Biomedical Engineering Reference
In-Depth Information
FIGURE 10.2
Reformatted sagittal view through a CT volume before (left) and after (right) correction of
a 22° gantry tilt. Any CT gantry tilt must be corrected either before or during the registration
process, or substantial errors result.
If the patient changes position during the acquisition, then for all slices
acquired before the motion the subject is in one position, and for all slices
acquired after the motion the subject is in a second position. There are, there-
fore, two registration transformations between this CT acquisition and an
MR acquisition: one for each patient position. A registration algorithm
might find either transformation or some average transformation, depend-
ing on the way the algorithm works. If the patient moves multiple times dur-
ing an acquisition, this problem gets worse. In general, patient motion
during a CT acquisition will result in reduced registration accuracy; depend-
ing on the amount of movement, this misregistration could be a centimeter
or more.
Motion during MR acquisition is more complicated. If the acquisition is 2D
multislice, like most spin-echo acquisitions, it is normal for the scanner to
acquire several slices essentially simultaneously. When the desired number of
slices exceeds the number the scanner can collect simultaneously, multiple sets
of interleaves are acquired. Patient motion during an MR scan of this type can
result in one interleave being transformed with respect to the other. As in the
CT case, this will result in degraded registration accuracy, for example by
calculating a transformation that is some sort of average of the required
transformation for the patient in the two positions.
Motion during a CT acquisition or multislice MR acquisition can quite
easily be identified by reformatting the images in a perpendicular plane.
