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THE US WAVE
WAVELENGTH
The US wave is a pressure wave. As it passes through tissues, it has mechanical and heating effects on the tissues. The mechanical effects are oscillation of tissue particles. The heating effect is due to a transfer of energy as the wave passes through.
As with any wave, an US wave has peaks and troughs. The length from one peak to the next trough is the wavelength of the US wave. A wavelength is usually measured in millimetres.
wavelength is the distance travelled by one cycle
PERIOD
The period is the duration of time for one wavelength to occur. A period is usually recorded in microseconds.
The period is the time taken for one wavelength to occur
FREQUENCY
Shorter wavelengths occurs in US waves with a higher frequency. This makes sense because frequency is the number of cycles (in this case wavelengths) per second. If something is of a higher frequency, there will be more wavelengths per second. Thus, the shorter the wavelength, the more wavelengths which can occur per second. Frequency is recorded for US in megahertz (MHz) or million cycles per second.
Frequency = propagation speed / wavelength
Low frequency US waves can penetrate deeper into the tissues than high frequency waves. However, because low frequency US has fewer wavelengths per second, the returning information is less detailed than a high frequency wave.
low frequency US has a deeper penetration at the cost of resolution, high frequency waves have better resolution at the cost of penetration
SPEED OF US
The US wave passes through tissues at a certain speed (propagation speed). In some tissues, it travels slowly and in other it speeds up. The average speed of US in tissues is 1540m per second.
The density and elasticity of a tissue affects how fast US moves through that tissue. Therefore very dense and non elastic tissues like bone allow the US waves to pass though quickly, whereas very loose and malleable tissues like fat slow down the passage of US.
The average propagation speed of US in tissues is 1540m/s
As the US moves through tissues it causes movement and heating. However, the tissues also affect the US wave by causing it to degrade (attenuation) and sometimes change direction.
ATTENUATION
This is where the US wave loses amplitude and intensity as it passes through tissues. High frequency waves attenuate faster than low frequency waves. This is why high frequency transducers have less penetration than low frequency transducers.
The US machine compensates for attenuation by enhancing the amplitude of echoes returning from deeper structures.
REFRACTION
Refraction is where the US wave changes direction at a tissue interface. It occurs when there is a difference in propagation speed of the two tissues. This leads to the US wave angling in a different direction to the original. Thus subsequent echoes from this wave will reach a different transducer crystal leading to misinterpretation and artefact formation.
"Glass" - Hamza Lafrouji: An example of refraction in real life.
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