Dear Shugao,
I cannot really diagnose the problem from this image, so please
upload your data to:
http://www.fmrib.ox.ac.uk/cgi-bin/upload.cgi
and send us the reference number.
As for the dwell time (or echo spacing) calculation, neither option
is perfect because scanners don't instantaneously move from
one line to another, and the time for the phase encoding blips
and any ramp sampling needs to be taken into account. You
will get something roughly right by using the bandwidth and the
number of readout points (your first calculation) but it will be
somewhat off. As for the second calculation, I do not know
exactly what "EPI factor" means on a Philips scanner but it seems
unlikely to me that this is the correct thing to do. It may refer to
the number of lines acquired per shot, but then I would expect it
to be a factor of 240. However, the number of lines acquired does
not affect the dwell time which is the time taken per line. It may
indicate that you have a segmented acquisition or an accelerated
acquisition. If you have an accelerated or segmented acquisition then
you also need to divide by the acceleration/segment factor (the ratio
of the line spacing in the phase-encode direction of k-space to what it
would be in a single-shot, unaccelerated acquisition) in order to get the
"effective" echo spacing (or dwell time, the two terms are interchangeable).
All the best,
Mark
On 23 May 2011, at 23:07, Shugao Xia wrote:
> Hi,
>
> I used the Feat GUI to correct EPI distortion and the results were wrong, the small portions of the fieldmap only appeared in space of EPI. I think this is because of the wrong registration between the magnitude image and EPI. So I tried to use flirt as following:
>
> flirt -in mag -ref EPI -dof 6 -omat rot.mat -o mag_in_EPI [ -usesqform ]
>
> the attached fig shows the image mag_in_EPI, does any one advise me to correct it? if you need the data, I will upload it.
>
> Another question: how to calculate the dwell time? I used 3T Philiphs to obtain EPI data wiht BW(Hz)=24.1, image resize=240*240, EPI factor = 61. So the dwell time = 1/(24.1*240) = 0.17ms or the dwell time = 1/(24.1*61) = 0.68ms. My colleague thinks we use the former one here, but I am confused. Can anyone help me clearify? Thank you very much
>
> Take care
>
> Shugao
>
>
> <1.jpg>
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