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USG Basics, Part 1: Key Principles in Medical Ultrasound Physics

Updated: Dec 7, 2023

Ultrasound Physics Dr Debjyoti Dutta

Article Highlights - Discover the key principles of medical ultrasound in this informative blog post on USG Basics. Learn about sound wave properties wavelength, frequency, period, speed, and amplitude

Introduction -

Sound is a type of wave that carries energy through a medium by making its particles vibrate. The medium can be a solid, a liquid, or a gas. Different media have different properties that affect how sound behaves. One of these properties is stiffness, which is how much a medium resists being deformed. The stiffer the medium, the faster sound can travel through it. For example, sound can travel about 15 times faster in metal than in air, because metal is much stiffer than air. Another property is density, which is how much mass a medium has in a given volume. The denser the medium, the slower sound can travel through it. For example, sound can travel about four times slower in water than in air, because water is much denser than air.

When sound encounters a boundary between two media with different properties, some of the sound will be reflected back to the original medium, and some of the sound will be transmitted to the new medium. The amount of reflection and transmission depends on the difference in acoustic impedance between the two media. Acoustic impedance is a measure of how much a medium opposes the passage of sound. It is calculated by multiplying the density and the speed of sound in the medium. The greater the difference in acoustic impedance, the more of the sound wave is reflected. For example, at the boundary between air and glass, about 96% of the sound wave is reflected, because air and glass have very different acoustic impedances. At the boundary between water and glass, only about 2% of the sound wave is reflected, because water and glass have similar acoustic impedances.

The reflected sound waves can be detected by the human ear or by a device. The detection of reflected sound waves is called echolocation. Echolocation is used by some animals, such as bats and dolphins, to navigate and hunt in their environment. Echolocation is also used by humans for various purposes, such as measuring distances, locating objects, and imaging structures. One of the most common applications of echolocation is ultrasound imaging. Ultrasound imaging uses high-frequency sound waves to create images of internal organs, tissues, and blood vessels. Ultrasound imaging works by sending pulses of sound waves into the body and receiving the echoes that bounce back from different boundaries. The echoes are then processed by a computer to form an image on a screen. The image shows the shape, size, and position of the structures inside the body.

Ultrasound imaging relies on the principles of sound wave reflection and transmission. The quality of the image depends on the acoustic impedance of the media involved. For example, at the boundary between fat and muscle, a little sound is reflected, because the difference in impedance is small. This means that most of the sound is transmitted to the muscle, and the image shows a clear contrast between the two tissues. At the boundary between muscle and bone, a lot of the sound is reflected, because the difference in impedance is large. This means that most of the sound is bounced back to the device, and the image shows a bright spot where the bone is. Images created by an ultrasound machine depend on these principles.

Sound has a frequency, which is how many times it repeats in a second. We use Hertz (Hz) to measure frequency. 1 Hz means 1 cycle per second. Ultrasound is a sound that has a very high frequency, above 20,000 Hz. Doctors use ultrasound to see inside the body. They use sound waves that have a frequency between 2,000,000 Hz and 15,000,000 Hz. When ultrasound waves go through a medium, they make the pressure change. This makes the molecules move closer together or farther apart. We call these areas compression and rarefaction. Compression is when the pressure is high and the molecules are dense. Rarefaction is when the pressure is low and the molecules are sparse. The ultrasound device sends pulses of sound that create compression and rarefaction in the medium.

Characteristics of Sound Wave

Properties of Sound Wave -   wavelength, frequency, time period, speed and amplitude
Sound waves possess various properties such as wavelength, frequency, time period, speed, and amplitude.

Frequency - A wave’s frequency is how many cycles it completes in one second. Waves that complete more cycles in one second have a higher frequency than waves that complete fewer cycles. We measure frequency in Hertz (Hz). 1 Hz means one cycle per second.

Wavelength - The wavelength is how long a wave is from one peak to the next or from one trough to the next. We measure it in units of distance. To find the speed of sound, we multiply the wavelength by the frequency of the wave.

Amplitude- The amplitude is how far a wave goes up or down from the wave node, which is the middle line. It is related to the height of the wave, or how much the particles of the medium vibrate when the sound passes through. The higher the amplitude, the louder the sound. The lower the amplitude, the softer the sound. The wave node has an amplitude of zero. The amplitude changes the acoustic pressure, which is how much force the wave has. We measure the acoustic pressure in Pascal.

Wave velocity -The wave velocity is how fast the wave moves through a certain medium. We measure ultrasonic velocity in meters per second and we find it by multiplying frequency and wavelength. We can also use millimetres per microsecond (mm/microsecond) to measure the speed of sound in the human body. This is the same as meters per second.

Interference -Interference is what happens when waves meet. Waves can be in sync or out of sync. When two waves are in sync, they match up perfectly and make a bigger wave. This is called constructive interference. When two waves are out of sync, they don’t match up and make a smaller wave or no wave at all. This is called destructive interference. Destructive interference happens when two waves have the same frequency and wavelength but are out of sync or opposite to each other.

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