(HVL) The attenuation of ultrasound waves in human tissue is characterized as the half value layer, or the half power distance. Half value layer means the distance the sound beam will travel in a tissue before its amplitude or energy is attenuated to half its original value. Air and lung tissue have extremely short half-power distances and represent severe obstacles to the transmission of acoustic energy.

Attenuation is the reduction of power, for example due to the passage through a medium or electrical component. In ultrasound imaging, attenuation means the decrease in amplitude and intensity as a sound wave travels through a medium. In ultrasound attenuation is often characterized as the half-value layer, or the half-power distance. These terms refer to the distance that ultrasound will travel in a particular tissue before its energy is attenuated to half its original value.

Attenuation originates through:
A thick muscled chest wall will offer a significant obstacle to the transmission of ultrasound. Non-muscle tissue such as fat does not attenuate acoustic energy as much. The half-value layer for bone is still less than muscle, that's why bone is such a barrier to ultrasound.

See also Attenuation Coefficient, and Derated Quantity.

Through diffraction and refraction on intersections edge acoustic shadowing can be created. The acoustic shadowing artifact is the loss of information below a dense object because the majority of the sound energy was reflected back by the object.
Shadowing artifacts occur if decreasing of the echo amplitude is not exponential with penetration depth caused by inhomogeneous tissue layers and fluid or air-filled regions. Bone, air, foreign bodies and calcification stop the transmission of sound waves producing a 'sonic shadow' which is a dark region distal to the echogenic obstructing region. This artifact occurs also in objects like e.g. prosthetic valves.

See also Boundary Layer, and Half-Value Layer.

Enhancement artifacts occur if decreasing of the echo amplitude is not equal with penetration depth caused by inhomogeneous tissue layers and fluids like cysts or air-filled regions. The enhancement artifact appears as a hyperintense (hyperechoic) signal. The attenuation of the ultrasound wave in fluids is much lower as the attenuation in other tissues, therefore tissues distal to fluid are enhanced. Artificial enhancement may also be found distal to a homogeneous solid tumor surrounded by adipose tissue, due to the comparatively high attenuation in fat.

See also Boundary Layer, and Half-Value Layer.

Laminar flow is non turbulent flow in arteries with blood traveling in layers or laminae. In a straight vessel, the layer in the center flows at a greater speed with subsequent layers toward the vessel wall flowing at slower speeds.

See also Boundary Layer, and Half-Value Layer.