Transmural myocardial strain in mouse: quantification of high-resolution MR tagging using harmonic phase (HARP) analysis.

MR tagging allows noninvasive examination of regional myocardial function with high accuracy and reproducibility. The current tagging method is limited by low tagging resolution for accurate transmural strain quantification. Previously, a spatial modulation of magnetization (SPAMM)-based method was proposed to increase the tagging resolution by combining two or more tagged images with different tagging grid positions. However, there has been limited application due to the challenge in image processing of multiple data sets. In the current study, we propose a harmonic phase (HARP)-based method for automated and fast analysis of high tagging resolution images. First-order harmonic peaks from low tagging resolution images were combined to generate the composite second-order harmonic peak for strain computation. The combined images reached a tagging resolution of 0.3 mm. The proposed method was applied to the quantification of transmural myocardial wall strain in seven normal C57BL/6 mice. Principal strains, as well as radial and circumferential strains, were quantified using the current method.