Dear sm ke,
Your calculated dwell time looks fine - although if there is significant
non-linear sampling during the gradient ramps then it can be a bit
different. However, this is probably close enough for this purpose.
As for the your field-map image - is it in units of radians per second?
If it is not then you cannot use it. Many scanners output "fieldmaps"
which are not in these units, so be quite careful. Have a look through
the FSL documentation and the FSL course practical for more info
on the fieldmap calculations.
All the best,
Mark
On 10 Jan 2009, at 06:28, sm ke wrote:
> Hi,
> How are you?
> I'd like to ask if this calculation is right for dwell time.
> The bandwidth of the DTI sequence is 1775 Hz/pixel, Matrix is
> 128x128, and
> IPAT parallel imaging reduction factor is 2.
> Is the dwell time = (1/1775 x 128) / 2?
>
> The FSL FUGUE website provided two usage commands in FUGUE to unwarp
> EPI,
> the latter of which is used for a "previously calculated field map".
> I have
> a real field-map image output from the scanner. Is this equivalent
> to a
> "previously calculated field map", in which case, I use the following
> command which does not require the assym time.
> fugue -i epi --dwell=dwelltime --loadfmap=fieldmap -u result
>
> many thanks
>
> sm ke
>
|