Which statement best describes how power behaves as it travels through tissue?

Which statement best describes how power behaves as it travels through tissue?

• A. Power remains constant
• B. Power increases as it propagates
• C. Power decreases as it propagates
• D. Power fluctuates randomly

Answer: (C)

Energy decays as it travels through tissue. As ultrasound moves, part of the energy is absorbed by the tissue (often turning into heat) and part is scattered out of the forward path, with the remaining energy also spreading over a larger area. These losses cause the intensity to fall—usually in an exponential fashion—with depth. Interfaces between tissues can reflect some energy away due to impedance differences, further reducing what continues forward. Because of these attenuation effects, the power delivered to deeper tissue is less than at the surface, making the statement that power decreases as it propagates the best description.

Energy decays as it travels through tissue. As ultrasound moves, part of the energy is absorbed by the tissue (often turning into heat) and part is scattered out of the forward path, with the remaining energy also spreading over a larger area. These losses cause the intensity to fall—usually in an exponential fashion—with depth. Interfaces between tissues can reflect some energy away due to impedance differences, further reducing what continues forward. Because of these attenuation effects, the power delivered to deeper tissue is less than at the surface, making the statement that power decreases as it propagates the best description.