Original article
Radiofrequency and Microwave Ablation of the Liver, Lung, Kidney, and Bone: What Are the Differences?

https://doi.org/10.1067/j.cpradiol.2007.10.001Get rights and content

Radiofrequency (RF) ablation is becoming an accepted treatment modality for many tumors of the liver and is being explored for tumors in the lung, kidney, and bone. While RF energy is the most familiar heat source for tissue ablation, it has certain limitations that may hamper its efficacy in these new organ systems. Microwave energy may be a better source for tissue ablation but has technical hurdles that must be overcome as well. This article outlines the physics behind RF and microwave heating, discusses relevant properties of the liver, lung, kidney, and bone for thermal ablation and examines the roles of RF and microwave ablation in these tissues.

Section snippets

Radiofrequency Ablation

RF ablation relies on a complete electrical circuit created through the body to conduct RF current. RF current is able to pass through tissue because of the abundance of ionic fluid present; however, tissue is not a perfect conductor and RF current causes resistive heating (the Joule effect). Direct RF heating occurs within several millimeters of the applicator (electrode). The rest of the final ablation zone is created when thermal conduction pushes heat into more peripheral areas around the

Microwave Ablation

Microwave ablation is a special case of dielectric heating, where the dielectric material is tissue. Dielectric heating occurs when an alternating electromagnetic (EM) field is applied to an imperfect dielectric material. In tissue, heating occurs because the EM field forces water molecules in the tissue to oscillate. The bound water molecules tend to oscillate out of phase with the applied fields, so some of the EM energy is absorbed and converted to heat. The best EM absorbers contain a high

Tissue Properties

For RF ablation, the important tissue properties are electrical conductivity and thermal conductivity. It should be noted that the tissue impedance more commonly quoted in the literature and displayed by RF ablation systems is inversely proportional to conductivity. High electrical conductivities (ie, low impedances) allow more current flow and more power to be applied from the generator, while low electrical conductivities (ie, high impedances) inhibit current flow. Thermal diffusion is also

Liver

RF ablation has become a popular treatment modality in the liver, particularly for tumors less than 3.0 cm in diameter. Many centers now consider it a first-line treatment option for small hepatocellular carcinomas and colorectal metastases to the liver.1, 2 However, RF energy has been limited in its ability to heat larger volumes of tissue (>3 cm in diameter) or to heat tissues in high-perfusion areas, such as tumors that abut vessels larger than 3 mm in diameter.13 Device and technique

Conclusions

While RF ablation has been an effective tool for treating tumors of the liver, lung, kidney, and bone, there are substantial drawbacks fundamental to RF heating. In particular, RF heating is limited in areas of high perfusion (kidney and liver), in tissues with poor electrical and thermal conductivity (lung and bone), and in areas near large heat sinks (liver, lung, and kidney). Microwaves offer all of the same benefits as RF energy for thermal ablation but are not as dependent on tissue

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    The author discloses financial interest (shareholder and consulting salary) in Micrablate, LLC, a developer of microwave ablation technologies.

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