Biomedical Engineering Reference
In-Depth Information
energy to the targeted tissue despite the presence of coagulum,
temperature control of the ablation electrode is important to
avoid thromboembolic risks (van der Brink et al. 2000). Reviews
of microwave invasive microwave techniques include those by
Rhim (2004) and Ryan et al. (2010).
A common antenna design is based around an insulated
resonant dipole. In this case the choice of frequency becomes a
compromise between energy deposition at increased radial dis-
tance from the antenna and the length of the antenna. Antennas
designed for use at 915 MHz are of a practical length (Trembly
1985). The use of helical antennas provides greater flexibility in
choosing the length of the antenna, and these have been custom-
ized for various applications (Stauffer 1988; Sherar et al. 2001;
Reeves et al. 2005, 2008).
both on the same side of the body part to be treated. The elec-
trodes are often small metal plates mounted in cushion-like
enclosures, but may also be made of a flexible material such
as wire mesh so that they may conform to the shape of the
region of interest. The tissue temperature is raised predomi-
nantly through Joule (
I
2
R
) heating by conduction currents. An
example is shown in Figure 4.8 in which two plate electrodes
are positioned around a plane, layered fat-muscle-fat phan-
tom, offset from the fat layers by a water bolus, although other
means of spacing the electrodes from tissues may be used. This
is necessary to avoid exposing the tissue to the very high elec-
tric field close to the electrodes. Relatively high values of SAR
are restricted to the region immediately below the small elec-
trode; higher values are located in the fat-like layer because
the electric field is essentially perpendicular to the interface
between fat and muscle layers and therefore greater in the low
permittivity fat than the high permittivity muscle in order
to satisfy boundary conditions. The ability to heat at depth is
determined by the size of the small electrode.
Inductive coupling of RF energy to tissues may be achieved
by placing RF current carrying coils near to or around the body
part to be treated. The RF magnetic field produced induces elec-
tric fields and circulating currents in the body tissues, which
give rise to Joule heating. Figure 4.9 illustrates the method. A
three-turn coil is modeled by three concentric circular current-
carrying loops (in this case the radii are 10, 8, and 6 cm) located
4.8 External Heating techniques
4.8.1 Capacitive and Inductive techniques
RF heating techniques can be classified into two groups: those
that use the electric field primarily and the energy is coupled
to the tissues through capacitance, and those that use the
magnetic field primarily to couple energy to the tissue induc-
tively. A basic capacitive method involves applying power to
two electrodes that may be placed either one on each side or
Z
X
0
-8
-16
-24
Z
-32
X
-40
FIGURE 4.8
Example of capacitive plate technique. Top: Two plate electrodes, 5 × 5 cm (upper) and 15 × 15 cm (lower), each placed on a water
bolus 2 cm thick, are placed around a phantom consisting of a muscle-like central region 80 × 80 × 26 cm with fat-like layers (80 × 80 × 1 cm) placed
on the top and bottom surfaces. The electrodes are connected to a current source and tissue properties at 30 MHz are assumed. Bottom: SAR dis-
tribution (in dB relative to the peak SAR) in plane × = 0.
