Cardiac Magnetic Resonance Imaging for the Investigation of Cardiovascular Disorders. Part 2: Emerging Applications

Abstract

Cardiac magnetic resonance imaging has emerged as a robust noninvasive technique for the investigation of cardiovascular disorders. The coming-of-age of cardiac magnetic resonance-and especially its widening span of applications-has generated both excitement and uncertainty in regard to its potential clinical use and its role vis-à-vis conventional imagC ardiac magnetic resonance imaging (CMR) is unique in its ability to provide comprehensive evaluation of cardiovascular disorders in a single session. Part 1 of this review highlighted the mainstream applications of CMR that are now routinely used for diagnosis, risk stratification, and case management.1 In this section, we provide an overview of the principles and potential applications of emerging CMR techniques that seek to characterize the myocardial substrate in both health and disease. These techniques offer the ability to noninvasively delineate pathophysiologic processes in vivo and, therefore, to expand our understanding of cardiovascular disorders. We also present a synopsis of innovative applications of validated CMR techniques that have the potential to expand the usefulness of CMR as a clinically relevant, cost-effective tool for reliable investigation and accurate case management in a wide variety of clinical situations. T1 Mapping and Extracellular Volume QuantificationDetection of regional or replacement myocardial fibrosis on CMR arises from the visual evaluation of well-delineated areas of late gadolinium enhancement (LGE) on delayed-enhancement MR (DE-MR) imaging. As discussed in Part 1 of this review, 1 high contrast-to-noise ratio in DE-MR imaging is achieved by suppressing the signal from the normal, unaffected myocardium. However, in patients with diffuse interstitial fibrosis, defining the unaffected myocardium can be difficult, which limits the usefulness of DE-MR imaging for detecting diffuse myocardial processes and for quantifying the extent of extracellular matrix (ECM) expansion. Therefore, direct quantification of longitudinal relaxation time of tissue (T1) has been proposed as a novel alternative imaging biomarker of myocardial fibrosis.Depending on the composition of the tissue, T1 relaxation times vary across tissue types. Deviation from established ranges can therefore be used to quantify the effects of pathologic processes.2 The generation of parametric maps that encode T1 value in each pixel of an image is called T1 mapping (Fig. 1).Not only does T1 mapping provide quantitative estimation of diffuse myocardial disease processes and extracellular volume (ECV), but it has the advantage (when compared with DE-MR imaging) of circumventing the influences of windowing, of variations in signal enhancement, and of variability of observer input in characterizing the diseased myocardium.3 Recent robust developments in acquisition and postprocessing techniques have given T1 mapping significant traction.

Cardiac Infections

“…Contrast enhancement of the thickened pericardium usually suggests acute pericarditis (11). Transudative pericardial effusions exhibit low TI signal, whereas exudative effusions typically have relatively higher signal (12).…”