5 - Microwave ablation: physical principles and technology

Abstract

Microwaves are nonionizing electromagnetic waves in the frequency range ∼300MHz–300GHz. Systems using microwave power have been applied for localized thermal ablation of targeted tissue for treatment of a range of indications, including malignant tumors, cardiac arrhythmias, and tissue reshaping. In an ablation procedure, a device, often referred to as an applicator or antenna, delivers microwave power from an external microwave power source to the targeted tissue. The applicator may be inserted into the targeted tissue via a surgical procedure or minimally invasively under image guidance. The dimensions, form factor, and constituent materials of the applicator are a function of the anatomic characteristics of the target tissue, frequency of applied power, physical properties of the target and adjacent tissue, and the procedure by which the applicator is introduced to the target. Microwave power radiated by the applicator is absorbed in the tissue surrounding the applicator, leading to heating. When an ablative thermal dose has been delivered to the target volume, as determined by intraprocedural monitoring or based on estimations from preclinical studies, power applied to the applicator is terminated, and the applicator is withdrawn. This chapter provides an overview of the physical principles of microwave tissue heating with application to thermal ablation and discusses engineering design considerations for microwave applicators and systems, and methods for assessment of candidate applicator designs.