Classification of myofibers using statistics of the helix angle: a novel approach to characterize the structure of the human heart
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KeywordsFiber Length Helix Angle Fiber Tracking Continuous Tract Cardiovasc Image
To characterize the microstructure of human/mammalian hearts using a statistical definition of fiber helix angle in the myocardial continuum.
The integration of primary eigenvectors in a diffusion field yields continuous tracts, to which it is useful to assign a single helix angle. However, the optimal approach to derive the helix angle of a continuous tract in the myocardium remains unclear. Strategies used in the brain and in prior tractographic studies in the heart (Circ Cardiovasc Imaging. 2009; 2(3): 206-12) may not be optimal for the myocardial continuum.
Excised human, sheep and rat hearts (n=12) were studied. Diffusion tensor MRI (DT-MRI) of the human and sheep hearts was performed at 3T using 6 gradient-encoding directions; a b-value of 2000s/mm2; voxel-size=2x2x2mm3; TR/TE=8430/96ms; and 24 averages. DT-MRI of the rat hearts was performed at 4.7T using a b-value of 2000 s/mm2 and spatial resolution of 0.4x0.4x0.4 mm3. Fiber tracking was performed with a fourth-order Runge-Kutta approach. The helix angle assigned to each tract was defined in three ways: 1) Each voxel in the tract had its own (original) helix angle defined by its primary eigenvector, 2) the entire tract was classified by its median helix angle, or 3) by its mean helix angle.
The helix angle along a given myofiber is not constant and is highest at the midventricular level. The classification of myofibers based on their median helix angles strikes the optimal balance between clarity and preserved angular resolution.
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