Biomedical Engineering Reference
In-Depth Information
interventions in the spine, pelvis, or head, or in endoscopic or microscopic
surgery. More details of potential applications are provided in Chapter 12.
The two main classes of methods for 2D-3D registration are feature-based
and direct intensity-based. In feature-based methods of x-ray image align-
ment, silhouettes of bony structures are delineated in the x-ray image, and
the algorithm aligns the projections of these silhouettes with the surface of
the same structure delineated from a CT (or MR) volume. One algorithm
makes use of geometric properties of tangent lines of projected silhouettes
and tangent planes of 3D surfaces.
32
This type of algorithm is fast, but is
highly dependent on the integrity of the segmentation in both images.
An alternative method is to match the pixel and voxel intensities directly.
The method is based on digitally reconstructed radiographs (DRRs), first pro-
posed for stereotactic neurosurgical applications.
18
DRRs are computed by
integrating (summing intensities) along rays through the CT volume to sim-
ulate the process of perspective projection in x-ray imaging. New DRRs can
readily be calculated for trial poses and then compared with the true x-ray
image using an appropriate measure of similarity.
33,34
The confounding effect
of soft tissue movement is minimized by removing the soft tissue from the CT
image by intensity thresholding.
Video images may be used to register visible surfaces to MR or CT vol-
umes. Again there are two methods for establishing registration, those based
on matching a reconstructed surface and those that are intensity based. In the
first, a pattern of light (lines, random dots, etc.) is projected onto the visible
surface. Correspondence of the pattern in the two calibrated video images is
used to reconstruct the surface in 3D. This surface is then registered to the
corresponding MR or CT derived surface using an appropriate surface regis-
tration method such as the iterative closest point algorithm. Recently, alter-
native methods have been proposed which do not rely on the initial step of
reconstructing a surface. In these methods the voxel intensities are used
directly to match to the 3D surface derived from MR or CT. Viola has pro-
posed using the mutual information between optical image intensities and
directions of the MR or CT surface normals.
35
An alternative approach uses
the observation that intensities of a given point on a surface appear very sim-
ilar or “photoconsistent” when viewed under the same lighting conditions
with two or more cameras.
36
Trial registrations are iteratively tested until an
appropriate photoconsistency criterion is satisfied.
2.5
Nonrigid Registration Algorithms
As already stated, the two main application areas where nonrigid registra-
tion algorithms are required are when establishing correspondence between
an image of one individual and atlas, computer model, or image of another
individual (intersubject registration), and when establishing correspondence
between images of tissues that have deformed, shrunk, or grown over time
