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

590 Image-Guided Surgery

A. Interstitial Laser Therapy The feasibility of MRI-guided FUS to monitor the
therapy has been demonstrated (Cline et al., 1994; Chung
Interstitial laser therapy (ILT) is a minimally invasive ab-
et al., 1996; Hynynen et al., 1996, 1997, n.d.). MRI rep-
lative procedure designed for tumor coagulation. ILT uses
resents major advantages over other imaging techniques
a laser as a heating source and applies near-infrared emis-
for targeting, monitoring, and controlling ultrasound
sion (such as via a neodymium–yttrium–aluminum garnet,
exposures. MRI’s excellent tissue characterization can be
Nd:YAG) laser to deliver energy directly to tissue through
exploited for localization and accurate targeting. Tempe-
optical fibers. The distribution of energy depends on both
rature-sensitive MRI sequences can be used to monitor
the optical and thermal properties of the tissue (scattering,
temperature changes and to detect irreversible tissue
absorption, thermal conductivity, and perfusion). Primary
damage (Young et al., 1994; Dickinson et al., 1986; Patel
optical absorption and subsequent thermal conduction re-
et al., 1998; Ishihara et al., 1995; Kuroda et al., 1997;
sult in irreversible tissue coagulation. This coagulation
Stollberger et al., 1998; Bertsch et al., 1998). The location
◦
occurs at and above 60 C.
of the focus can be depicted at low power levels to verify
MRI is well suited for monitoring ILT (Jolesz et al.,
accurate targeting. The tissue changes induced by the
1988). The optical fibers generally have small diameters
sonications can be detected using T1- and T2-weighted
and can pass through thin needles. Therefore, laser treat-
MR images. The occlusion of the microvasculature can
ment is convenient and adaptable for converting a biopsy
be detected by the lack of MRI contrast agent uptake.
procedure into a treatment session. The optical fibers and
The temperature history of the treated tissue volumes can
the light itself are fully compatible with MRI, which can
be used to calculate the biological effect or thermal dose
provide fast and relatively accurate temperature-sensitive
induced by the exposure. In addition, the imaging an be
images with appropriate temporal resolution. Soon after
used to monitor normal tissue temperatures for safety.
the original suggestion of MRI-guided ILT, clinical appli-
The feasibility of using a single, focused ultrasound
cations for brain tumor treatment were initiated (Kahn
transducer guided by MRI has been demonstrated in clin-
et al., 1998; Bettag et al., 1991; Ascher et al., 1991;
ical treatments of fibroadenomas of the breast (Fig. 8)
Schwabe et al., 1997). The treatments were mostly for
(Hynynen et al., n.d.). Phased array ultrasound transducer
malignant gliomas and brain metastases.
systemscanincreasethefocalvolumeandreducethetreat-
Preoperative localization can be complemented by elec-
ment time. The utilization of the phased arrays allow one
trophysiological methods and by fast MRI. Monitoring
to make the thermal exposure distribution uniform and use
can be accomplished by T1-weighted, or phase-sensitive,
the minimum amount of power to reduce the total treat-
MRI. Control by on-line monitoring makes ILT particu-
ment time. The results so far indicate that one can aim
larly suitable for brain tumors that are located in areas of
the ultrasound beam into the tumor accurately through
functional relevance. Soon after the introduction of MRI-
the breast tissue and that the temperature can be elevated
guided ILT in neurosurgery, other clinical applications
enough to coagulate the tumor tissue. These treatments
were tested. MRI-guided thermal therapy has become an
have shown that MRI can detect temperature elevation
accepted, minimally invasive treatment option for liver
during the sonication, and thus the basic concept of MRI-
tumors, breast cancer, and head and neck malignancies
monitored ultrasound surgery is valid (Fig. 8).
and more recently for the treatment of uterine fibroids. It
is a relatively simple and straightforward method which VI. CRYOABLATION
can be well adapted to the MRI environment (Kettenbach
et al., 1998; Vogl et al., 1997; Kahn et al., 1998).
The MRI signal from frozen water is minimal or ab-
sent. This lack of signal has been exploited for control
of cryosurgery by monitoring the signal void of the evolv-
B. Focused Ultrasound Surgery
ing ice ball with standard fast MRI sequences (Silverman
One of the most attractive approaches for thermal ablation et al., n.d.). Using special an MRI-adapted cryosurgery
is based on the use of focused ultrasound (FUS). Ultra- unit, percutaneous treatment of soft tissue tumors (liver,
sound penetrates through soft tissues and can be focused breast, kidney, muscle, and bone) is possible (Fig. 9).
to a few millimeters. The acoustic energy is absorbed and The MRI-guided percutaneous cryotherapy approach is
causes temperature elevation with a relatively narrow ther- feasible because monitoring of the developing iceball in
mal gradient. It is possible to achieve sharply demarcated multiple planes is possible. During the freezing process,
target volumes without damaging the adjacent normal tis- dynamic MRI demonstrates the slow expansion of the ice-
sues. Similar well-controlled focusing of thermal energy ball, while thawing images confirm the receding effect.
cannot be achieved by other heating methods especially Permanent tissue changes have been clearly identified and
without an invasive probe. can be followed by serial MRI (Silverman et al., n.d.).