Biomedical Engineering Reference
In-Depth Information
FIGURE 10.3
In this multislice spin-echo T1-weighted MR image, slices in the axial plane of acquisition
are motion free (left), but motion during the study has resulted in jagged boundaries of
tissues in the reformatted coronal slice (right). Motion of this type might not reduce the
diagnostic value of the MR scans but can introduce unacceptably large errors into registered
volumes. It is advisable always to view reformatted MR and CT images prior to registration
to check for the presence of motion artifacts of this type.
For example, by reformatting axial images in the coronal or sagittal plane,
motion during the scan can be easily identified. Figure 10.3 shows an example
of MR image with motion between the interleaves.
Motion during a 3D volume MR acquisition will result in a ghost artifact
throughout the images which will be spread out in the phase-encoded directions
of the images. This is likely to be clear from inspection of any slice from the
volume. Ghost artifacts in 3D acquisitions can make it hard to accurately delin-
eate points or surfaces in the images, if these are needed for registration.
For imaging the head, the sort of motion that causes problems is normally
a nodding or rolling of the head, which causes a rotation. In the abdomen or
chest, breathing during the scan can cause similar problems.
For images that are to be registered, it is important to check for signs of
motion as part of the routine quality assurance process; if this is not done,
clinically significant errors may arise.
10.2.1.5
Data Transfer
An essential step prior to registration of MR and CT images is the transfer of
both images onto the same computer. With increased use of picture archiving
and communication systems (PACS), image transfer is becoming more
straightforward. There are, however, many hospitals where the relevant
scanners are not all networked, or compatible file formats or network proto-
cols are not used. In these cases, the logistics of data transfer can be compli-
cated. It may be necessary to transfer images from one computer to another
using tape or removable disks, and dedicated software may be needed to
convert the data to compatible formats prior to registration. This process can
