It has been more than 100 years since ultrasound has been 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 and physical properties like density and tension.
In medicine, ultrasound visualization is mainly used as a non-invasive medical imaging technique using high-frequency sound waves to produce images of internal body structures. It is commonly used to view and diagnose conditions related to pregnancy, the abdomen, and the heart. The images are created by transmitting sound waves into the body, which bounce off internal structures and are detected by a transducer, then processed by a computer to create real-time images. Ultrasound is generally considered safe and painless and does not involve ionizing radiation.
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 the 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, fibers, lesions, and organs. Those irregularities are detected due to different density areas.
Ultrasound waves can propagate in two different waves. One type is longitudinal,
Longitudinal ultrasound waves are characterized by particle vibrations in the same direction as the direction of wave propagation. These waves are often used in medical imaging because they can easily penetrate soft tissues and fluids, allowing for the visualization of structures such as organs and blood vessels. Longitudinal ultrasound waves can propagate through various media, including solids, liquids, and gases.
Transverse ultrasound waves, also known as shear waves, are characterized by particle vibrations perpendicular to the direction of wave propagation. Shear waves are typically used in industrial applications, such as testing the integrity of materials and detecting flaws in metals. They can propagate through solids and are sensitive to material properties such as stiffness and density.
In medical applications, transverse ultrasound waves can be used to measure the elasticity of tissues, which can provide information about the presence of disease or injury. These waves are typically propagated through soft tissues, such as muscles and tendons.
There are other subtypes of shear waves, Lamb, Love, and Rayleigh, that are different modes of ultrasonic wave propagation that are used in various applications:
The Lamb wave is a type of guided wave that propagates along the surface of a solid structure, such as a plate. It is commonly used in non-destructive testing (NDT) to detect cracks or other defects in structures such as aircraft wings or pipelines.
The Love wave is also a guided wave that propagates along the surface of a solid, but it is sensitive to changes in the medium above the surface. This makes it useful for biosensing applications, such as monitoring changes in the mass or viscosity of a fluid.
The Rayleigh wave is a type of surface wave that travels along the surface of a solid. It is commonly used in geophysical exploration to locate and characterize subsurface features such as oil and gas deposits.
Those are more complex topics and may need a separate post on each.