Biomedical Engineering Reference
In-Depth Information
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Frequency (GHz)
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Frequency (GHz)
FIGURE 2.6
Real and imaginary parts of permittivity of beef blood.
volume fraction of extracellular space. For small temperature increases, it has
been reported that the conductivity properties of tumor and normal tissue, at
44 kHz and 1 MHz, varied reversibly, with a temperature coefficient of about
2%/°C. These changes reflect thermally induced changes in the conductivity
of tissue electrolyte. For large temperature increases (above about 44.5°C),
however, the dielectric properties exhibit abrupt and irreversible changes,
reflecting thermal damage to the tissue. In excised tissue maintained at 44°C,
the low-frequency conductivity decreased initially by approximately 10%
during the first hour, then gradually increased by approximately 50% during
the next 8 h; 10-fold changes were reported in the permittivity over similar
periods [12]. Hence, monitoring temperature is accompanied with a lot of
uncertainty.
At RF/microwaves, the conductivity of tissues reflects that of the tissue elec-
trolyte. There are two (sometimes opposing) effects. Increasing temperature
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