<div dir="ltr">The last point, smaller k-space coverage, seem to be related to a parameter called "halfscan" in Philips:<div><br></div><div><div>Halfscan is a method in which approximately only one half of the acquisition matrix in the phase encoding direction is acquired. </div>
<div><br></div><div>Effects if Halfscan is set to `Yes' </div><div><br></div><div>Signal-to-Noise Ratio: Reduced with ÷2 </div><div>Spatial resolution: Not affected </div><div>Scan time Shorter (almost a reduction by a factor of 2) </div>
<div>Susceptibility effects: More sensitive to field inhomogeneities and susceptibility </div><div>Artifacts: More prominent flow and motion artifacts </div></div><div><br></div><div>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.</div>
<div><br></div><div>Is this what you were referring for fourier transform? Do you have any advise on this?</div><div><br></div><div><br></div><div>Thank you.</div><div><br></div><div>Dorian</div><div>TJU</div><div><br></div>
<div class="gmail_extra"><br><br><div class="gmail_quote">2013/11/6 Watts, Richard <span dir="ltr"><<a href="mailto:Richard.Watts@vtmednet.org" target="_blank">Richard.Watts@vtmednet.org</a>></span><br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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.<br>
<br>
Warning... here comes the physics!<br>
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.<br>
<br>
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!<br>
<br>
Whole-slice dropout is most likely to occur when:<br>
1. Your subject moves a lot (obviously!)<br>
2. You use a high b-value (bigger phase shift for a given motion), especially in directions away from the z-axis<br>
3. You use a smaller k-space coverage (as Jesper/Romain noted 5/8 partial Fourier is worse than 3/4)<br>
<br>
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.<br>
<br>
Cheers -<br>
<br>
<br>
Richard Watts<br>
University of Vermont<br>
<div><div><br>
<br>
<br>
_______________________________________________<br>
Mrtrix-discussion mailing list<br>
<a href="mailto:Mrtrix-discussion@www.nitrc.org" target="_blank">Mrtrix-discussion@www.nitrc.org</a><br>
<a href="http://www.nitrc.org/mailman/listinfo/mrtrix-discussion" target="_blank">http://www.nitrc.org/mailman/listinfo/mrtrix-discussion</a><br>
</div></div></blockquote></div><br></div></div>