by using conventional T1-weighted imaging. Advances in cardiac magnetic resonance (CMR) imaging techniques over the last few years have rendered it feasible to quantify T1 in the heart and to generate color-encoded T1 maps, in which the pixel values represent the T1 in each voxel (rather than a signal intensity in arbitrary units). T1 maps can depict even relatively small variations of T1 within the heart muscle to highlight tissue pathology. Through the use of extracellular paramagnetic contrast agents, structural changes in the myocardium can be “amplified. ” For example, it is possible with T1 mapping performed before and after injection of a contrast agent to measure a parameter called the extracellular volume (ECV), which quantifies the relative expansion of the extracellular matrix as a result of diffuse reactive fibrosis in multiple cardiac pathologies; the result is creation of a noninvasive alternative to myocardial biopsies and histochemical analysis. Furthermore, T1 mapping without administration of a paramagnetic contrast agent, referred to as “native” T1 mapping, is sensitive to myocardial edema, iron overload, and the presence of myocardial infarcts and scarring. For these reasons, T1 mapping is becoming an essential tool for the CMR imager trying to understand myocardial tissue pathology and its prognostic implications. In addition, by quantifying tissue characteristics through T1 mapping, it becomes feasible to follow longitudinal changes, an essential aspect for using these novel markers in treatment trials. With these rapid developments, the term T1 mapping encompasses a number of approaches to assess tissue remodeling by CMR involving some form of T1 imaging, and either only the native T1 (i.e., without giving contrast) or in combination with a contrast agent. Similarly, the list of indices derived from these measurements has grown longer with time, although native T1 and the ECV fraction are arguably the mostoften derived measures. The present review, although by no means exhaustive, presents an overview of the field, starting from the physical principles to the clinical applications of T1 mapping in the heart. PHYSICAL CONCEPTS OF T1 RELAXATION The measurement of the T1 relaxation time requires measuring the spin magnetization component in the direction of the magnetic field in the magnetic resonance scanner (“longitudinal” component) after it AB BR E V I A T I O N S