Biomedical Engineering Reference
In-Depth Information
10.3
Applications
10.3.1
Planning Surgery of the Brain and Skull Base
The combination of MR and CT images of the head can be useful in planning
certain types of neurosurgical and ENT surgical procedures. In particular, the
relationship between the soft tissue contrast provided by MRI and bone
detail provided by CT can be useful where a single modality is insufficient.
One example is planning procedures within the posterior fossa, where CT can
usefully provide information about the most suitable approach but MR is needed
for soft tissue detail, because beam-hardening artifacts in the CT lead to streaks
that can degrade the quality of soft tissue information. Another example appli-
cation is lesions that involve bone, such as cysts in the petrous apex and glomus
jugulare tumors, where soft tissue detail and bone contrast are both desir-
able.
8,26,27
The clinical motivation of registration is to provide the surgeon with
an improved understanding of the relationship among the lesion, adjacent crit-
ical structures, and possible surgical approaches. This can result in better posi-
tioning of craniotomies, reduced craniotomy size, quicker operations with less
time under anesthetic, and, consequently, improved patient outcomes.
Figures 10.4 and 10.5 show example combined MR and CT images used in
planning resection of skull-base tumors. The benefits of combining multi-
modality information can be even greater for guiding surgical procedures, as
discussed in Chapter 12.
10.3.2
Localizing Electrodes in the Brain
Functional neurosurgical procedures include implantation of mat electrodes
over the surface of the brain or depth electrodes into brain parenchyma to
localize an epileptogenic region or focus by subsequent neurophysiological
recording in patients with intractable epilepsy. This is done to plan surgical
resection of the epileptogenic area of the brain. Good anatomical localization
of the electrode shown to be closest to the focus is crucial, as this will deter-
mine the success of the operation. Another type of functional neurosurgical
procedure is the implantation of electrodes into the subthalamic nucleus (a
small structure) in patients with Parkinson's disease, to alleviate tremor.
Assessment of success of the procedure is dependent on knowing that the
electrodes are satisfactorily positioned. The localization of electrodes can be
achieved using CT scans, but the soft tissue contrast is relatively poor, and
streak artifact from the electrodes degrades the images. While MR can be
obtained with the electrodes in place, there is a potential risk from electrical
current generation from changing magnetic gradients and heating from the
use of electrical conductors while applying radiofrequency pulses with the
MR scanner. For many patients, the risk of ionizing radiation from a CT scan
may be more acceptable than the less well understood risk of an MR scan
