Microwave ablation technology: what every user should know

Abstract

Microwave ablation is a relatively new technology under development and testing to treat the same types of cancer that can be treated with radiofrequency ablation. Microwave energy has several possible benefits over radiofrequency energy for tumor ablation but, because clinical microwave ablation systems are not widespread, the underlying principles and technologies may not be as familiar. The basic microwave ablation system contains many of the same components as a radiofrequency ablation system: a generator, a power distribution system, and an interstitial applicator. This article attempts to provide an overview of each of these components, outline their functions and roles, and provide some insight into what every potential microwave ablation user should know about systems in development.

Figures

Figure 1

Microwave tissue heating relies on the interaction of an electromagnetic field with water molecules in the tissue.

Figure 2

Microwave ablation system schematic. The three basic components are the microwave generator, power distribution system and antenna. The generator frequencies allowed by the Federal Communications Commission (FCC) are, most commonly, 915 MHz or 2.45 GHz. The power distribution system may contain one or more of several components, including cables, power splitters, phase shifters and switches. The antenna delivery system may also contain one or more antennas of various designs, each with the goal of creating a large and reproducible zone of ablation.

Figure 3

System schematic (left) and image of a typical 1000 W magnetron (right) generator. The advantages of using a magnetron are simple design, high power output, high efficiency and excellent robustness. The disadvantages include bulky power supplies and monitoring systems.

Figure 4

Basic schematic of a solid-state generator. In contrast to a magnetron generator, power is generated in stages using solid-state devices. Advantages of solid-state generators include low-power (pre-amplified) control, a more stable output and often a smaller, lighter unit. Disadvantages include reduced efficiency and lower output power.

Figure 5

Basic coaxial cable. The inner and outer conductor are usually copper, but can be any good conductor. The inner conductor is often silver plated to improve performance. Solid conductors improve rigidity while braided conductors improve flexibility. The dielectric material is typically polytetrafluoroethylene (PTFE) or a similar polymer. The jacketing material is optional but helps to protect the cable from mechanical stress, especially if the outer conductor is braided.

Figure 6

Power ratings of common coaxial cables in air at 20 °C. Power rating increases for cables in tissue for several reasons, including the enhanced conductive heating offered by tissues and the increased convective cooling offered by blood flow. Actively cooling antennas can increase their power ratings as well.

Figure 7

Simulated heating pattern of a three-antenna array with constructive phase overlap at the array center (left) along with a plot comparing the heating power through the array center for one or three antennas (right). The use of phase shifters in the power distribution system could allow 9× more power deposition at the array center than with one antenna.

Figure 8

Comparison of the simulated electric fields produced by three microwave ablation antennas: a) a 17-gauge triaxial antenna (14), b) a 10-gauge floating sleeve antenna (16) and c) a 17-gauge coaxial slot antenna (8). The triaxial antenna sacrifices some localized heating for higher efficiency, while the sleeve antenna sacrifices invasiveness to reduce heating along the feedline.

Figure 9

Comparison of simulated reflection coefficients of the antennas in Figure 8. The triaxial antenna is 99+ % efficient, the sleeve antenna is ~ 93 % efficient and the slot antenna is ~ 70 % efficient at the operating frequency (2.45 GHz). Tissue properties caused by heating and ablation will change the reflection coefficient of each antenna. Ideally, an ablation antenna would have a low reflection coefficient for a broad frequency range at and below the operating frequency to counteract tissue property changes.

Figure 10

Example of feedline heating created by uncooled antennas operated at high powers.