DRAFT Recommended Practice for Measurements and ...

1/29/98 66 C95.3-1991 Revision — 2 nd Draft
10/98 Draft
spheres [B15, B116] and in waveguides filled with lossy dielectric liquids, e.g. salt water,
[B59]. These lossy media are exposed to a known external E-field and then placed in the
dielectric object in which the internal E-field distribution has been calculated using
electromagnetic field theory. The internal E-field calibration factor for the probe is
determined from its response compared with the calculated internal field. If this is done
at several frequencies for a specific probe design, the calibration uncertainty will be
typically 1 to 2 dB when implanted in any lossy, high dielectric-constant object, such as
biological tissue that contains a large percentage of water (muscle, brain, and internal
organs, but not bone or fat). (See also 5.5.2.)
Another technique for calibrating probes in tissue equivalent material is based on the
boundary condition that the tangential component of the electric field is continuous
across any interface [B59]. This technique utilizes a waveguide section with a thin
plastic septum separating two segments, one containing air and the other segment
containing the tissue simulant. (See Figure 4.10.) Various recipes for tissue-equivalent
material can be found in the literature, e.g., [Hartsgrove, 1987] and [Chou, 1984].
The probe output is measured in various positions on each side of the septum and the
resulting curves are extrapolated to the septum interface. The field in the tissueequivalent
material decays exponentially whilst the field in air varies sinusoidally due to
reflections at the air-tissue simulant interface. To minimize interaction with the field, the
probe is passed through the narrow wall of the waveguide to place the probe axis and
lead wires normal to the E-field. Measurements should be made as close as possible to
the interface. The resulting curves should plot as straight lines on semi-log paper.
When distances between the measurement points and the distance between the septum
interface and the closest measurement point are equal, the following equation may be
used to measure the probe-tissue enhancement factor F TE :
Fig 4.10 Boundary Condition Method for Calibrating Implantable E-field Probes in
Tissue-Equivalent Material
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