When imaging a rounded mass that propagates slower than surrounding tissue, which sound-tissue interactions will occur?

• A. Refraction
• B. Reflection
• C. Absorption
• D. All of the above

Answer: (D)

When a region has a slower propagation speed than the surrounding tissue, the boundary between them creates several simultaneous sound-tissue interactions. The impedance mismatch at that curved boundary reflects part of the incident energy back toward the transducer, producing the characteristic echo from the lesion edge. At the same boundary, the transmitted wave changes direction as it enters the slower medium, bending toward the normal—this refraction alters the beam path inside the mass. While the wave travels through the slower tissue, some energy is attenuated by absorption (and scattering), so the transmitted signal loses amplitude. With a rounded boundary, these effects occur around the entire margin, leading to the combined result: reflection, refraction, and absorption. All of the above is observed in this scenario.