Biomedical Engineering Reference
In-Depth Information
MRI registration of gated images by transforming to reference images acquired
in the same temporal phase. They employed a surface- fitting iterative tech-
nique developed by Pelizzari et al.
11
20
also based their registration
technique on the heart surface extracted from both PET and MR images.
Other protocols for registering images from head and neck,
Nekolla et al.
21,22
breast, and
23
parts of the abdomen
employ interactive point-based registration tech-
21
niques,
multiple interactive steps supported by reference to the normal
tracer distribution of FDG,
22
or have relied on external and internal land-
23
marks. Wahl et al.
used reconstructed transmission images from the PET
examination (see Chapters 5 and 11) in the registration procedure. Indeed,
Kuhl et al. had already pointed out the usefulness of transmission images for
the interpretation of emission images in 1966.
24
Transmission data are rou-
tinely acquired in order to correct for photon absorption. The absorption
coefficients can be reconstructed into images which resemble low-resolution
x-ray CT images. Despite the relatively low resolution and the reduced
sensitivity of the 511 keV photons to small variations in tissue type and den-
sity, the images show a clear body outline and gross anatomical details, espe-
cially in the thorax. They can provide the physician with enough details to
help him orient the emission images obtained in the same examination, even
when only a few spots with high tracer uptake scattered across the imaged
volume are visible. These transmission images can provide a means to ease
registration of whole body images.
Recently, Pietrzyk et al.
25
used transmission images during 3D rendering
processes when no complementary morphological imaging in an appropriate
orientation was available. A limitation in the combined analysis of PET emis-
sion and MR images is the absence of a body outline on most PET images. This
may be overcome by including transmission images in the analysis. Figure 9.7
shows an example of such an application for images from the head and neck
region, and Color Figure 9.8* shows an example in the thorax.
A further complication in combining PET and MR images is differences
in the shape of the patient bed and positioning. This is not a problem for
brain imaging, but is a major limitation in body imaging, where different
bed shapes can lead to different positioning of the patient's spine. Also,
organs under study do not remain in the same position with respect to each
other once the patient has left the scanning bed or has to be moved to
another imaging modality. This is demonstrated in Figure 9.7 and Color
Figure 9.8. In each of these examples the range covered by the two imaging
modalities is different, that of the MRI being considerably smaller. Also,
as shown in Figure 9.7, the positioning of the neck may be different in the
two devices, leaving the registration with some remaining systematic
uncertainty. It has been recently shown by Theissen et al.
26
that a carefully
designed clinical protocol can lead to successfully registered PET and MR
images even in the case of extracranial images. Special care has been taken
to assure identical positioning of arms in both modalities. This assumes that
* Color Figures follow page 22.
