Biomedical Engineering Reference
In-Depth Information
Figure 12.2:
Sequential spatial filtering. (A) results of a 10
×
10 pixel mask flat
blur to suppress the local detail in the original image (D). (B) 5
×
5 mask. (C)
2
×
2 pixel mask, and (D) original image.
the template intensities changes according to the computed deformation field.
Therefore, all image operations on the template during the registration process
(including spatial filtering techniques) must be performed on the deformed tem-
plate image, rather than the static template image before deformation. Since,
in most cases, the template finite element mesh nodes are not co-located with
the template image voxels, the computed deformation field must be interpo-
lated onto the original template image in order to apply the image operations
accurately.
12.2.8
Regular Versus Irregular Meshes
Hyperelastic Warping accommodates an FE mesh that corresponds to all or part
of the template image. A “regular mesh” is a rectilinear structured mesh that
corresponds to the entire image domain. This mesh may be a subsampling of
the actual image voxel boundaries. An “irregular” mesh conforms to a particular
structure of interest in the template image. The template intensity field
T
is
interpolated to the nodes of the FE mesh. As the FE mesh deforms, the values
of the target intensity field
S
are sampled at the current location of the nodes of
the template FE mesh.
Regular meshes are used primarily for nonphysical deformable image prob-
lems (Fig. 12.3). Regular meshes are simple to construct and can easily span
the entire image space or a specific region of interest. However, since the mesh
does not conform to any structure in the template imaged, these analyses are
susceptible to element inversion prior to the completion of image registration.
Typically, only a single material type is used for the entire mesh.
