Assessment of Heart Microstructure
From Mouse to Man
Stem cell therapy has undergone a rapid translation from bench research to clinical trials as a promising approach for the regeneration of the injured myocardium.1,2 Magnetic resonance imaging (MRI) plays a pivotal role in the assessment of stem cell therapy efficacy and elucidation of the mechanisms behind therapeutic effects.3 One important aspect of stem cell therapy, however, remains missing: There are currently no noninvasive methods to evaluate the restoration of myocardial tissue microstructure. A study by Sosnovik et al4 published in this issue of Circulation fills this gap and demonstrates the feasibility of evaluating the integrity and spatial organization of myofibers after cell therapy.
Article see p 1731
The microstructure of the heart was described histologically >40 years ago in landmark studies by Streeter and Hanna.5 The myoarchitecture of a healthy heart is made up of 3 layers of crossing spiral myofibers. The subendocardium fiber orientation is a right-handed helix, the subepicardium is a left-handed helix, and fibers in the midmyocardium are circumferential.5 This structure allows for maximal contractile force to ensure effective blood pumping. Despite the discovery of the complex cardiac myoarchitecture and its role in heart function, opportunities to study this aspect of the cardiac anatomy noninvasively were not available for several decades.
Diffusion tensor imaging (DTI), the first MRI method capable of visualizing cardiac microstructure, was developed in the mid 1990s.6 DTI allows characterization of anisotropic diffusion of water molecules in tissues. Diffusion anisotropy arises from natural barriers, such as cell membranes, and is most prominent in tissues consisting of coherent fiber bundles, such as brain white matter or muscle. The magnetic resonance signal can be sensitized to diffusion by applying sufficiently strong magnetic field gradients, which cause the loss of phase coherence of individual molecular magnetizations in the …