How Many Gradients are Sufficient in High-Angular Resolution Diffusion Imaging (HARDI)?

We scanned 61 healthy adults with 105-gradient HARDI at 4 Tesla, and examined how the number of diffusion gradients affects the signal-to-noise ratio (SNR) for several common DTI-derived scalar measures: the fractional and relative anisotropy (FA, RA) mean diffusivity (MD), and volume ratio (VR). HARDI applies diffusion-sensitive magnetic field gradients to the brain at a range of spherical angles (typically >100) to analyze white matter microstructure and integrity. We optimized the angular distribution energy on gradient image subsets of size 1≤N≤94, to artificially reduce the angular sampling. 7 gradients are mathematically sufficient to determine FA/RA/MD/VR, but by increasing the number of diffusion-sensitized gradients from 20 to 94, SNR improved by 69.23% and 19.93% for VR and RA, and by 12.24% and 8.77% for FA and MD. Measures involving products of 3 eigenvalues (e.g., VR) were noisier, requiring more gradients to determine. FA SNR rose rapidly with more gradients than are routinely collected, suggesting advantages of HARDI even for standard neuroscientific studies.

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