Biomedical Engineering Reference
In-Depth Information
FIGURE 11.4
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MRI (left), [
Tc]-DMSA(V) (right) coregistration. The SPECT
studies were aligned to the MRI scan by using fiducial markers, which were visible on both
the MRI and SPECT studies. The fiducial markers are not visible in the MR slice shown, as
they lie in a different plane.
In]-Octreotide (center), and [
scaling, in some cases, is all that is usually required. Two examples of inter-
modality automated registrations are shown in Color Figure 11.3*.
Figure 11.4 shows an example of an MRI scan of the head and neck coreg-
istered with SPECT reconstructions of the uptake of [ ]-octreotide,
which maps somatostatin receptors on certain types of cancer cells, and
[ ]-DMSA(V) which can demonstrate calcification in medullary
tumors. The registration problem in this case is not a trivial one, as the recon-
structed volumes are vastly different in nature. The registration was done
with a point-landmark method using fiducial markers visible on both the
SPECT and MRI scans. In the absence of the MRI, it would be difficult to
interpret precisely where the uptake is localized on the SPECT scan. The MRI
scan, of course, gives no indication of the functional status of the receptors
expressed by this cancer. An alternative method for registration would be to
use the transmission scans acquired simultaneously with the SPECT scans,
automatically coregister the reconstructed anatomical scans to the MRI to
derive the transformation matrix, and apply these transformations to the
SPECT emission data.
111
In
99m
Tc
19
Coregistration between PET or SPECT and anatomical modalities outside
the brain has been less accurate. This is mainly due to the range of different
conformations the body can assume on a scanning bed, as well as internal
movement of organs within the chest and abdomen. For example, x-ray CT
scans of the chest are usually taken with a breath-holding maneuver. This
raises the diaphragm and affects not only the thoracic contents but abdominal
contents as well. By contrast, nuclear medicine scans are usually acquired over
minutes or tens of minutes, and therefore the patient will normally breath tid-
ally. This tends to blur the organ boundaries and will cause an intrinsic misreg-
istration internally even if the outside boundary of the patient were exactly
* Color Figures follow page 22.
