An ultrasound transducer is used to image body tissues. A thin layer of coupling gel is placed between the transducer and the skin, as shown in the diagram.
The acoustic impedance of the coupling gel is Z1=1.5×106 kg m−2 s−1Z_1 = 1.5 \times 10^6 \text{ kg m}^{-2} \text{ s}^{-1}Z1=1.5×106 kg m−2 s−1. The acoustic impedance of the skin is Z2=1.6×106 kg m−2 s−1Z_2 = 1.6 \times 10^6 \text{ kg m}^{-2} \text{ s}^{-1}Z2=1.6×106 kg m−2 s−1.
What is the main behavior of the ultrasound wave intensity at the boundary between the coupling gel and the skin?
Approximately 99.9%99.9\%99.9% of the wave intensity is transmitted into the skin because the acoustic impedances are closely matched.
Approximately 99.9%99.9\%99.9% of the wave intensity is reflected back into the gel because of the high density of the skin.
Approximately 3.2%3.2\%3.2% of the wave intensity is transmitted into the skin, with 96.8%96.8\%96.8% reflected.
Exactly 51.6%51.6\%51.6% of the wave intensity is transmitted, as determined by the ratio Z2Z1+Z2\frac{Z_2}{Z_1 + Z_2}Z1+Z2Z2.