Measurement of Strain in Physical Models of Brain Injury: A Method Based on HARP Analysis of Tagged Magnetic Resonance Images (MRI)

The utility of harmonic phase (HARP) analysis has recently been demonstrated in humans and large animals as a technique for rapid and automatic analysis of tagged MR images. In the current study, the applicability and accuracy of HARP analysis for automatic strain quantification in small animals were investigated. A validation study was performed on seven post-infarct rats and seven age-matched controls. A method for direct computation of 2D Lagrangian strain fields from spatial derivatives of HARP images was also developed in this paper. The results of HARP analysis were evaluated by comparison with those of homogeneous strain analysis employing finite element method and manual tag tracking. Both methods were validated with simulated digital images. Compared to conventional homogeneous strain analysis, HARP analysis yielded similar results in the assessment of regional strain patterns in both control and infarct rats. Both methods detected a reduction in maximal stretch and shortening in infarct rats. Our results suggest that HARP analysis can also be applied to quantify alterations in regional myocardial wall motion in small animals.