Page 78 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

HEAT TRANSFER APPLICATIONS IN BIOLOGICAL SYSTEMS 55

High energy DC shock (Scheinman et al., 1982) has been used as an implanted energy source for
the treatment of drug-refractory supraventricular arrhythmias. During the catheter ablation, the peak
voltage and current measured at the electrode-tissue interface are typically higher than 1000 V and
40 A, respectively. The high voltage pulse results in a very high temperature at the electrode surface.
Explosive gas formation and a shock wave can occur, which may cause serious complications,
including ventricular fibrillation, cardiogenic shock, and cardiac peroration.
Alternating current in the radio frequency range has been investigated as an alternative to shock
for heating applicator (Huang et al., 1987; Nath et al., 1994; Nath and Haines, 1995; Wonnell et al.,
1992; Zhu and Xu, 1999). Radio frequency ablation has been successfully used to treat liver neo-
plasms, solid renal mass, and osteoid osteomas. In recent years, this technique has been applied to
destroy brain tissue for the treatment of motor dysfunctions in advanced Parkinson’s disease
(Kopyov et al., 1997; Linhares and Tasker, 2000; Mercello et al., 1999; Oh et al., 2001; Patel et al.,
2003; Su et al., 2002). The current clinical practice of inducing RF lesions in the brain involves
implanting a microelectrode-guided electrode and applying RF current to the targeted region, in
order to relieve symptoms of the Parkinson’s disease in patients whose symptoms cannot be con-
trolled with traditional pharmacological treatment. RF energy is readily controllable, and the equip-
ment is relatively cheap (Hariharan et al., 2007a). The standard RF generator used in catheter
ablation produces an unmodulated sinusoidal wave alternating current at a frequency of 200 to 1000
kHz. Two electrodes are needed to attach to the tissue and a current is induced between them. The
2
passage of current through the tissue results in resistive or ohmic heating (I R losses). Resistive cur-
rent density is inversely proportional to the square of the distance from the electrode. Thus, resistive
heating decreases with the distance from the electrode to the fourth power. Maximal power occurs
within a very narrow rim of tissue surrounding the electrodes. The heating typically leads to desic-
cation of tissue immediately surrounding the catheter electrodes, but diverges and decreases in-
between, which can cause broad variations of heating. Improved RF hyperthermia systems have been
proposed to reduce the heterogeneity of the RF heating, including implanting a feedback power cur-
rent system (Astrahan and Norman, 1982; Hartov et al., 1994) and using electrically insulating mate-
rial around the electrodes (Cosset et al., 1986).
Microwave hyperthermia uses radiative heating produced by high-frequency power. High-
frequency electromagnetic waves may be transmitted down an appropriately tuned coaxial cable and
then radiated into the surrounding medium by a small antenna. The mechanism of electromagnetic
heating from a microwave source is dielectric rather than ohmic. The heating is due to a propagating
electromagnetic wave that raises the energy of the dielectric molecules through which the field passes
by both conduction and displacement currents. While maintaining alignment with the alternating elec-
tric field, neighboring energized dipole molecules collide with each other and the electromagnetic
energy is transformed into thermal energy. The main limitation of microwave heating is that the energy
is absorbed within a very narrow region around the microwave antenna. Typically, the generated heat
2
decays fast and can be approximated as proportional to 1/r . The highly absorptive nature of the water
content of human tissue has limited the penetration of electromagnetic energy to 1 to 2 em.
Laser photocoagulation is a form of minimally invasive thermotherapy in which laser energy is
deposited into a target tissue volume through one or more implanted optical fibers. Laser is used in
medicine for incision and explosive ablation of tumors and other tissues, and for blood vessel coagu-
lation in various tissues. Laser light is nearly monochromatic. Most popular lasers utilized in the lab-
oratory include argon laser (488 nm), pulsed dry laser (585 to 595 nm), Nd:YAG lasers operating at
1064 nm, and diode lasers operating at 805 nm. Laser-beam power ranges from milliwatts to several
watts. Usually the laser energy is focused on a small tissue area of a radius less than 300 μm, resulting
in a very high heat flux. Because there is minimal penetration of laser energy into the tissue, suffi-
cient energy is delivered to heat tissues surrounding the point of laser contact to beyond 60°C or
higher, leading to denaturation and coagulation of biomolecules. Because of the high temperature
elevation in the target tissue, laser photocoagulation may produce vapor, smoke, browning, and char.
A char is usually formed when temperature is elevated above 225°C or higher (LeCarpentier et al.,
1989; Thomsen, 1991; Torres et al., 1990; Whelan and Wyman, 1999).
Laser ablation has been used primarily in two clinical applications, one is dermatology and the
other is ophthalmology. Laser treatment for port wine stain with cryogen spray cooling has been