Ultrasound Transmission Through Tissues

Propagation of ultrasound is basically traveling of waves through different

structures. The ultrasound is directed towards a structure, 

a part of it is reflected back and a part of it is transmitted. 

Propagation through structures is dependent on the angle at which the 

ultrasound wave hits the structure i.e. the angle of insonation.

When ultrasound waves hit a structure, a part of the wave is reflected. 

If reflected off a smooth surface, specular reflection occurs, meaning 

the angle of reflection is equal to the angle of incidence. However, 

when it hits a rough surface, it scatters. The part of the scattered 

wave that returns back to the probe is termed ‘back-scatter’, while the 

remainder of the wave is transmitted in various directions. The 

back-scatter is directly proportional to the ultrasound frequency. 

Back-scatter is responsible for most of the ultrasound images produced 

during echo examination.

Another term to remember is acoustic impedance. Acoustic impedance 

is the product of propagation speed and density of the medium. At an 

interface between two media, there occurs an acoustic impedance 

mismatch. A higher mismatch enhances reflection and a lower 

mismatch enhances transmission of ultrasound waves.

As ultrasound waves pass through tissues, a part of its energy is lost. 

This loss of energy is called attenuation. The lost energy is reflected 

as a change in amplitude.