Biomedical Engineering Reference
In-Depth Information
uptake pattern are used, or when the MR pulse sequence is altered. Hence, the
similarity measure has to be very robust but still sophisticated enough to cope
with this problem, i.e., detect similarity without making strict assumptions
about the intensity distribution in the two modalities. Other approaches
explore geometrical features, such as outlines of surfaces of the head or the
brain, and minimize the distance between contours extracted from the two
types of image.
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To do this, the images should allow a precise definition of sur-
faces; this may not be possible for certain types of images which do not allow
a clear structural definition.
The alternative subgroup encompasses interactive techniques which incor-
porate the selection of multiple internal landmarks, the interactive definition of
contours, or a combination of both.
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Similar prerequisites regarding image ori-
entation, image quality, etc. apply for interactive techniques as for automated
registration methods. Landmark-based techniques assume the ability to define
intrinsic landmarks in both image sets to be registered. Again, the situation
tends to be more difficult for PET images with tracers other than FDG. As
shown in Figure 9.2, images of these types pose a challenge to the operator,
with the task of unambiguously identifying landmarks. When suitable features
can be identified, techniques utilizing overlay and exchange of contours have
been shown to be very robust for detecting even subtle misregistrations.
During interactive registration the operator will use visual criteria like matching
of contours around morphological structures. Multiple simultaneously con-
trolled cursors can be applied in three dimensions to check proper alignment
of isolated intrinsic landmarks. Color Figure 9.5* shows an example of a typical
display used during interactive registration. Figure 9.5a displays the initial
state with unregistered studies and Color Figure 9.5b shows the result after
registration has been completed. It also provides an impression of what can
be achieved with image registration and what forms the basis for further
image analysis.
The advantage of interactive techniques is that they are widely applicable in
various situations combining imaging modalities of different types and with
arbitrary combinations. Also, they are not restricted to brain images, but can
be applied to whole-body studies as well. Hybrid procedures are also available
which draw on the strengths of both automated and interactive techniques.
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9.5
Discussion of Applications in a Clinical Setting
9.5.1
Introductory Remarks; Definitions
Image registration of PET and MRI is feasible and provides important additional
information often not available from a single modality alone. To date, most
applications have taken place under research protocols. There is, however, also
* Color Figures follow page 22.
