Ultrasound imaging is a technique used to examine the organs and tissues inside the body using sound waves with a frequency higher than 20,000 Hertz (or 20 kilohertz). This frequency is too high for us to hear.

The accuracy of the details that ultrasound can detect is determined by its wavelength. The wavelength is the distance between two consecutive crests of a wave and is inversely proportional to the frequency of the wave. This means that we can't observe details that are smaller than the wavelength of the wave used to probe an area. For example, we can't see individual atoms with visible light because atoms are much smaller compared to the wavelength of light.

Ultrasound imaging definition

Ultrasound imaging, also known as sonography, is a cutting-edge technology that utilizes high-frequency sound waves to examine the inside of a body. The most amazing thing about ultrasound is that it captures images in real-time, allowing doctors to observe internal organ movement and flow through blood vessels.

How does it work? Well, pictures are created when sound waves are sent into the body and then reflected back to a scanner. This creates a visual representation of the internal organs and tissues, which can then be analyzed by a doctor or radiologist.

Ultrasound imaging is used for a variety of purposes, from detecting medical conditions to monitoring the growth and development of a fetus during pregnancy. It is a safe and non-invasive technique that does not require exposure to ionizing radiation, makes it a preferred choice for many medical professionals.

If you're interested in learning more about ultrasound imaging, we recommend consulting with your doctor or healthcare provider to see if it's right for you.

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Calculating depth with ultrasound imaging

How deep can ultrasound imaging scan? It depends on the frequency of the wave.

For example, a frequency (f) of 7 megahertz (MHz) is commonly used for abdominal scans. If the speed of sound in tissue (vw) is around 1540m/s, we can calculate the wavelength (λ) of the ultrasound as follows (don’t forget your conversions!):The prevailing assumption is that ultrasound imaging can scan tissue to a depth of roughly 500λ. That is 500 ⋅ 0.22mm = 0.11m for 7MHz.

Lower frequencies can scan larger depths in the body but with less resolution. Higher frequencies can produce better resolution but have a restricted scanning depth.

The physics of ultrasound imaging

The piezoelectric effect is a phenomenon where a material, such as a crystal, expands and contracts when a voltage is applied across it. This causes the crystal to vibrate, emitting ultrasonic waves in ultrasound imaging. The transducer, which is a crystal that exhibits the piezoelectric effect, is the hand-held part of the ultrasound machine that is responsible for the production and detection of ultrasound waves. It consists of five main components: a crystal/ceramic element with piezoelectric properties.

When the transducer emits the ultrasonic waves, any tissue in contact with it receives the high-frequency vibrations. In addition, when pressure is applied to the crystal in the form of a wave reflected off tissue layers, it produces a voltage, allowing the crystal to act as both a sound transmitter and receiver.

Ultrasound is absorbed by tissue in its path, and the duration between the transmission of the initial signal and the reflections received from different barriers between mediums is used to determine the type and location of each boundary between tissues and organs.

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The colours in ultrasound imaging

How do the black, white, and grey colours occur in ultrasound imaging? This happens via a characteristic called the acoustic impedance Z (measured in kg/m2⋅s). Here is the equation:

The table below shows the density, acoustic impedance, and speed of sound through various mediums.

The intensity reflection coefficient (a) is the ratio of the reflected wave’s intensity to the incident (transmitted) wave’s intensity. We can express this mathematically as follows:

Z1 and Z2 are the acoustic impedances of the two mediums making up the boundary (the border between two different tissues). We can use the intensity reflection coefficient to determine the reflection’s intensity:

Unlike X-rays or CT scans, ultrasound imaging cannot identify tissue density. Instead, it looks for sonotransmission (the passage or reflection of sound).

Check out our explanations on Diagnostic X-Rays and CT Scanners.

What are the applications of ultrasound imaging?

Ultrasound is used in various applications, including burglar alarms, cleaning sensitive objects, and bat navigation systems. In medicine, it is widely used for diagnosis and therapy. The following table shows common ultrasound imaging procedures in medical physics.

Ultrasound Imaging - Key takeaways Ultrasound imaging is a method that examines tissues and organs in the body using high-energy sound waves. Ultrasound is defined as any sound with a frequency of more than 20,000Hz (or 20kHz).The accuracy of the detail that an ultrasound can detect is limited by its wavelength. Depending on the strength of the reflection, an ultrasound will be grey for low reflections and white for high reflections. The acoustic impedance (Z) of tissue is a physical characteristic. Different tissues have different acoustic impedances. Ultrasound is widely used in medical physics, particularly for diagnosis and therapy.

Ultrasound Imaging

What is ultrasound imaging used for? 

Ultrasound imaging is used in a wide range of applications, including burglar alarms, cleaning sensitive objects, and bat navigation systems. However, it is most commonly used in medical fields for both diagnosis and therapy.

What is the principle of ultrasound imaging?

The diagnostic ultrasound, also known as a sonography test, converts reflected sound energy into pictures using the Doppler effect or echoes.

What is ultrasound imaging?

Ultrasound imaging is a method that examines tissues and organs within the body using high-energy sound waves.

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