Tissue Characterization: T₁, T₂ and T₂* Techniques

Abstract

Noninvasive characterization of tissue has long been the unique domain of magnetic resonance imaging (MRI) when compared to other imaging modalities. Techniques for such typically emphasize one or more MR-based relaxation parameters and the corresponding image contrast or weighting. With or without administration of an exogenous contrast agent, cardiac MRI affords detailed myocardial tissue characterization via various segmented as well as single heart beat approaches. The workhorse technique for myocardial characterization has been late gadolinium enhancement (LGE); LGE is routinely performed in MRI centers around the world as an integral part of nearly every cardiac MRI exam. While originally developed to characterize infarct scar, LGE has since become an important technique to delineate other features of myocardial disease such as fibrosis in nonischemic cardiomyopathy and infiltrates such as sarcoid granuloma and amyloid protein. LGE usually provides robust myocardial characterization, but has two major limitations. First, it requires administration of gadolinium-based contrast, which may not be suitable for individuals with known allergy to such agents or patients with advanced kidney disease. Second, it may be insensitive to more diffusely diseased myocardium where one loses the ability to ‘null’ normal tissue via this technique’s key inversion time parameter. To overcome these limitations, as well as to characterize other myocardial features, imaging techniques that capture intrinsic contrast in T₁, T₂ and other MR-based relaxation parameters are often incorporated into the cardiac MRI examination. Accruing evidence suggests that quantitative approaches, also known as tissue mapping techniques, are helping to further advance MR-based myocardial characterization.