Biomedical Engineering Reference
In-Depth Information
FIGURE 8.3
Time series plots of stimulation model (dotted line, arbitrary units) against estimated rota-
tion about the
x
axis (solid line, arbitrary units).
8.2.4
Nonrigid Motion Correction
Of course, head motion does not just occur between fMRI (multi-slice) images
but also during their production, resulting in distortion of the ima-ges, as dis-
cussed in Chapter 5. Thus rigid-body motion correction is imperfect,
although it is widely accepted, since the combination of the low resolution of
the images and the relative speed of their capture makes the nonrigid com-
ponent small. Some initial work has been undertaken
8
where each slice is reg-
istered separately to the target, but such approaches are not widely used at
present. These nonrigid correction schemes are likely to become more viable
as the resolution of fMRI images increases in the future.
A secondary point to note here is that if non-rigid-body motion correction
is to be applied, then it is desirable to integrate the slice-timing correction
(normally applied before motion correction) into this procedure. Thus, rigid-
body motion, nonrigid distortions, and slice-timing effects could all be
corrected at the same time.
8.3
Geometric Distortion
The acquisition of MR images is imperfect for a number of physical reasons. In
particular, the magnetic field inside the head is usually inhomogeneous, even
after careful shimming, and this results in geometric distortion of the image.
The magnitude of distortion depends upon the parameters of the imaging
