The scattered echo originates from relatively small, weakly reflective, irregularly shaped objects, and is less angle dependent and less intense. The mathematical treatment of non-specular reflection involves the Rayleigh probability density function (i.e. blood cells).

An echo is defined as the repetition of a sound by reflection of sound waves from a surface.
Echo types used in ultrasound imaging:
Specular echoes are created from relatively large, regularly shaped objects with smooth surfaces. Specular echoes are relatively intense and angle dependent.
Scattered echoes are created from relatively small, weakly reflective, irregularly shaped objects. Scattered echoes are less angle dependant and less intense.

See also Specular Echo, and Scattered Echo.

Rayleigh scattering is the backscattering of ultrasound from blood. The echoes detected from blood are created through interference between scattered wavelets from numerous point scatterers. Rayleigh Scatterers are objects whose dimensions are much less than the ultrasound wavelength. Rayleigh scattering increases with frequency raised to the 4th power and provides much of the diagnostic information from ultrasound. Doubling the ultrasonic frequency makes the echoes from blood 16 times as strong. The intensity of the backscattered echoes is proportional to the total number of scatterers, which means that the echo amplitude is proportional to the square root of the total number of scatterers.
At normal blood flow, the number of point scatterers in blood is proportional to the number of red blood cells. When blood flow is turbulent, or accelerating fast (e.g. in a stenosis), the number of inhomogeneities in the red blood cell concentration will increase.

See also Scattered Echo.

The perfect image quality is dependent on some assumptions of the propagation of ultrasound waves in tissues after generating in an imaging system. These assumptions are important for the developing of optimal ultrasound imaging systems.
A lot of steps can be taken to prevent artifacts and to improve image quality, for example beamforming is used to focus the ultrasound beam, and contrast agents decrease the reflectivity of the undesired interfaces or increase the backscattered echoes from the desired regions.

See also Coded Excitation, Validation and Refraction Artifact, Q-Value, Ultrasound Phantom, Dead Zone, Narrow Bandwidth.

The specular echo originates from relatively large, strongly reflective, regularly shaped objects with smooth surfaces. These intense reflections are angle dependent, and are described by reflectivity equation. This type of reflection is called specular reflection (i.e. IVS, valves).

See also False Distance Artifact, and Scattered Echo.