[Mrtrix-discussion] Slice collapse problems

[Watts, Richard]

My explanation for whole-slice signal dropout on diffusion is that it's an interaction between head rotation, the diffusion-weighting gradients and the k-space coverage.

Warning... here comes the physics!
Head rotation produces a position-dependent velocity (zero at the center of rotation, negative on one side, positive on the other).  The diffusion gradients convert this into a linear variation of phase with position (same mechanism as phase contrast imaging).  Applying the Fourier shift theorum makes this equivalent to a shift of the center of k-space.  If the center moves out of your k-space coverage, then you will end up dropping the low spatial frequencies and signal dropout.

Incidentally, the phase variability due to very small head movements is why we can't generally do multishot DW-EPI.  Think of diffusion as phase-contrast MRI on steroids!

Whole-slice dropout is most likely to occur when:
1. Your subject moves a lot (obviously!)
2. You use a high b-value (bigger phase shift for a given motion), especially in directions away from the z-axis
3. You use a smaller k-space coverage (as Jesper/Romain noted 5/8 partial Fourier is worse than 3/4)

I believe that the manufacturers have become a little more conservative with partial Fourier to try to avoid these problems, at the expense of increased TE.

Cheers -


Richard Watts
University of Vermont