Biomedical Engineering Reference
In-Depth Information
practice; energy is absorbed instead in the near-field or Fresnel
zones of the antenna, where waves must be treated as spherical.
For this reason, comparing frequencies based upon plane-wave
assumptions alone may not be appropriate. More investigation is
needed to address this concern. As a result, reports comparing
915 MHz and 2.45 GHz systems have provided mixed results;
some conclude that 915 MHz produces larger ablation volumes,
while others suggest that equally large ablations can be achieved
with 2.45 GHz (Hines-Peralta 2006, Sun 2009, Brace 2007b,
Strickland 2002, Hope 2008). None of the existing literature has
employed a well-controlled design to test equal powers delivered
to the tissue. In one notable exception, frequencies as high as
9.2 GHz have been used to ablate the endometrial lining of the
uterus. Development of that system demonstrated that 9.2 GHz
energy provided an appropriate balance of energy penetra-
tion and rapid heating required for that particular application
(Feldberg 1998).
9.5.1 power Generation and Distribution
9.5.1.1 Generator Design
Microwave power in clinical ablation systems is generated
using either solid-state semiconductor sources or vacuum-
tube devices such as the magnetron. Magnetron sources have
the advantage of relatively high-power conversion efficiency
(typically greater than 70%), the ability to produce substantial
powers from a single device, and a high tolerance to imper-
fect load matching without device failure. However, microwave
generators that employ magnetron sources distribute power to
multiple antennas by using passive splitters. Phase control of
each channel can be achieved using passive phase adjusters, but
these must be able to distribute the full power of each channel.
Phase adjusters capable of distributing typical microwave abla-
tion powers of 50-150 W tend to be relatively bulky and may
not be ideal for clinical implementations.
More recently, solid-state devices have become available that
may satisfy the requirements for clinical microwave ablation
systems. A solid-state generator consists of a low-power fre-
quency source, or oscillator, followed by control and amplifica-
tion stages that boost power output by 4-5 orders of magnitude.
In general, solid-state systems suffer from lower-power efficiency
(typically less than 40%), less tolerance to high-reflected powers
produced by imperfect load matching, and lower-output power
per channel than magnetron sources. However, solid-state sys-
tems utilize smaller components, can be phase adjusted and
power controlled in the preamplification stages, and can have a
cleaner output spectrum than magnetron sources with compa-
rable control features.
Both solid-state and magnetron generators are used in micro-
wave ablation systems currently in clinical use. Some of these
systems allow for a single antenna to be operated by a single-
generator system, requiring additional generators to power each
element of a multiple-antenna array. Other systems provide
external power splitters to divide power between two or more
antennas, but without any control of the relative phase between
antennas. Still other systems provide multiple power channels
from a single-generator system, with or without control of the
relative phase between antennas. Even without the ability to
control antenna phase, most single-generator, multiple-antenna
systems produce a coherent output in each output channel.
9.5.1.3 Coaxial Cables
Microwave power is carried from the generator to the antenna
through coaxial cables, due to their relative flexibility, excellent
propagation characteristics, and ease of connectivity. Cable flex-
ibility is a function of cable diameter and construction materials,
with improved flexibility in braided conductors and low-density
dielectrics. Power handling—the ability of a cable to safely trans-
fer power without overheating or failure—is related to these same
factors, as well as the frequency of the applied microwave power.
Therefore, coaxial cables used to distribute microwave power
may have a greater diameter and increased stiffness when com-
pared to their counterparts in RF or laser ablation systems.
Coaxial cables also comprise the underlying structure of most
interstitial microwave ablation antennas. Those intended for per-
cutaneous use are 1.5 mm to 2.5 mm in diameter, while antennas
greater than 5 mm in diameter have been reported for surgical
applications or endometrial ablation (Feldberg 1998, Hines-Peralta
2006, Strickland 2002). Data from the biopsy literature suggest
that larger needle diameters are associated with an increased risk
of complications such as bleeding and pneumothorax, providing
motivation to decrease percutaneous antenna diameter (Geraghty
2003). However, small-diameter coaxial cables absorb more
microwave energy, which reduces power throughput and, in turn,
increases heat generation within the cable. Such heating can lead
to potentially dangerous thermal damage along the antenna shaft.
Yet, increased power delivery has been associated with faster and
potentially more effective microwave ablation treatments, par-
ticularly for large tumors. A balance between small cable diam-
eter, high power throughput, and low internal heating must be
achieved in the power delivery and antenna cables.
9.5.1.2 Frequency Considerations
Energy penetration and heating rates in biological tissues
depend on the frequency of the applied electromagnetic field.
The simplest way to compare frequencies is to assume plane-
wave propagation into the tissue. The plane-wave condition
implies that the radiating wavefronts lie along parallel planes,
which occurs in the far-field of the antenna. The far-field is typi-
cally defined as radial distances greater than 2
D
2
/λ, where
D
is
the largest dimension of the antenna and
λ is the wavelength in
tissue. This distance is about 4 cm from the antenna in tissue.
The fact that 4 cm is greater than the radius of a typical abla-
tion zone implies that the far-field condition is rarely achieved in
9.5.2 Microwave ablation antenna Design
The microwave ablation antenna can be defined in a number of
different ways. In surgical and percutaneous applications, the
antenna is typically defined as the entire applicator beyond the
flexible coaxial power delivery cable. With this definition, the
