[Mrtrix-discussion] Slice collapse problems

[Dorian P.]

The last point, smaller k-space coverage, seem to be related to a parameter
called "halfscan" in Philips:

Halfscan is a method in which approximately only one half of the
acquisition matrix in the phase encoding direction is acquired.

Effects if Halfscan is set to `Yes'

Signal-to-Noise Ratio:  Reduced with ÷2
Spatial resolution:  Not affected
Scan time  Shorter (almost a reduction by a factor of 2)
Susceptibility effects:  More sensitive to field inhomogeneities and
susceptibility
Artifacts:  More prominent flow and motion artifacts

We have been advised to increase the halfscan factor (currently 0.702), but
in doing so I have to loose another 4ms TE (going to 98ms). I tried this
and effectively the scan with higher halfscan factor had no slice problems
compared to our regular hardi on the same subject (2-3 bad slices). The
problem is that the signal is weaker and I was not sure the benefit would
be consistent in the future.

Is this what you were referring for fourier transform? Do you have any
advise on this?


Thank you.

Dorian
TJU



2013/11/6 Watts, Richard <Richard.Watts at vtmednet.org>

> 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
>
>
>
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