The depth of tissue that results in 3 dB of attenuation to the intensity is:

• A. Half-value layer thickness
• B. Quarter-value layer thickness
• C. Double-value layer thickness
• D. Attenuation depth

Answer: (A)

3 dB of attenuation means the intensity drops to half of its original value, since 3 dB corresponds to a power ratio of about 0.5 (10 log10 0.5 ≈ -3 dB). The depth of tissue that reduces the intensity to this half value is defined as the half-value layer thickness. This standard term describes how much tissue is needed to halve the signal as it attenuates. For comparison, a depth that produces a 6 dB reduction would bring the intensity to one quarter, not one half, and terms like attenuation depth aren’t the standard designation for this specific half-intensity measurement. Therefore, the depth that yields 3 dB attenuation is the half-value layer thickness.