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 wide spread due to number of benefits. Probably one of the most important feature is that it is non destructive. Other that follow are 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 diagnosis ultrasound is mostly used for visualization of tissues and inner organs.
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 wave length 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 visualization of structures inside materials its resolution depends on wavelength. Normally it is possible to detect λ/2 – half wave length 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 very limited. 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, fibers, lesions, organs. Those irregularities are detected due different density areas.
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Ultrasound waves can propagate in two different waves. One type is longitudinal
In case of longitudinal waves, the particles are oscillating along wave propagation axis. Normally longitudinal waves are used in solid, liquid and gases.
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 wide topic.
Next time we will focus on ultrasound propagation principles.