The term "half layer" in ultrasound refers to what?

• A. Depth where 50% of the energy has been absorbed by the tissues
• B. Depth at which energy absorption is maximal
• C. Distance energy travels before absorption
• D. The skin depth at which energy first appears

Answer: (A)

Half layer is the depth where the ultrasound intensity has fallen to 50% of its initial value, meaning about half of the incident energy has been absorbed by the tissue. This is a practical way to quantify tissue attenuation because ultrasound loses energy exponentially as it travels (I(x) = I0 e^{-αx}). If I(x) is half of I0, the depth x is the half-value layer, defined as x = (ln 2)/α. This concept also explains how higher frequencies attenuate more quickly: increasing frequency raises the attenuation coefficient α, so the half-value layer becomes shorter and penetration decreases. The other ideas don’t fit because attenuation isn’t characterized by a single depth of maximal absorption, and skin depth is an electromagnetic concept not used for ultrasound energy in tissue.