Hi, Dr Jenkinson
So, if the gradient echo as you said "the asym time is simply the factor required to convert the phase difference in the scans to a rad/s field value", does that mean the asym time is the time causing the phase difference? if this is the case, I think, for gradient echo, the asym time should be (TE2-TE1), where TE2> TE1, since the phase accumulation difference is caused by the different TE. However, if I use this rule to your spin-echo case, the asym time should be twice of the time 180 degree pulse shifted, but actually this is not the case. Now, I am a little confused, could you please tell me why the time cause for your spin echo scan is only the time 180 degree pulse shifted? what should be the corresponding "asym time" for gradient echo sequence when using youe fugue?
Maybe it's a stupid question, but I will really appreciate your help!
Minzhi
----- Original Message -----
From: Mark Jenkinson <[log in to unmask]>
Date: Sunday, April 25, 2004 5:13 pm
Subject: Re: [FSL] Using gradient echo sequence, which time should be the "asym time" in FUGUE ?
> Hi,
>
> I've just responded to the question about using gradient-echo vs
> spin-echo.
> Unfortunately it is not easy or optimal to use our existing software
> for this.
> It is much better to use a spin-echo sequence.
>
> As for the asym time, it is the timing difference between the 180
> degreepulses in the two spin-echo scans. The echo time (TE) is
> kept the same
> for both scans, but the 180 degree pulse is set at TE/2 for the
> symmetric
> scan (normal spin-echo) and set to (TE/2 + asym time) for the
> asymmetricscan. If you do want to use gradient echo, then the
> asym time is simply
> the factor required to convert the phase difference in the scans
> to a
> rad/s field value. However, for gradient echo scans, be aware
> that the
> field map itself will be distorted and needs correction before
> applyingit to the EPI.
>
> All the best,
> Mark
>
>
> On Wednesday, April 14, 2004, at 03:27 am, Minzhi Gui wrote:
>
> > Hi,
> > Did anyone use the gradient echo sequence to undistort the
> images> before, by using Prelude & FUGUE? I'm now wondering what
> is the
> > corresponding "asym time" for gradient echo sequence, is it
> (TE2-TE1)
> > or (TE2-TE1)/2 or somethinf else?
> >
> > Thanks
> >
> > Minzhi
>
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