DRAFT Recommended Practice for Measurements and ...

1/29/98 111 C95.3-1991 Revision — 2 nd Draft
10/98 Draft
value of the resistor should be small enough so as not to perturb the body current, i.e.,
small compared with the capacitive reactance of the plate and standing subject to
ground. From the voltage drop V across the resistance R, the current I may be
computed from the expression I = V/R. SAR is then obtained by using the expression
SAR = ρJ 2 /σ W/kg (Eq C1)
where:
ρ = tissue resistivity ( Ω·m)
J = current density (A/m 2 )
σ = mass density (kg/m 3 )
J is determined by dividing the measured current by the cross sectional area of the
conductive tissues in the region of interest. Measurements of SAR derived from current
measurements are most accurately related to anatomical areas where the effective
conduction cross-section and current are best known. At frequencies of 50 MHz and
below, this is typically the legs and ankles, or arms and wrists.
Localized SAR can be derived from current measurements in extremities. A simplified
"effective cross-section" related to realistic anatomical features can be used to determine
current density. For many years, a 9.5 sq. cm effective cross-section for the adult
human ankle was assumed. That value represents about 15% of total area. However,
recent experimental evidence points to an effective cross-section of 60% of the total.
[Olsen and Van Matre]
C3 . Fabrication of Simulated Tissues
Phantom models are often used together with temperature probes, E-field probes and
thermographic cameras. They are composed of materials with dielectric, thermal and
geometric properties identical to the biological subject they represent. Phantom
materials have been developed that simulate human fat, muscle, brain and bone. The
dielectric properties of the phantom can be varied over a wide range by varying the
percentage of constituent materials. In one formulation that was developed by [C6 ], the
relative amounts of polyethylene powder and the salinity of the water used for making the
simulated muscle material can be varied to simulate specific tissues of high water
content. This particular form of phantom material is very viscous, while pliable and puttylike,
and is well suited for the construction of bisected phantoms for thermographic
evaluation. Mixing techniques and the exact grain size of the polyethylene powder affect
the homogeneity and dielectric properties significantly. Once poured into a mold, the gel
has a tendency to entrap air pockets (see Tables 5.4 and 5.5 ).
A less viscous gel has been developed using hydroxethylcellulose (HEC) gelling agent
and salt water (saline Polyethylene powder or sugar should be added for use above 100
MHz [C8 ]. The HEC gel does not retain permanent voids and air bubbles as readily as
the putty-like formulations [C1]. Visual inspection for air bubbles and the position of
implanted probes can be performed simply in the optically-transparent HEC gel.
Preservation with a bactericide, and refrigeration are necessary when making either of
these simulated soft-tissue gel formulations. Also, evaporation of the water should be
prevented by proper sealing of the containers, or the molds or shells that contain the gel.
Copyright © 1998 IEEE. All rights reserved. This is an unapproved IEEE Standards Draft,
subject to change.