What sound-tissue interaction is necessary to form an ultrasound image?

• A. Rarefaction
• B. Refraction
• C. Reflection
• D. Diffraction

Answer: (C)

The essential idea is that ultrasound imaging relies on echoes. The transducer sends a brief sound pulse into the body and then listens for echoes that return from tissue interfaces where the acoustic impedance changes. These reflections travel back to the transducer, and the machine uses their time delay to determine depth and their strength to gauge how much sound was reflected, creating the image you see. Other wave phenomena—rare faction is just part of the wave cycle, refraction bends the beam at boundaries, and diffraction spreads the beam—affect the beam in different ways, but they aren’t what actually form the image by themselves. It’s the reflections from tissue boundaries that provide the data used to map out structures.