ULTRASOUND IMAGING : A DIAGNOSIS BY SOUND WAVE

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 It is also called ultra Sonography (USG).

Sound is a form of energy resulting in form transmission of mechanical vibrations through a  medium.

Ultrasound is a sound having frequencies greater than20,000 Hz, which can't be detected by human ear.

Ultrasound used to formation of images from static and moving organs.



Basic of ultrasound 

Propagation of ultrasound

US is produced by piezoelectric effect and it travels in straight line in a medium, in the form of wave with compression and rarefaction.
Diagnostic ultrasound uses 1-20 MHz frequency.

Sound waves travel faster in solids and slower in gases.
The average velocity in soft tissue is 1540 m/sec.

The intensity of ultrasound is a measure of beam energy and is proportional to the square of the amplitude. It expressed in watts per mm².

Relative intensity is measured in a lograthmic scale in bels .
                    1B = 10 decibels (dB )

Interaction of ultrasound with medium 

When the transducer is placed in contact with the patient, the particles of the organ begins to vibrate backward and forward about their mean position.
The ultrasound wave interacts with a medium and undergoes
 reflection, 
refraction, 
absorption and
 scattering.
Reflection may be a boundary or tissue reflection  useful to produce image.

Reflection

Reflection is divided into specular and non specular reflection.
A specular reflection is one which ultrasound strike a large smooth boundary, where structure size > wavelength.
this is major contributor of ultrasound imaging.
Ultrasound obeys snell's law.

Refraction

Refraction describes the change in direction of the transmitted US beam, when passing from one medium to another.
Refraction effects gives rise to artifacts 
It is not use ful for imaging.

Attenuation

Attenuation is the loss of acoustic energy with distance, and it is exponential in nature.
It is due to absorption and scattering of ultrasound beam in the medium.
Attenuation produce shadowing.


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