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Ultrasound waves and propagation

It is more than 100 years when ultrasound is used for analyzing materials and echolocation. A bit later it was adapted for medical diagnostics. Ultrasound became popular and widespread due to the number of benefits. Probably one of the essential features is that it is non-destructive. The other is simplicity and precision. Of course when we talk about price and simplicity we have X-ray or tomography in mind. Ultrasound can be used in many areas and environments. You can measure geometrical properties of internal structures, physical properties like density, tension. In medical diagnostics, ultrasound is mostly used for visualization of tissues and inner organs.

(source https://www.ultrasound-images.com/vascular.htm)

Ultrasound waves and parameters

Ultrasound waves are mechanical waves that propagate through media. In solid materials, ultrasound waves propagate faster while in liquids and biological tissues slower (~1400 m/s). In air due to low-density sound waves propagate relatively slowly (~340 m/s). Ultrasound speed depends on temperature and pressure.

Ultrasound wavelength is strictly tied to its speed and frequency:

λ = v/f;

where v – ultrasound speed inside material; f – frequency.

Since ultrasound is mainly used for the visualization of structures inside materials its resolution depends on wavelength. Usually, it is possible to detect λ/2 – half-wavelength size objects. It may seem logical to increase ultrasound frequency to get better resolution and sensitivity, but in practice, the other limiting factors appear. Higher frequency ultrasound is strongly attenuated; this is why penetration depth is minimal. There is always a compromise between resolution and scanning depth.

Ultrasound visualization is mainly based on its ability to reflect from any irregularities in media. If we take biological tissue, those are blood vessels, fibres, lesions, organs. Those irregularities are detected due to different density areas.

Ultrasound waves can propagate in two different waves. One type is longitudinal,

longitudinal-wave

(source)

In the case of longitudinal waves, the particles are oscillating along the wave propagation axis. Usually, longitudinal waves are used in solid, liquid and gases.

The second type of waves is transverse (shear) when particles oscillate perpendicular to wave propagation direction. Those are used in solid object analysis and not possible on liquids or gases.

We are not focusing on other subtypes of shear waves that include various types of surface waves like Lamb, Love, Rayleigh – as this is already a broad topic.

Next time we will focus on ultrasound propagation principles.

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