*1.3.2. Microwave ablation*

A variety of probes have been proposed for use in MWA, with the majority being based on a coaxial structure due to the deep-seated location of many tumors and the angular symmetry of the tumor. Initial antennas based upon a coaxial waveguide structure include designs, such as the monopole, dipole and slot antennas, often encased in a polytetrafluoroethylene (PTFE) catheter to minimize adhesion of the probe to desiccated ablated tissue. A number of challenges, characteristics and trade-offs have been identified in the design of MWA probes. Challenges include the reduction of backward heating, minimization of probe diameter and impedance matching of the antenna to the surrounding tissue. Trade-offs in design involve probe diameter versus maximum application of power and ablation power versus ablation time. Early coaxial antennas developed for MWA yielded ablation zones resembling a 'tear drop', as opposed to the desired spherical shape [9]. More recent MWA probes were designed to minimize probe size, maximize ablation zone size, minimize detrimental heating of the feedline and yield more spherical lesions, by minimizing impedance mismatch [10-12].
