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MRI-based finite-element analysis of left ventricular aneurysm

¹Joint Graduate Group in Bioengineering, University of California at Berkeley/San Francisco; Departments of ²Surgery, ³Anesthesia, and ⁴Radiology, University of California, San Francisco; ⁵Department of Veterans Affairs Medical Center, San Francisco; ⁶Department of Biomedical Engineering, University of Southern California, Los Angeles, California; ⁷Department of Biomedical Engineering, Duke University, Durham; and ⁸The Center for In Vivo Microscopy, Duke University Medical Center, Durham, North Carolina

Submitted 6 December 2004 ; accepted in final form 15 March 2005

Tagged MRI and finite-element (FE) analysis are valuable tools in analyzing cardiac mechanics. To determine systolic material parameters in three-dimensional stress-strain relationships, we used tagged MRI to validate FE models of left ventricular (LV) aneurysm. Five sheep underwent anteroapical myocardial infarction (25% of LV mass) and 22 wk later underwent tagged MRI. Asymmetric FE models of the LV were formed to in vivo geometry from MRI and included aneurysm material properties measured with biaxial stretching, LV pressure measurements, and myofiber helix angles measured with diffusion tensor MRI. Systolic material parameters were determined that enabled FE models to reproduce midwall, systolic myocardial strains from tagged MRI (630 ± 187 strain comparisons/animal). When contractile stress equal to 40% of the myofiber stress was added transverse to the muscle fiber, myocardial strain agreement improved by 27% between FE model predictions and experimental measurements (RMS error decreased from 0.074 ± 0.016 to 0.054 ± 0.011, P < 0.05). In infarct border zone (BZ), end-systolic midwall stress was elevated in both fiber (24.2 ± 2.7 to 29.9 ± 2.4 kPa, P < 0.01) and cross-fiber (5.5 ± 0.7 to 11.7 ± 1.3 kPa, P = 0.02) directions relative to noninfarct regions. Contrary to previous hypotheses but consistent with biaxial stretching experiments, active cross-fiber stress development is an integral part of LV systole; FE analysis with only uniaxial contracting stress is insufficient. Stress calculations from these validated models show 24% increase in fiber stress and 115% increase in cross-fiber stress at the BZ relative to remote regions, which may contribute to LV remodeling.

tagged magnetic resonance imaging; computational modeling; diffusion tensor magnetic resonance imaging; ventricular remodeling; congestive heart failure

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