DRAFT Recommended Practice for Measurements and ...

1/29/98 88 C95.3-1991 Revision — 2 nd Draft
10/98 Draft
Table 5.4 Specific Heat Capacity and Mass Density of Tissue-Equivalent
(Phantom) Materials and Actual Biological Tissues §
Specific Heat
(kJ/kg/ o C)
Mass density
(kg/m 3 x10 3 )
Muscle Simulant* 3.7 1.0
Muscle Simulant † 3.6 1.1
Brain Simulant* 3.4 0.98
Adipose Tissue (fat) Simulant †# 1.1 1.4
Muscle in-vitro 3.5 1.1
Brain in vitro 3.5 1.1
Adipose Tissue in vitro 1.2-1.6 1.05
Bone 1.25-3.0 1.25-1.8
Notes:
* Data for tissue formulated for use at 2450 MHz
† Data for tissue formulated for use at 27 MHz
# Simulated fat material has dielectric properties that are almost identical to living bone
§ Data for this table was taken from [B87]
There are several sources of error when using temperature probes for SAR
measurements. First, it is difficult to obtain repeatable results at a location of a large
spatial SAR gradient. Small changes in probe location can cause large SAR changes in
such situations, and the linear portion of the slope of ∆T/∆t is rather brief (with respect to
the initiation of irradiation). This is due to the presence of high-thermal gradients and the
resulting thermodynamic effects that cause measurement degradation. For these
reasons, it is desirable to determine locations in the subject where maximum values of
SAR exist. Data should be taken at and on either side of the maxima. Second, regions
of relatively high SAR (> 20 W/kg) should be scrutinized closely to ensure that the proper
value is actually obtained, because such regions often exhibit high-thermal conduction
losses, as well as high SAR. In high SAR regions, it is appropriate to halve the irradiation
time and ensure that ∆T is also reduced by one-half; if it is not, thermodynamic effects,
such as those associated with thermal conduction, are present. It is also important to
note that dielectric constants will generally change with temperature, which of course will
alter the measured SAR. This would be relevant, say, if the measurements were being
made inside a dead animal that was allowed to equilibrate to room temperature to obtain
∆T/∆t =0 in order to null out the spatial thermal gradients.
Significant errors can occur when SAR is measured, using temperature probes, at a
single point in an object with one or more ‘’hot spots’’ near, but not coincident with the
probe tip. The temperature, as measured by the probe, will behave as follows: The
temperature may not rise immediately after the RF irradiation of the test object. After
several seconds, the temperature, as monitored by the probe, begins to rise more quickly
as heat is conducted from a nearby hot spot. When the irradiation ceases, the
temperature continues to rise as heat is conducted away from the hot spots to cooler
regions where the probe is located. Figure 5.1 illustrates this phenomenon. The rate of
temperature rise seen by the probe (apparent slope) could be falsely interpreted as the
Copyright © 1998 IEEE. All rights reserved. This is an unapproved IEEE Standards Draft,
subject to change.