FUGUE EPI Unwarping
FMRIB's Utility for Geometrically Unwarping EPIs - v2.2 (in c# minor)
|  |
INTRODUCTION
Standard functional images, using the EPI sequence, are distorted
due to magnetic field inhomogeneities. These inhomogeneities are
caused by magnetic susceptibility differences in neighbouring tissues
within the head - particularly for air/bone or air/tissue interfaces
in the sinuses. Consequently, the functional (EPI) images suffer
from geometrical distortion and signal loss, particularly in the
inferior frontal and temporal regions.
It is possible to measure the field inhomogeneities with a fieldmap
sequence, use the measured field values to calculate the geometric
distortion and signal loss, and then use the calculated information to
compensate for these artefacts. Compensating for these artefacts is
done by geometrically unwarping the EPI images and by applying
cost-function masking in registrations to ignore areas of signal loss.
Note that areas where signal loss has occurred cannot be restored
with any form of post-processing, as the signal has been lost - only
different acquisition techniques can restore signal in these areas.
Furthermore, there are two situations where dealing with these
distortions: (1) for correcting motion-dependent changes; and (2) for
registration with non-distorted images (e.g. structurals). Within
FEAT, only the second case is dealt with. The issue of
motion-dependent signal changes (due to motion-dependent changes in
field inhomogeneity and distortion directions) is not dealt
with in the current version.
Fieldmap Acquisition
Unfortunately, there is no standard sequence for fieldmap acquisitions
and different scanners return different images. Normally these
images require processing before they represent images with field values
in the desired units (of radians/second) in each voxel.
The most common sequence acquires two images with different echo
times. The change in MR phase from one image to the other is
proportional to both the field inhomogeneity in that voxel and the
echo time difference. The field value is therefore given by the
difference in phase between these two images divided by the echo time
difference. This is true for Spin Echo, Gradient Echo or EPI
sequences. However, EPI-based fieldmaps suffer from the same
distortions (more or less) as the functional images, while Spin Echo
or Gradient Echo based fieldmap images do not. Within FSL
you cannot use EPI-based fieldmaps with the standard
processing, and their use in general is very problematic. We strongly
recommend that Spin Echo or Gradient Echo fieldmap sequences are used
to acquire the images.
MR phase is the most important quantity in a fieldmap sequence,
whereas in normal imaging this phase is not of interest and is
normally not saved when reconstructing the images. As a consequence,
raw fieldmap scans are somewhat different from most scans, and may
contain images of complex values, or separate phase and magnitude
images. Furthermore, some scanners/sites may do the full
reconstruction of acquired scans to yield a real-valued map of field
inhomogeneities (in units of Hz, radians per second, Tesla or ppm).
Alternatively no reconstruction may be done, and the raw phase and
magnitude (or complex) images may be saved instead. It is important
for each different scanner/site/sequence to know what form your data
is in. If they have been converted to NIFTI or ANALYZE format, then
you can use the FSL tools (particularly fslinfo
) to
determine the types of images present. To obtain fieldmaps that can
be used within FSL using the FSL tools (in particular, PRELUDE and
FUGUE), please refer to the page on
preparing fieldmaps for FEAT.
Software Documentation
The rest of this document gives a brief description of the
individual command line programs available in the FUGUE component of FSL
- used for unwarping geometric distortion in EPI images. At present
no GUI interface exists for direct application of FUGUE (or PRELUDE),
but it is now possible to run within FEAT. See the detailed
documentation on FEAT and
documentation on using PRELUDE/FUGUE for
preparing fieldmaps for FEAT.
For each of the programs described here, a full list of available options
can be obtained by running the command with the -h
option.
Note that for all programs the options follow the normal convention that
"single minus" options are separated by a space from their arguments (if any)
whilst "double minus" options are separated by an equals sign and no space.
For example,
prelude -c data --unwrap=result
or
prelude --complex=data -u result
PRELUDE
prelude (Phase Region Expanding Labeller for Unwrapping
Discrete Estimates) performs 3D phase unwrapping of images. The input
can either be a single complex analyse file, or a pair of real analyse
files giving the phase and absolute values separately. If the files
are 4D files, then each 3D volume is unwrapped separately, and the
result saved as a 4D file of unwrapped phase images. The output in
either case is a real, unwrapped phase image (in radians).
The two main forms of usage are:
- prelude -c data -u result
- uses a single complex input file
- prelude -a data_abs -p data_phase -u result
- uses separate phase and absolute input files
Additional options that are useful are:
- -m mask
- uses the user defined mask
- -n num
- specifies the number of phase partitions
the algorithm uses for labelling - a larger value is likely to be more
robust but slower
- -s
- unwrap in 2D, then stick slices together
with appropriate offsets - this is less robust but fast - mainly used for
very high-resolution images where speed is an issue
- --labelslices
- does labelling in 2D, but
unwrapping in 3D - the default for high-res images
FUGUE
fugue (FMRIB's Utility for Geometrically Unwarping EPIs)
performs unwarping of an EPI image based on fieldmap data. The input
required consists of the EPI image, the fieldmap (as an unwrapped
phase map or a scaled fieldmap in rad/s) and appropriate image
sequence parameters for the EPI and fieldmap acquisitions: the dwell time for EPI (also known as the echo spacing); and the echo time difference (called asym time herein).
The main forms of usage are:
- fugue -i epi -p unwrappedphase -d dwelltoasymratio -s 0.5 -u result
- fieldmap specified by a 4D file unwrappedphase containing two unwrapped
phase images - from different echo times - plus the ratio of
dwell time to echo time difference (asym time)
- fugue -i epi --dwell=dwelltime --loadfmap=fieldmap -u result
- uses a previously calculated fieldmap
Note the option -s 0.5 is an example of how
to specify the regularisation to apply to the fieldmap (2D Gaussian
smoothing of sigma=0.5 in this case which is a reasonable default).
There are many different forms of regularisation available which can
be applied separately or together. These are:
- -s sigma
- 2D Gaussian smoothing
- --smooth3=sigma
- 3D Gaussian smoothing
- -m
- 2D median filtering
- --poly=n
- 3D Polynomial fitting of degree n
- --fourier=n
- 3D Sinusoidal fitting of degree n
Some other uses are:
- fugue -i undistortedimage -p unwrappedphase -d dwelltoasymratio -s 0.5 -w warpedimage
- applies the fieldmap as a forward warp, turning an undistorted image into a distorted one - useful for creating a registration target for the EPI from the undistorted absolute fieldmap image
Additional options that are useful are:
- --mask=maskname
- uses a user-defined mask (called maskname) instead of deriving it from the phasemaps or fieldmaps
- --unwarpdir=dir
- specifies the direction of
the unwarping/warping - i.e. phase-encode direction - with dir being one of
x,y,z,x-,y-,z- (default is y)
- --phaseconj
- uses the phase conjugate correction method, rather than pixel shifts
- --nokspace
- for forward warping (only) - uses an image-space method for forward warping
- --icorr
- applies an intensity correction term
when using the pixel shift method - often poorly conditioned for standard
fieldmap acquisitions
Mark Jenkinson
Copyright © 2001-2006, University of Oxford