At an air-skin boundary about 99.9% of the ultrasound intensity is reflected, because the acoustic impedance of air is tiny compared with that of skin. If there were even a thin layer of air between the transducer and the skin, almost no ultrasound would enter the body and there would be no useful echoes.The solution is impedance matching (acoustic matching): making sure the ultrasound passes between materials of similar acoustic impedance, so that very little is reflected at each boundary and almost all of it is transmitted.A coupling gel is spread on the skin before the transducer is placed on it. The gel:• fills the gaps and pushes out the air between the transducer and the skin
• has an acoustic impedance similar to that of skin, so there is very little reflection at the gel-skin boundary
The front layer of the transducer is also chosen to have an impedance between that of the crystal and that of skin, which improves transmission out of the transducer.Worked example: gel has Z = 1.5 × 106 and skin Z = 1.7 × 106 kg m−2 s−1.Ir/I0 = (1.7 − 1.5)2 / (1.7 + 1.5)2 = (0.2 / 3.2)2 = 0.0039 Only 0.39% is reflected, so about 99.6% of the intensity is transmitted into the skin, compared with about 0.1% without the gel.
• has an acoustic impedance similar to that of skin, so there is very little reflection at the gel-skin boundary
The front layer of the transducer is also chosen to have an impedance between that of the crystal and that of skin, which improves transmission out of the transducer.Worked example: gel has Z = 1.5 × 106 and skin Z = 1.7 × 106 kg m−2 s−1.