Basic Physical and Technical Principles of ultrasound

 
Physics of Ultrasound
Properties of Sound Waves :
Propagation characteristics:Sound waves have several essential properties:
Propagation of ultrasound waves: Sound waves travel through air, fluids, and human tissue almost exclusively as longitudinal waves. These are zones in
which the molecules that make up the medium are alternately rarefied and condensed. Thus, sound waves must propagate through matter and cannot
exist in a vacuum.
Propagation speed:
The speed of sound is relatively slow in all materials (in tissue about 1540m/s). Consequently, its transit time can be accurately determined by electronic measurements and correlated with the distance traveled by applying the time–distance principle.
Reflection(partial or complete)of sound waves at interfaces:
The degree of reflection of incident sound waves at an interface depend sont he acoustic resistance (“impedance”) of the medium:
– Impedance = the ratio of the incident sound intensity to the portion that is transmitted.
– Acoustic resistance= the product of the density times the speed of sound.
Doppler effect:
The Doppler effect states that the frequency of the returning (received) sound waves changes when the source ofthe sound is moving toward or away from the receiver.According to the time–distance law,the produc to ftime and velocity equals the distance traveled. Thus, the frequency changes in the sound waves reflected from moving red blood cell scan beanalyzed to determine the direction and velocity of blood flowing through vessels and in the heart
 

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