What interaction of ultrasound and tissue is primarily responsible for imaging the internal structure of organs?

• A. Specular reflection
• B. Refraction
• C. Diffraction
• D. Scattering

Answer: (D)

The key idea is that imaging internal organ structure in ultrasound comes from backscatter produced by tissue microstructure. As the ultrasound pulse travels, it encounters countless tiny variations in acoustic impedance—cell membranes, fibers, and other small interfaces—that scatter the energy in many directions. A portion of that scattered energy travels back toward the transducer, where it is detected and used to form the image. The brightness in the image reflects how much backscatter a region produces, so the texture and details of the internal structure emerge from this scattered echoes.

Specular reflections from smooth, large interfaces can create strong echoes at obvious borders, but they don’t convey the fine internal texture. Refraction changes the beam’s path, and diffraction influences resolution, but neither is the primary mechanism for revealing internal tissue structure. Scattering, especially the backscattering from tissue microstructure, is what mainly carries the information needed to visualize organs.