Which medium would you expect to have the highest attenuation among the following?

Which medium would you expect to have the highest attenuation among the following?

• A. Water
• B. Blood
• C. Fat
• D. Air

Answer: (D)

Attenuation is the loss of ultrasound energy as it travels through a medium, caused by absorption, scattering, and reflection, and it tends to increase with frequency. Air shows the highest attenuation because sound propagates poorly in air and there is a huge impedance mismatch between air and tissue. When ultrasound encounters air, most of the energy is reflected at the interface rather than transmitted, and the little energy that does enter air is damped very quickly. Water, fat, and blood are all more capable of carrying ultrasound with relatively low absorption at diagnostic frequencies, so their attenuation is much less than that of air. In practice, this is why coupling gel is used to replace air between the transducer and skin, and why imaging through air (as in lungs or bowel gas) has much higher attenuation and poorer penetration.

Attenuation is the loss of ultrasound energy as it travels through a medium, caused by absorption, scattering, and reflection, and it tends to increase with frequency. Air shows the highest attenuation because sound propagates poorly in air and there is a huge impedance mismatch between air and tissue. When ultrasound encounters air, most of the energy is reflected at the interface rather than transmitted, and the little energy that does enter air is damped very quickly. Water, fat, and blood are all more capable of carrying ultrasound with relatively low absorption at diagnostic frequencies, so their attenuation is much less than that of air. In practice, this is why coupling gel is used to replace air between the transducer and skin, and why imaging through air (as in lungs or bowel gas) has much higher attenuation and poorer penetration.