DRAFT Recommended Practice for Measurements and ...

1/29/98 93 C95.3-1991 Revision — 2 nd Draft
10/98 Draft
using open coaxial line methods [Stuchly, et al., 1987; Athey, 1982], but the slotted
coaxial line method, if not more accurate, provides more repeatable results for liquid
simulants. Moreover, the slotted coaxial line method provides a means to examine the
attenuation of the RF wave as it progresses along the line so that the overall precision of
the measurement can be assessed more accurately than that for single surface point
measurement of the open coaxial line technique.
Accurate SAR measurements can only be made with probes that have been carefully
calibrated in the simulant being used to represent biological tissue (see 4.7.1.). The
calibration process, which is tedious and prone to error, requires simultaneous or
subsequent measurement of the magnitude of the E-field and the temperature rise at the
same location in a canonical model such as a flat or a spherical phantom of appropriate
tissue-equivalent material. The calibration in a flat model is usually is performed using a
relatively high power source coupled to a resonant dipole placed at a specified distance
from the phantom [Kuster and Balzano, 1992]; calibration in a spherical model is usually
performed under plane wave irradiation conditions.
As pointed out above, the experimental error associated with SAR measurements can be
substantial (±2 dB) because of the multi-step nature of the process. The following
factors contribute to the overall experimental accuracy that can be expected to be
realized: accuracy of the electrical characteristics of the tissue simulant – ±3 % (if the
measurements are limited to a narrow band of frequencies); accuracy of the temperature
rise measurements during probe and errors associated with calibration – ±3 %; accuracy
of the RF power measurements – ±5 %; positioning errors the non-isotropic response of
the probe – ±6 %. This leads to a total relative error of approximately ±17 % or about
±0.6 dB.
Even for a narrow frequency band, the achievement of a total relative error of only ±17 %
requires specialized equipment for measuring the dielectric properties of the tissue
simulant, accurately calibrated RF power meters, temperature probes, and well-trained
personnel to measure rises in temperature of the order of 0.10 0 C with a 0.03 0 C
measurement error. The procedure is time-consuming and the calibration of a single E-
filed probe at a single frequency in two different media, e.g., brain and muscle tissue
simulant, can take as long as two working days.
5.5.2.1 Automated SAR Scanners. The measurement of 3-dimensional SAR
distributions within a phantom involves measurements at hundreds of points. At higher
frequencies, especially with near field exposures from small localized sources which
produce rapid spatial variations in the SAR distribution, the locations of the measurement
points with respect to the phantom must be determined precisely. High precision is also
necessary to accurately measure the spatial peak SAR. Automated scanning systems
allow such measurements to be performed routinely. In order to move a small probe
along unrestricted continuous paths, such systems are generally restricted to shell
phantoms filled with liquids that simulate human tissue.
Although automated scanners based on temperature probes are possible, the maximum
measurement speed would be unacceptably slow. Because of this and the low
sensitivity of temperature probes, scanning systems that have been implemented are
based on miniature E-field probes. Such systems range from one-dimensional
Copyright © 1998 IEEE. All rights reserved. This is an unapproved IEEE Standards Draft,
subject to change.