An exploration of diffusion tensor eigenvector variability within human calf muscles

Abstract

Purpose To explore the effect of diffusion tensor imaging (DTI) acquisition parameters on principal and minor eigenvector stability within human lower leg skeletal muscles. Materials and Methods Lower leg muscles were evaluated in seven healthy subjects at 3T using an 8-channel transmit/receive coil. Diffusion-encoding was performed with nine signal averages (NSA) using 6, 15, and 25 directions (NDD). Individual DTI volumes were combined into aggregate volumes of 3, 2, and 1 NSA according to number of directions. Tensor eigenvalues (λ1, λ2, λ3), eigenvectors (ε1, ε2, ε3), and DTI metrics (fractional anisotropy [FA] and mean diffusivity [MD]) were calculated for each combination of NSA and NDD. Spatial maps of signal-to-noise ratio (SNR), λ3:λ2 ratio, and zenith angle were also calculated for region of interest (ROI) analysis of vector orientation consistency. Results ε1 variability was only moderately related to ε2 variability (r = 0.4045). Variation of ε1 was affected by NDD, not NSA (P < 0.0002), while variation of ε2 was affected by NSA, not NDD (P < 0.0003). In terms of tensor shape, vector variability was weakly related to FA (ε1:r = -0.1854, ε2: ns), but had a stronger relation to the λ3:λ2 ratio (ε1:r = -0.5221, ε2:r = -0.1771). Vector variability was also weakly related to SNR (ε1:r = -0.2873, ε2:r = -0.3483). Zenith angle was found to be strongly associated with variability of ε1 (r = 0.8048) but only weakly with that of ε2 (r = 0.2135). Conclusion The second eigenvector (ε2) displayed higher directional variability relative to ε1, and was only marginally affected by experimental conditions that impacted ε1 variability.