Which of the following has extremely low attenuation?

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

Answer: (B)

Attenuation in ultrasound describes how much the sound signal loses strength as it travels through a medium due to absorption and scattering. Water has extremely low attenuation because its molecules interact very weakly with diagnostic-frequency ultrasound, so there’s minimal energy loss as the wave passes. Air causes a large impedance mismatch and strong reflections at interfaces, so very little energy actually propagates, which feels like high attenuation in practice. Bone is dense and highly absorptive, leading to substantial signal loss. Blood does attenuate too, but not as much as bone or air; water still attenuates the least among these options. So water is the medium with the lowest attenuation.