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Compact Directional Microwave Antenna for Localized Heating

Heating is concentrated on one side.

A directional, catheter-sized cylindrical antenna has been developed for localized delivery of microwave radiation for heating (and thus killing) diseased tissue without excessively heating nearby healthy tissue. By “localized” is meant that the antenna radiates much more in a selected azimuthal direction than in the opposite radial direction, so that it heats tissue much more on one side than it does on the opposite side. This antenna can be inserted using either a catheter or a syringe. A 2.4-mm prototype was tested, although smaller antennas are possible.

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Figure 1. A Coplanar Waveguide With a Taper is rolled into a cylinder andjoined with a coaxial-cable adapter to form a narrow antenna that radiatespredominantly to one side.

Prior compact, cylindrical antennas designed for therapeutic localized hyperthermia do not exhibit such directionality; that is, they radiate in approximately axisymmetric patterns. Prior directional antennas designed for the same purpose have been, variously, (1) too large to fit within catheters or (2) too large, after deployment from catheters, to fit within the confines of most human organs. In contrast, the present antenna offers a high degree of directionality and is compact enough to be useable as a catheter in some applications.

The antenna design is a hybrid of monopole-antenna and transmissionline design elements. The antenna (see Figure 1) is formed from an open-ended coplanar waveguide in which the gap between the middle conductor strip and the two outer (ground) conductor strips tapers from (1) a smaller value more characteristic of a transmission line to (2) a larger value more characteristic of a leaky transmission line or an antenna. The coplanar waveguide is wrapped around a polytetrafluoroethylene (PTFE) tube, and its abutting edges are soldered together to form the cylindrical antenna structure, now denoted a cylindrical coplanar waveguide (CCPW), in which there is only one ground conductor. In operation, the wide-gap region between the middle conductor strip and the ground conductor permits radiation into the top side, while the larger ground side limits radiation on the back side.

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Figure 2. The Temperature Rises on Opposite Sides of the antenna wereunequal, as desired, in a mass of simulated tissue heated with microwavepower by use of the antenna.

For a test of directionality, the antenna was inserted in a piece of biomedical simulation material, called “phantom” in the art, formulated to have thermal and electromagnetic properties similar to those of human tissue. The phantom was instrumented with two fiber-optic temperature probes: one at 3 mm radially outward from the middle conductor of the CCPW and one 3 mm radially outward from the ground conductor on the opposite side. The catheter was excited with a power of 5 W at a frequency of 2.45 GHz. The temperature measurements, plotted in Figure 2, showed that, as desired, there was considerably more heating on the middle- conductor side. As indicated in Figure 2, the temperature difference between the targeted direction and the back side is about 13°C. This difference is sufficient to provide localized ablation (killing of targeted diseased cells) while preserving the healthy tissue.

This work was done by Patrick W. Fink, Gregory Y. Lin, Andrew W. Chu, Justin A. Dobbins, G. Dickey Arndt, and Phong Ngo of Johnson Space Center.

This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to the Patent Counsel, Johnson Space Center, (281) 483-0837. Refer to MSC-23781.