What occurs when the tissue interface is small, equal to or less than the wavelength of the beam?

When the tissue interface is small, equal to or less than the wavelength of the beam, scattering occurs. This phenomenon happens because the sound wave interacts with small structures or interfaces that are comparable in size to the wavelength of the sound waves being employed.

In the case of scattering, the energy of the sound wave is dispersed in many directions rather than being reflected back or transmitted straight through the tissue. The extent and directional nature of this scattering are largely dependent on the size of the interface and the wavelength of the sound. When the tissue interface dimensions are similar to or smaller than the wavelength, the ultrasound waves are unable to maintain a coherent path, resulting in scattering.

Other processes, such as refraction, absorption, and interference, operate under different circumstances and are influenced by factors like the angles at which the waves strike surfaces or the properties of the tissues they encounter, making them unsuitable for this particular scenario.