What term refers to the thickness of tissue required to reduce the incident sound intensity by one half?

• A. Half value layer
• B. Fractional bandwidth
• C. Frequency
• D. Energy

Answer: (A)

Attenuation of ultrasound in tissue follows an exponential decay, and the thickness needed to reduce the incident intensity by half is called the half-value layer. The intensity decreases as I = I0 e^{-μx}, where μ is the attenuation coefficient for the tissue at the chosen frequency. Setting I = I0/2 gives e^{-μx} = 1/2, so x = ln(2)/μ. This thickness depends on the tissue and the frequency: higher frequency increases attenuation (larger μ), so the half-value layer becomes thinner. This term specifically quantifies how thick a layer must be to cut the intensity in half.

The other terms don’t describe this thickness. Fractional bandwidth refers to the spread of frequencies in a pulse, frequency is the rate of oscillation, and energy is the overall capacity to do work or the total energy carried, none of which define a thickness that halves intensity.