The breast model and the matching layer are immersed in a water bath for acoustic coupling. Oil can also be used as the acoustic coupling liquid [10], [11], [17] for its low microwave and acoustic losses. But a water environment is superior to oil because it is easier to be matched with human breasts due to impedance similarity and easier to work with in a bench-top or clinical setting. In this particular model setup, using water or mineral oil as the coupling medium for acoustic signal detection makes no difference in terms of microwave excitation because the microwave power directly goes into the sample without going through the coupling medium .However,for certain practical configuration in which microwave propagates through the coupling medium, the much higher loss of water will attenuate the incident microwave power significantly.For example,at 3 GHz, water has σ = 2S/m, while oil has σ = 7 × 10−5 S/m, which will lead to an attenuation constant of 43 and 0.009 m−1 for water and mineral oil cases, respectively. In addition, since water has much higher εr (77.4 at 3 GHz) than oil, the corresponding EM model with water as the coupling medium is electrically larger(a factor of 6)assuming the same physical volume so that the EM simulation takes longer time. Microwave attenuation in water can be reduced by decreasing the distance between the matching device and breast model.
The breast model and the matching layer are immersed in a water bath for acoustic coupling. Oil can also be used as the acoustic coupling liquid [10], [11], [17] for its low microwave and acoustic losses. But a water environment is superior to oil because it is easier to be matched with human breasts due to impedance similarity and easier to work with in a bench-top or clinical setting. In this particular model setup, using water or mineral oil as the coupling medium for acoustic signal detection makes no difference in terms of microwave excitation because the microwave power directly goes into the sample without going through the coupling medium .However,for certain practical configuration in which microwave propagates through the coupling medium, the much higher loss of water will attenuate the incident microwave power significantly.For example,at 3 GHz, water has σ = 2S/m, while oil has σ = 7 × 10−5 S/m, which will lead to an attenuation constant of 43 and 0.009 m−1 for water and mineral oil cases, respectively. In addition, since water has much higher εr (77.4 at 3 GHz) than oil, the corresponding EM model with water as the coupling medium is electrically larger(a factor of 6)assuming the same physical volume so that the EM simulation takes longer time. Microwave attenuation in water can be reduced by decreasing the distance between the matching device and breast model.
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