Analysis of microwave ablation antenna optimization techniques
Christopher L. Brace
Department of Biomedical Engineering
and Radiology, University of Wisconsin-
Madison, Madison, Wisconsin
Christopher L. Brace, Department of
Biomedical Engineering and Radiology,
University of Wisconsin-Madison, Madi-
son, WI, USA.
Turkish Ministry of National Education,
Grant/Award Number: 1416.
Microwave ablation is a minimally invasive treatment modality for malignant and
benign tumors in several organs. While many microwave ablation antennas have
been described in the literature, most have been designed assuming normal ambient
tissue and have not accounted for tissue property changes that occur during intense
heating. We analyzed three optimization approaches for canonical monopole and
dual-slot antennas: minimal reflection, spherical specific absorption rate (SAR) pat-
tern, and spherical ablation zone. Simulated ablations with each optimal design were
also validated in ex vivo liver tissue. Optimized designs for minimal reflection
matched previously published results, while designs optimized for spherical SAR and
spherical ablation yielded novel geometries. Surprisingly, optimization for spherical
SAR rendered the most spherical ablation zones in ex vivo tissue. Optimizations for
minimal reflection and spherical ablation zone did not achieve the most spherical
ablation zones in experiments. These results point to the need for greater accuracy in
dielectric and thermal tissue models to improve simulation-aided design, and to the
potential for continued refinement in microwave ablation antenna design.
antenna optimization, computational modeling, microwave ablation
Thermal ablation modalities rely on local tumor destruction
by heat and include focused ultrasound,
modality has distinct energy–tissue interactions. Ablative
techniques can be applied at open surgery, laparoscopy, and
percutaneously under imaging guidance. Ablations are per-
formed by placing an applicator within the tumor, then deliv-
ering enough energy through the applicator to heat the
surrounding tissue to at least 50–608C. The objective is cellu-
lar death by rapid coagulative necrosis, ischemia secondary
to microvascular coagulation, or apoptosis.
While several energy sources have been evaluated to
heat tissue, microwaves (300 MHz to 300 GHz) have
recently become a mainstay in the thermal ablation arma-
mentarium. The mechanism of microwave heating is primar-
ily rotation of polar molecules (e.g., water) in along with
some ionic displacement at lower frequencies. Microwave
energy penetrates through all biological tissues (including
desiccated or aerated tissues that inhibit radiofrequency
energy), allowing current devices to heat tissues faster and
more continuously than many previous devices based on
Contemporary studies show that
microwaves can create larger ablation zones in only a few
and that microwave ablation is less susceptible to
the deleterious effects of blood flow on heat transfer.
A shortcoming of many microwave ablation devices has
been elongated ablation. Many antennas have excessive radi-
ation along the antenna axis and create an ablation zone that
is ellipsoidal, with a short axis transverse to the antenna.
As many liver, lung, and kidney tumors are spherical, elon-
gated ablation zones can cause unnecessary damage in adja-
cent healthy tissues, and complicate the planning and
execution of ablation procedures.
For this reason, a number
of microwave ablation antenna developments have recently
focused on controlling the shape of the ablation zone.
The antenna structure dictates the shape of heating pro-
duced by a microwave ablation antenna and, by extension,
the shape of the resulting ablation zone. Many variants of
Int J RF Microw Comput Aided Eng. 2018;28:e21224.
2017 Wiley Periodicals, Inc.
Received: 20 July 2017
Revised: 2 December 2017
Accepted: 4 December 2017