[Mrtrix-discussion] Slice collapse problems

[romain valabregue]

Hello

Yes signal dropouts is a common feature of Siemens dti acquisition. but 
it is not clear what is the exact cause. I think there may be different 
cause
_ subject motion (when there is a lot of subject motion then I see 
several dropout on the same volume
_ bed vibration a know problem on TRIO 3T (I heard they had a solution 
but we could not convince them to install it ...)
_ gradient heating
_ partial fourier : Jesper on fsl list : " My personal experience is for 
example that 5/8 is to severe and that 3/4 is better for reducing drop-out."

For the gradient heating you can try to increase the TR (take 1 or 2 
second abouve  the minimal TR ). The bvec order may also play a role : 
make sure you the directions where the absolute amplitude on a given 
axis (it is often the case)

in the next mrtrix release there will be a correction for those 'bad' slice

Cheers

Romain

Le 05/11/2013 15:13, Dorian P. a écrit :
> Dear MRtrix list,
>
> My HARDI sequence is frequently contaminated by slices with collapsed 
> signal. This happens in 2-3 slices of 61 volumes. Slices and volumes 
> affected are a bit random, not always the same.
>
> In concomitance I see shifts in anterior-posterior (phase) direction 
> from one volume to the other. I have been told this may happen because 
> gradients heat to much.
>
> My question at this point is:anybody had the same experience with 
> slice dropouts?
>
> Do you think slice dropouts are related to the b-value. Would 
> decreasing b from 3000 to 2500 make a difference? Could slices 
> collapse because of motion causing bigger artifacts with high b-values?
>
> Finally, do you think the slice collapse is related to the 
> anterior-posterior shift (gradient heating) problem?
>
> Any comment may be useful.
> Thank you.
>
> Dorian
> TJU
>
>
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