Biomedical Engineering Reference
In-Depth Information
1996, Hurter 1991, Ahn 2005). These antennas are highly effi-
cient, coupling over 95% of input power into the tissue, with
respectable broadside radiation patterns. However, they are rela-
tively narrowband, and backwards radiation along the antenna
shaft can lead to undesirably long ablation zones. Tip loading
can be used to increase the electrical length of an antenna or
alter its heating pattern.
Coaxial slot antennas radiate from one or more annular slots
(Lin 1987, Ito 1990, Saito 2004). Since energy radiates from a
smaller aperture than linear element designs, they may be desir-
able for ablation of small volumes. However, single-slot antennas
also suffer from excessive backward heating along the antenna
shaft without design modification. Multiple-slot antennas show
promise for reducing this backwards heating (Saito 2004). Coaxial
chokes have also been proposed to reduce unwanted backwards
radiation by enforcing an open boundary at the antenna feed
point, thereby eliminating current on the outer surface of the
coaxial cable (Bertram 2006, Longo 2003, Lin 1996, Wong 1993).
However, chokes add to the total antenna diameter, making them
less attractive for use in percutaneous applications.
Looped or helical designs are less common in practice, but
have been described in the literature (Gu 1999, Shock 2004, Liu
1996). Notably, looped designs operating in the axial mode have
been described for cardiac ablation, while catheter-based helices
have been proposed for microwave-assisted angioplasty. Single
and multi-loop designs have also been tested for microwave
ablation in the liver. Deployable loop designs are more problem-
atic to use in practice since they are more difficult to visualize
completely on ultrasound during guidance to the target and can
be difficult to retract at the end of the procedure.
Handle
Shaft
Radiator
FIGURE 9.13
Cartoon schematic of a typical microwave ablation
antenna. Coaxial cable runs the length of the shaft, with the radiating
element at the distal end of the antenna. Energy is produced around
the radiating element.
antenna consists of a rigid shaft and a radiating section at the
distal aspect (Figure 9.13). In catheter-based designs, the term
antenna
usually refers only to the distal radiating element,
rather than the entire catheter.
Antenna properties relevant to thermal ablation include both
the pattern of radiation and reflection coefficient, or return loss.
In general, the lowest return loss is desirable to maximize energy
transfer from the antenna into the tissue. Energy reflected from
the antenna reduces tissue heating, while increasing unwanted
heating of the antenna shaft. In extreme cases, high return loss
may necessitate short ablation times to prevent thermal damage
along the antenna shaft (Sato 1996). The desired radiation pat-
tern is largely dependent on the clinical application. Most anten-
nas in use currently radiate in the normal (broadside) mode,
with propagation directed radially outward from the antenna.
This is especially true of antennas designed for tumor ablation
applications, where the ideal radiation pattern is focused and
omnidirectional to match the approximately spherical shape of
many tumors. Antennas designed in the axial (end-fire) mode
have been developed for cardiac applications to produce local-
ized heating of a spot at the distal tip of a catheter (Gu 1999).
To achieve the goals of low return loss and focused energy
radiation, several designs have been proposed (Figure 9.14).
Broadly, these can be classified as designs that utilize a linear ele-
ment, coaxial slot, loop, or helix as their primary mode of radia-
tion. Designs primarily comprised of a linear element include
monopoles, dipoles, and triaxial antennas (Brace 2005, Labonte
9.5.2.1 antenna Cooling
Antenna cooling is an effective solution to the problem of
excessive internal heating in the antenna shaft (Figure 9.15).
Water and gas-cooled antenna designs have all been described
in the literature, and most current clinical systems use some
type of cooling along the antenna shaft (Kuang 2007, Yeh 1994,
Knavel 2010). Sufficiently cooling the antenna shaft prevents
unwanted thermal damage that may be produced by heat con-
duction from inside the antenna shaft, heat conduction from
the hot ablation zone along the antenna shaft, or backward
heating from the radiating segment. By eliminating excessive
heat produced by these three sources, higher powers can be
passed through even very small-diameter antennas. In some
implementations, cryogenic gas cooling can also be used to
create a small ice ball at the distal tip of the applicator to pre-
vent applicator migration during placement or pre-procedural
imaging (Knavel 2010).
Slot
Monopole
Dipole
Triaxial
Choked slot
9.5.3 Multiple-antenna arrays
9.5.3.1 Limitations to Single-applicator ablation
Despite technical advances and optimization, power delivery
from a single antenna is inherently limited. First, consider
FIGURE 9.14
Schematic cross sections of five antenna designs for
microwave ablation. From top to bottom: slot, monopole, dipole, triax-
ial, and choked slot. Metallic components are represented in dark gray,
dielectric insulators in light gray.
