Microwave energy is well known for its heating effects at certain frequencies/wavelengths. One of the keys for using microwave ablation (MWA) for medicinal reasons, specifically to heat and destroy cancerous tumors in patients, is finding the optimum antenna for each case. With the wrong antenna and coaxial feed lines, healthy tissues can be heated (and overheated) along with the cancerous tissues, causing unwanted damage to those tissues. To complicate matters, when using minimally invasive interstitial antennas to apply microwave energy to cancerous tumors, the antenna’s input impedance depends on the insertion depth.
Among the antennas designed for MWA treatments, a triaxial antenna employs a biopsy needle in a floating sleeve over the outer conductor. For a given insertion depth, the position of the needle is adjusted to create the best impedance match for the antenna. Another design, a balun-free antenna, operates at the second resonant frequency of a monopole to achieve a given specific-absorption-rate (SAR) pattern and ablation zone for the applied microwave energy without using a coaxial balun. Yet another antenna configuration, a choke antenna, uses short-circuited quarter-wavelength sleeves as part of the MWA process.
In the interest of finding an optimum antenna configuration for MWA on humans, researchers from the University of Wisconsin-Madison performed experiments with the different antennas on ex vivo bovine livers to examine the ablation zones obtained from each antenna. The bovine livers were exposed to microwave radiation at a power level of 40 W for a total of 5 min. for each antenna. Test signals were produced by boosting the outputs of a commercial microwave signal generator through a solid-state power amplifier and then feeding each antenna.
After each five-minute application of microwave energy at 1.9 GHz, the bovine liver was cut along the insertion path of the antenna and the ablation zone of each antenna was measured. Each antenna’s ablation zone was ellipsoidal in shape. By means of visual inspection and careful measurements, the aspect ratio of each ablation zone was ascertained to determine the most spherical of the ablation zones for the three antennas, since many tumors are spherical in shape.
The experiments revealed that the choke dipole antenna and the balun-free base-fed monopole antenna produced the most spherically shaped ablation zones of the three types of MWA antennas, with the triaxial antenna providing the least spherical of the ablation zones. The triaxial and choke antennas are both currently used in FDA-approved commercial MWA systems.
Electromagnetic (EM) computer software simulations showed that each of the antennas could provide a good impedance match with the bovine liver at the desired operating frequency. However, the choke and balun-free antennas provided the most compact and localized power absorption patterns, with much less extra heating of surrounding tissues compared to the triaxial antenna.
See “Tools for Attacking Tumors,” IEEE Antennas & Propagation Magazine, Vol. 60, No. 6, December 2018, pp. 52-57.