Measurement of tissue permittivities using a microwave “pencil” probe

A variety of probe radiator configurations have been investigated for use in measurement of the complex dielectric permittivity of tissue. We report here on tests of a configuration which employs a small-diameter, open-ended, semi-rigid coaxial line which has been machined at the open, radiating end to a conical termination, so that it resembles a pencil tip. The multiple rationale for this choice is as follows. First, a coaxial radiator is preferred because it permits wideband measurements. Second, although noninvasive devices which do not require penetration in the tissue have an obvious advantage in ease of use, they are subject to errors created by a variety of surface effects. Third, an axially symmetric radiator prevents the generation of higher-order H-modes. Fourth, the pencil shape is useful, since it simplifies insertion into harder biological tissue types. Of particular interest to us is the possible application of this form for measurements of the permittivity of tough meat and soft bone. Based on the results achieved so far, the pencil probe appears to be a viable configuration with a number of possible applications. Precise calibration of the probe depends on a proper choice of standards so that the radiation term can be accurately compensated. Permittivity measurements to within 2% appear to be feasible over the microwave band