Hi Martin,
There are indeed two methods for unwarping. I certainly
recommend the pixel-shift for unwarping, but the
current distributed version uses Fourier for forward warping
(of the magnitude fieldmap image in order to create a
distorted target image for registration purposes). The reason
for this is that it avoids the necessity to invert the warp
field, as taking a warped image and unwarping it is opposite
to warping an unwarped image, and what space the warp
is in matters, so I either needed an inverse of the warp field
(in general difficult to impossible) or a different technique
for forward warping. I chose the latter and used a Fourier method.
In summary, both methods are used in fugue, but just got with the
defaults as they automatically choose the best method.
The second question is all about units.
My fieldmap is in radians per second, obtained by
taking phase diff in radians and dividing by asym_time
in seconds. My pixel_shift is then in voxels (dimensionless)
which is obtained by taking the fieldmap in rad/s and
multiplying by dwell time in seconds, and dividing by 2*pi
(radians).
In contrast, the field in Jezzard et al is in Telsa, so he takes phase_diff
(radians) and divides this by 2*pi (radians) and gyr_ratio
(Hz/T) and asym_time (seconds). Since Hz = 1/seconds
then the result is a quantity in units of Tesla. By taking my
fieldmaps in rad/s I just avoid the need for using the
gyromagnetic ratio, as I never need to convert back to Tesla.
However, you can certainly do it with mine in which case you
need to insert the gyromagnetic ratio again.
All the best,
Mark
Martin Kavec wrote:
>Hello everybody,
>
>I have two questions regarding FUGUE.
>
>1. FUGUE tutorials
>( http://www.fmrib.ox.ac.uk/fslcourse/lectures/flirt_fugue/s_7063.htm)
>says that Unwarping can be done as Fourier phase shift and Pixel shift. I
>assume that Pixel shift is a shift of a pixel together with interpolation. I
>also assume that it is faster than Fourier phase shift, so why would I want
>to use the latter one? Which one is used in FUGUE?
>
>2. FUGUE tutorials
>( http://www.fmrib.ox.ac.uk/fslcourse/lectures/flirt_fugue/s_7057.htm )
>says that
>FIELDMAP = PHASE_DIFF/ASYM_TIME (a)
>and
>PIXEL_SHIFT = FIELDMAP*N*DWELL/(2PI). (b)
>
>Jezzard et al. 1995 in their MRM paper say at page 66 that
>FIELD = PHASE_DIFF/(2PI*GYR_RATIO*ASYM_TIME)
>
>This almost the same as (a), however I am missing gyromagnetic ratio in either
>of the equations (a) and (b).
>
>
>I would appreciate if someone could shed some light into this for me.
>Thanks.
>
>Martin
>
>
