Dyssynchrony Evaluation: MRI and CCT

Abstract

Cardiac resynchronization therapy (CRT) has been shown to improve outcomes in clinical trials for appropriate selected patients with heart failure; however, there is still a significant nonresponse rate when the electrocardiographic QRS duration is used to identify mechanical dyssynchrony. From a physiological standpoint, the appropriate cardiac substrate for CRT is most appropriately identified by assessment of mechanical dyssynchrony and myocardial scar burden, but QRS duration has poor sensitivity and specificity in this regard. For this reason, cardiac imaging has been evaluated in order to improve CRT candidate selection, but results using echocardiography have been disappointing. In contrast, clinical studies show great promise for CMR in this regard. As the gold standard for assessment of cardiac scar and strain, cardiac magnetic resonance (CMR) offers clear advantages over other imaging modalities for assessment of the cardiac substrate for resynchronization. In particular, CMR is regarded as the best imaging modality for assessment of circumferential strain, which corresponds to the primary orientation of cardiac myofibers. This chapter reviews important principles of CMR dyssynchrony and scar imaging, then discusses in detail clinical and technical aspects of myocardial tissue tagging, displacement encoding with stimulated echoes (DENSE), strain-encoded MR (SENC), velocity-encoded MR, and contour tracking methods. The role of CMR in patients with pacemakers and defibrillators is also discussed. The potential role of cardiac CT (CCT) for dyssynchrony evaluation is also discussed. Cardiac CT provides excellent assessment of coronary venous anatomy, while methods for assessment of dyssynchrony and scar based on CCT are being developed.