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Encyclopedia of Physical Science and Technology EN007O-865 July 6, 2001 17:0

Image-Guided Surgery 589

lesions treated include intracranial hemorrhages, cysts, as et al., 1986; Patel et al., 1998; Ishihara et al., 1995; Kuroda
well as malignant and benign brain tumors, cavernous et al., 1997; Stollberger et al., 1998; Bertsch et al., 1998).
hemangiomas, and arteriovenous malformations (Black Thermal ablations are effective, minimally invasive meth-
et al., 1997; Schwartz et al., 1999). During open surgery ods for tumor treatment if appropriate guidance of the
the surgeon cannot see beyond the visible surface, so it is thermal deposition is achieved. For a successful thermal
very helpful to use intraoperative volumetric imaging to therapy the tumor should be localized and targeted and
depicttheentireoperationalvolumeduringtheintracranial heated or cooled. The treatment is unsuccessful unless
surgery. Today the major benefit of intraoperative imag- the right temperature range is achieved and maintained
ing is to control tumor resections and to reduce the pos- for an appropriate time period. In addition, the damage
sibility of residual tumor. More precise definition of tar- to the adjacent normal tissue must be minimized. These
get tissue, as well as functional and structural areas to be goals cannot be accomplished without image guidance. If
avoided, with functional MRI and diffusion-weighted im- the thermal treatment can be combined with temperature
ages and diffusion tensor representation of the white mat- monitoring, the full potential of thermal therapy can be
ter tracts will continue to improve IMRI. Improvements demonstrated. The development of temperature-sensitive
to MR imaging will include faster acquisition times and MRI techniques which improve the efficacy of various
refinement of imaging sequences for neurosurgical guid- thermal treatments can help in the resurgence of thermal
ance such as continuous imaging (Kacher et al., 2000). ablations. The field is now best characterized as being in
Most importantly, in the future, intraoperative guidance an early stage of development and mostly involves the
may result in major changes in operative approaches and testing of feasibility.
in introducing novel surgical techniques. MRI’s high sensitivity for localizing tumor margins
In neurosurgery the introduction of real-time image and the surrounding anatomic structures can be used for
updates has improved localization and targeting and the targeting. The multiplanar capability helps in trajectory
completeness of tumor resections. The same fundamen- optimization and in correct targeting by various probes.
tal approach can also be used in other surgical fields. Multiple temperature-sensitive MRI parameters (T1, dif-
MRI provides high-sensitivity identification of the mar- fusion, and chemical shift) are relevant for thermal map-
gins of breast cancer. The approach that was refined for ping and monitoring. MRI can demonstrate thermally in-
brain tumor detection and removal can be directly applied duced changes in diffusion and perfusion and characterize
for lumpectomy. One of the major challenges in perform- tissue injury.
ing lumpectomies is the intraoperative detection of tumor The biological mechanisms of heat-mediated tissue
boundaries. Because the breast is less rigid than the brain, damage are well known, but the entire spatial extent of the
the use of preoperative image data is even more restric- tissue damage can only be demonstrated using volumetric
tive than in the case of brain. In lumpectomy it is highly imaging. This requires not only accurate target definition,
desirable to use intraoperative image updates to identify but also sensitive monitoring. Temperature sensitive MRI
margins and to recognize residual tumor. monitoring can be used to control the deposition of ther-
mal energy and can detect potential energy spread to the
surrounding normal tissue. This way a thermal ablative
V. THERMAL ABLATIONS treatment can be highly effective and safe.
Imaging is an important, but it is not the only factor
Originally, image guidance was limited to the deployment of image guidance. The integration of imaging and ther-
of various probes through which the biopsy and/or the apy is requisite for the control of interventional manip-
treatment was accomplished (targeting). Although cor- ulations and for optimal energy delivery during thermal
rect targeting is very important, imaging can continue dur- ablations. This control can be accomplished only if the
ing the procedure (monitoring). The resurgence of local characteristics of the imaging systems and the features
tissue-killing techniques using various forms of energy of the therapy devices are matched and their functional
(chemical, thermal) is justified by increasing capabilities properties are coordinated. Both the temporal and spatial
of monitoring and control by advanced image-guidance resolution of imaging have to satisfy the requirements for
methods (US, CT, MRI). the particular intervention. The time constants of thermal
The unique potential of MRI to detect temperature diffusion or the identification of the tip of thermal en-
changes initiated and inspired the evolution of interven- ergy delivery probe (optical fiber, radiofrequency needle,
tional MRI from simple MRI-guided biopsy method to a etc.) are equally important to perform a safe and efficient
sophisticated tool to monitor or potentially control thermal image-guided therapy. These factors all should be seri-
ablations (Silverman et al., 1995; Morrison et al., 1998; ously considered before an image-guided therapy proce-
Matsumoto et al., 1994; Young et al., 1994; Dickinson dure has been conceived, developed, and implemented.