Biomedical Engineering Reference
In-Depth Information
FIGURE 5.2
Illustration of image distortions in EPI: (a) conventional spin-echo; (b) single-shot EPI;
(c) eight-shot EPI. TR
80 msec. Both signal loss (dephasing) and geometric
distortions can be seen in the EPI. The distortions are drastically reduced in multi-shot vs.
single-shot EPI due to shorter echo train and resulting reduced accumulated phase error
(images taken from
2600 msec, TE
http://www.nmr.ion.ucl.ac.uk/
~
marks/epi/shots.html
with permission).
5.3.3.1
Correction
Correction schemes can be categorized into two groups: double acquisition
with inverted gradients and phase unwarping based on field maps. Bakker
et al.
14
29
described methods based on two acquisi-
tions identical except for inverted gradients. This approach requires the iden-
tification of corresponding points in the two images. Using this approach, a
reduction in the stereotactic localization error from 3.8 to 2 mm was obtained.
Moerland et al. found a similar improvement using Bakker's method.
and Chang and Fitzpatrick
25
Maurer
5
29
et al.
measured the effect of applying the method by Chang and Fitzpatrick
on the accuracy of CT-MRI registration and found an improvement in the
range of 30 to 40%.
Sekihara et al. were the first to propose field mapping to correct for static
field inhomogeneity.
13
Sumanaweera et al.'s approach is based on a field map
(requiring two volume acquisitions) followed by the application of a phase
unwarping algorithm to correct for positional errors throughout the imaged
volume
15
In a later development, Sumanaweera et al. extended their
technique to remove the need for connectivity, which makes it applicable to
images containing stereotactic frames.
in vivo
.
19
This method results in an improve-
ment of localization accuracy from 3.7 to 1.1 mm. The field mapping approach
