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.

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