DRAFT Recommended Practice for Measurements and ...

1/29/98 90 C95.3-1991 Revision — 2 nd Draft
10/98 Draft
during each day of laboratory experimentation. Therefore, a compromise should be
made that provides good SAR data and also conserves time. A useful rule of thumb in
deciding when to start another irradiation is to wait until the slope of the cooling curve is
relatively constant, (about 5% of the prior RF-induced rate of temperature rise over the
period to be used for the next irradiation), and the decrease in temperature before
irradiation is relatively small compared with the expected irradiation-induced ∆T/∆t.
Repeated experiments using consistent techniques are essential for obtaining accurate
results in SAR studies utilizing temperature probes. Finally, after several RF irradiations
of the same object, its temperature may have increased above acceptable limits, and the
phantom or biological material may degrade.
The majority of SAR measurements are made using temperature probes. However,
many researchers are not fully aware of the many factors that degrade accuracy of these
measurements. For example, thermodynamic factors will always limit the accuracy and
precision of SAR measurements as will any uncertainty in the value of the specific heat
capacity of the actual or simulated tissue being evaluated. The specific heat capacity is
often mistakenly cited as that of water (about 15% higher than that of most high-water
content tissue), which, even under optimal usage conditions, leads to uncertainties of at
least ±1 to 2 dB in the local SAR distribution in an object when measured by sampling the
tissue volume with a temperature probe.
5.5.2. SAR Measurement with Miniature Electric-Field Probes.
Miniature isotropic, implantable E-field probes with high impedance feed lines, which
have been commercially available for a number of years [Cheung, et al., 1975], have
been used to measure SAR distributions in phantom models and in living, anesthetized
animals [C2, C12]. [See also 4.6.1.] These probes have much higher sensitivity than
thermal probes and are especially suitable for measuring E-fields within simulated or
actual biological tissues of moderate to high water content, i.e., brain and muscle. While
it is possible to measure SARs of the order of 1 W/kg using sensitive and precise
thermal measurements (∆T/∆t ≈ 0.1 0 C/30 s), it is well within the domain of E-field probes
to measure SARs as low as 10 mW/kg [Balzano, 1995]. SAR can be calculated using
Eq. 5.5 and the data in Tables 5.4 and 5.5 that show typical values for simulated and
actual tissues.
where:
SAR = 1 2
ωε
0ε"
Eint W/kg
2ρ = σ 2ρ
ρ = mass density (kg/m 3 )
2
E int
(Eq. 5.5)
ε 0 = permittivity of free space (F/m)
ε” = imaginary part of the complex relative permittivity
ω = radian frequency (= 2πf)
σ = conductivity (S/m)
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