Hi,

Sorry, but the strict limits on attachments to the list have
removed the pdf.  Could you send it to me directly.

Also, it isn't correct to say that dwell time is the "total time
taken to acquire all the lines of k-space for a single image".  In
fact dwell time is the time between successive lines, but with the
spacing required for a non-aliased image.  What we use it for is to
figure out the rate that phase is accrued as k-space is tranversed.
This rate controls the distortion and is easily specified via the
"dwell time" for non-accelerated scans.  However, if SENSE is used
then lines are skipped and images from other coils used to compensate
for this in the reconstruction. The effect is that the distortion is
reduced (since the phase accrued is less per unit k-space in the PE
direction).  So the easiest solution is to put in the "effective
dwell time" which is the time between successive k-space lines
(which are now more widely spaced) divided by the acceleration which
is normally equal to the change in spacing of the k-space lines.
If the ratio between the spacing of the k-space lines of the accelerated
and non-accelerated sequences were non-integer for any reason then
it would still be correct to use the non-integer factor as the divisor.

Of course, what terminology each manufacturer uses for terms like
acceleration factor, SENSE factor, etc as well as dwell time, varies
and it is necessary to pin down the exact definition of any numbers
pulled from the scanner output files or protocol specifications.

It is always useful to check various possibilities and look at the
results the first time you try unwarping, just to check that the
values you are using seem to give the best results.

All the best,
	Mark


On 7 Dec 2006, at 10:04, Chris Miall wrote:

> Liz Moore from Philips sent the attached PDF which explains how to  
> calculate
> the effective dwell time for unwrapped the phase etc. She says
>
> "This helps me to understand what they mean by "dwell time" too!  
> They mean
> the total time taken to acquire all the lines of k-space for a  
> single image.
>
> Referring back to my previous mail, the best way to calculate this  
> for a
> Philips system is by multiplying the echo spacing by the EPI  
> factor. The EPI
> factor takes care of the sense factor properly, you do not need to  
> do any
> further calculation. Be warned: if you do start just dividing by  
> the sense
> factor you will run into difficulty because sense factors are not
> necessarily integers. Try dividing 64 by 2.5 and figuring out what  
> the dwell
> time is! Much safer (and more correct) to take the EPI factor off the
> scanner for your protocols.
>
> The pdf attached contains all this information, and you can feel  
> free to
> post it on the FSL listing.
>
> Kind regards,
>
> Liz
> Elizabeth A Moore PhD
> MR Clinical Scientist"