Biomedical Engineering Reference
In-Depth Information
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FIGURE 4.7 Invasive needle electrodes. Upper: Two electrodes, diameter 1 mm, are implanted 5 cm into a muscle-like phantom with center-
to-center spacing of 10 mm. An ELF source is used to apply a voltage between the electrodes. Lower left: SAR distribution in the plane y = 0 that
intersects the electrodes (in dB relative to the peak SAR). Lower right: SAR distribution in the z plane at a depth of 26 mm into the phantom. Both
SAR distributions are shown between 0 and -30 dB.
Blouin and Marcus 1989). Attempts at producing larger lesions
have included the use of larger electrodes (Tsai et al. 1999) and
saline-irrigated and cooled electrode tips (Demazumder et al.
2 0 01).
Another common application of RFA is in the treatment of
liver tumors. Patients with colorectal liver metastases often pres-
ent with unresectable disease, and RFA is among several mini-
mally invasive treatments under investigation as an alternative
or complementary tool in the management of some of these
patients. RFA can be delivered percutaneously, laparoscopically,
or through open surgical approaches. Typically, Joule heating
caused by a 450-500 kHz current results in coagulative necro-
sis at a temperature between 50°C and 100°C. RFA procedures
involve the insertion of a needle electrode with a noninsulated
tip and an insulated needle shaft into the tumor. The method can
be either monopolar or bipolar. The dimensions of the volume
of ablated tissue are determined largely by the magnitude of the
RF current, the length and diameter the electrode tip, and the
time for which RF energy is applied (Goldberg et al. 1995). In the
event that the tissue temperature at the electrode becomes exces-
sive, carbonization occurs around the electrode, which results
in a sharp rise in tissue impedance and interruption of the RF
current. This effectively limits the volume of tissue that can be
ablated. To avoid this, RFA systems incorporate techniques that
monitor and control the temperature at the electrode and/or
monitor the impedance, keeping it below an acceptable value.
Volumes up to approximately 20 mm in diameter can be ablated
with a conventional single needle electrode, but ablation of larger
tissue volumes is possible with recent technical improvements
(Goldberg and Gazelle 2001). A cooled-tip electrode avoids char-
ring of tissue immediately around the electrode by cooling the
internal chamber of the needle via cold saline infusion, thus
allowing the use of a higher power than the conventional needle.
Other devices capable of ablating larger volumes use multiple-
prong (clustered) electrodes or an expandable electrode with
multiple retractable J hooks to create overlapping ablation fields
(Livraghi et al. 2000). Pulsing of the RF current and the use of
saline are other ways of increasing the ablated volume. Technical
aspects of the various approaches are described in Haemmerich
et al. (2001, 2002).
4.7.3 Invasive Microwave techniques
The use of either a small implantable microwave antenna or
an array of such devices has the potential for producing larger
lesions than is the case with RF electrodes, although the vol-
ume remains small for a single antenna. Antenna designs have
included monopoles (Nevels et al. 1998), helical structures
(Wonnell et al. 1992), and other structures (Lin 1999, Shetty
et al . 1996, Liem et al. 1996). Since microwave antennas deliver
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