IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. BME-22, NO. 6, NOVEMBER 1975

477

A Comparison of Electrically Short Bare and Insulated Probes for Measuring the Local Radio Frequency Electric Field in Biological Systems GLENN S. SMITH, MEMBER,

Abstract-The problems associated with measuring the local electric field in a medium with spatally inhomogeneous electrical constitutive parameters like biological tissue are discussed. Simple theoretical analyses that describe the operation of electrically short bare and insulated electric field probes in a dissipative medium are presented. The conditions that must be satisfied for each of these probes to have an electric field response that is independent of the electrical constitutive parameters of the surrounding tissue are determined. The results of a systematic experimental study of both types of probes are presented. In the study the probes were immersed in a succession of liquids with dielectric properties similar to those for biological tissues at radio frequencies.

I. INTRODUCTION

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jjj~~~~~ INCIDENT RADIATION

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LOCAL ELECTRIC

FIELD

ELECTRIC FIELD PROBE.

BIOLOGICAL BODY WITH INHOMOGENEOUS ELECTRICAL PROPERTIES ) I E. e)

Fig. 1. Probing the local electric field in a biological body with inhomogeneous electrical properties.

C URRENT interest in the effects of nonionizing electromagnetic radiation on biological systems has created a need for accurate methods for measuring the local electric field internal to biological specimens [11 -[6]. Measured values of the electric field in vivo are needed before any correlation can be established b-etween electric field strength and morphological changes in tissue, physiological effects, etc. Measurements of the spatial variation of the electric field internal to biological bodies or phantoms are necessary to determine if interior resonances exist [7], [8]. Field strengths at interior points of a biological body in general cannot be accurately determined from a simple measurement of the field external to the body and, therefore, electric field probes must be inserted directly into the body. The biological specimens studied are usually spatially inhomogeneous in the sense that the electrical constitutive parameters (e,, ee) of the specimens are not uniform throughout. Ideally, a probe is needed that will measure the electric field strength at different points in the specimen in a manner that is independent of the local constitutive parameters. Referring to Fig. 1, the response from the probe, i.e., the voltage V, should be proportional to the local, linearly polarized electric field E = Ee

probe with this kind of response would eliminate the need for dissecting a specimen to determine the constitutive parameters at each point where the probe was inserted. The probe could be calibrated absolutely by measuring a known field in one medium. Thermal probes (thermocouples, thermographs, etc.) have been used to determine average absorbed power densities and electric fields in the interior of biological specimens [21, [9]. While thermal probes provide a direct reading of the temperature increase in the specimen, knowledge of the effective electrical conductivity, specific heat and density at the probe location are needed before the local electric field can be determined. The electrically short antenna is an attractive alternative for this application since its small physical size allows high spatial resolution with a minimum field perturbation. In this paper the properties of the electrically short, bare, electric field probe in a dissipative medium are reviewed and a simple description is given of the operation of the same probe when enclosed in a concentric, insulating sheath. The conditions that must be satisfied to make each of the probes have an electric field response that is independent of the constitutive parameters of the surrounding tissue are described. Measured V = KeE (1) input admittances and received voltages for both types of and the proportionality constant Ke should be independent of probes are compared with those predicted by the respective the constitutive parameters of the surrounding medium. A theories. The measurements were made with the probes immersed in a succession of liquids with dielectric properties in a that of biological tissues at radio frequencies. Manuscript received June 14, 1974; revised November 18, 1974, and range similar to January 30, 1975. This work was supported in part by the National II. CIRCUIT PROPERTIES OF ELECTRICALLY Science Foundation under Grant GK-40575 with Northeastern University, and in part by the Air Force Office of Scientific Research under Contract F44620-72-C-0021 with Harvard University. The author was with the Gordon McKay Laboratory, Harvard University, Cambridge, Mass. 02138, and the Department of Electrical Engineering, Northeastern University, Boston, Mass. 02115. He is now with the Department of Electrical Engineering, Georgia Institute of Technology, Atlanta, Ga. 30332.

SHORT PROBES

Fig. 2(a) illustrates the cylindrical electric field probe of length 2h and radius a, center-driven by a voltage source of strength V. The insulated cylindrical probe in Fig. 2(b) is formed by adding a concentric sheath with effective dielectric

IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, NOVEMBER 1975

478

and can be represented by the circuit model shown in Fig. 3. Note that the terms corresponding to the radiation resistance 2ao HOMOGE NEOUS _ of the probe have not been kept in the expression for the adISOTROPIC MEDIUM ) (O(e Ee L0_ mittance. They are of order I kh 13 or less in comparison to the hi h terms retained and are, thus, insignificant under the assumptions made in (6). Dmm --V -V The insulated probe with the geometry shown in Fig. 2(b) not been analyzed for the general case where both the exhas INSULATION ternal medium and the insulation have arbitrary electrical PERFECT CONDUCTOR properties. The analysis that is available applies only when the magnitude of the propagation constant in the external medium BARE PROBE INSULATED PROBE is substantially greater than that in the insulation [1 1] . Useful Fig. 2. Details of bare and insulated cylindrical dipole electric field information about the operation of the electrically short inprobes. sulated probe can, however, be obtained from simple physical arguments. For an electrically short structure (6) the current constant eei = Co eri, length 2hi and radius b to the bare probe. distribution of the driven insulated probe will be approxiThe dissipative medium surrounding the probe is assumed to mately triangular and have the same form as on the bare probe. be a biological tissue. If the significant spatial variations in the The addition of the insulating sheath simply adds a capacitive electrical constitutive parameters of the tissue occur over dis- admittance jwC, in series with the admittance Gm + I&jCm taices large compared to the sampling volume of the probe, generated by the field in the external medium. This is illusthe material surrounding the probe can be considered to be trated in the circuit model of Fig. 3. If the radius of the inhomogeneous (for an electrically short probe the sampling sulation is small compared to the probe length, i.e., volume is roughly a sphere of radius h centered on the probe). (8) b

A comparison of electrically short bare and insulated probes for measuring the local radio frequency electric field in biological systems.

IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. BME-22, NO. 6, NOVEMBER 1975 477 A Comparison of Electrically Short Bare and Insulated Probes for...
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