What phenomenon occurs when ultrasound encounters a large reflector compared to its wavelength?

The phenomenon that occurs when ultrasound encounters a large reflector compared to its wavelength is specular reflection. This type of reflection takes place when the size of the reflecting surface is significantly larger than the wavelength of the ultrasound being used. In this case, the sound waves bounce off the surface in a predictable manner, allowing for clear and distinct returning echoes. Specular reflection is particularly important in diagnostic ultrasound imaging because it helps create well-defined images of structures such as organs and tissues, where the size of the structures is generally greater than the ultrasound wavelength.

Rayleigh scattering, on the other hand, occurs when ultrasound interacts with small particles that are comparable in size to the wavelength. This leads to scattering of sound in various directions rather than producing clear echoes. Diffuse reflection is associated with irregular surfaces reflecting sound waves in multiple directions, which does not provide a strong, coherent return signal for imaging purposes. Transmission refers to the passage of ultrasound waves through a medium without being reflected, and while it is a critical component of ultrasound imaging, it does not correspond to the interaction with large reflectors that results in specular reflection.