Biomedical Engineering Reference
In-Depth Information
FIGURE 10.6
A coronal slice through an MR volume dataset acquired from a patient with a motor disorder to
undergo functional surgery (note motion artifact), together with a registered post-implantation
MR scan (center) and the pre-implantation MR overlaid with high density features (bone
and electrodes) from a post-implantation CT scan (right). The combined pre-implantation
MR and CT avoids the susceptibility artifacts and potential risk to the patient resulting from
post-electrode implantation MR scanning.
with electrodes in place. The electrodes also cause some distortion of the MR
image.
Figure 10.6 shows an example coronal MR slice acquired post-implantation
of depth electrodes alongside a pre-implantation MR image in the same posi-
tion with the outline of the bone and electrodes imaged in post-implantation
CT overlaid.
A difficulty in registration of pre-implantation MR scans to post-implantation
CT scans is that the brain can deform substantially between these proce-
dures,
28
12,29,30
Brain defor-
mation remains poorly understood, but it is likely that deformation will be
greater if a large craniotomy is required (e.g., for insertion of electrode mats
on the surface of the brain), or if large amounts of CSF are lost during a pro-
cedure. When deformation of more than about 1 mm arises, a rigid-body reg-
istration algorithm is insufficient for accurately aligning the MR and CT
images. In this case a nonrigid algorithm could be used. Nonrigid registra-
tion algorithms are described in Chapter 13, but these are primarily used for
intramodality applications; reliable nonrigid registration of images from very
different modalities has not yet been demonstrated.
as it does during other neurosurgical procedures.
10.3.3
Radiotherapy Planning
The recent development of 3D CT-based radiotherapy planning has involved
the use of multiple CT slices to show a tumor in all three dimensions.
31
This
allows “conformal” radiotherapy to be planned, where multiple radiation
beams are used, configured as tightly as possible to the contour of a tumor to
spare adjacent potentially radiosensitive normal tissues from damage. This
technique has been of most use in the head and skull base and to preserve
brain and optic nerves, and has also been applied to the prostate. MR images
would seem ideal for the purposes of planning. The much greater soft tissue
contrast of MR allows better definition of the boundaries of a tumor from
