Ultrasound imaging is a method that examines tissues and organs within the body using high-energy sound waves.
Ultrasound is defined as any sound with a frequency of more than 20,000 Hertz (or 20 kilohertz). This is above the maximum audible frequency.
Ultrasound, like any wave, has features such as frequency and intensity. The accuracy of the detail that an ultrasound can detect is limited by its wavelength.
The wavelength is the distance between a wave’s consecutive crests and is inversely proportional to the frequency of the wave.
So, why is the wavelength important? It’s important because we can’t observe details that are smaller than the wavelength of the wave used to probe an area. A great example of this is us not being able to see individual atoms with visible light because atoms are much smaller compared to the wavelength of light.
Schematic diagram of sound waves
Ultrasound imaging definition
Ultrasound imaging (sonography) is a technology that uses high-frequency sound waves to examine the inside of a body.
Because ultrasound images are captured in real-time, they can reveal internal organ movement and blood flow through blood vessels. Pictures are created when sound waves are sent into the body and reflected back to a scanner.
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!):
\(\lambda=\frac{v_w}{f} = \frac{1540}{7 \cdot 10^6} = 0.22 mm\)
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
A transducer – a crystal that exhibits the piezoelectric effect – emits ultrasonic waves in ultrasound imaging. The piezoelectric effect occurs when a voltage is applied across a material. This material expands and contracts, causing the crystal to vibrate.
Any tissue in touch with the transducer receives these high-frequency vibrations. Similarly, applying pressure to the crystal (in the form of a wave reflected off tissue layers) produces a voltage. As a result, the crystal functions as both a sound transmitter and receiver.
Ultrasound is absorbed by tissue in its pathway. 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.
An ultrasound technician
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/m2s). Here is the equation:
\[Z = \rho \cdot v\]
Here, p is the density of the medium in kg/m3, and v is the speed of the sound through the medium in m/s.
The table below shows the density, acoustic impedance, and speed of sound through various mediums.
Medium | | | Acoustic impedance [kg/m2⋅s] |
Air | 1.3 | 330 | 429 |
Water | 1000 | 1500 | 1.5 ⋅ 106 |
Blood | 1060 | 1570 | 1.66 ⋅ 106 |
Fat | 925 | 1450 | 1.34 ⋅ 106 |
Muscle (average) | 1075 | 1590 | 1.7 ⋅ 106 |
Bone | 1400 - 1900 | 4080 | 5.7 ⋅ 106 to 7.8 ⋅ 106 |
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:
\[a = \frac{(Z_2 - Z_1)^2}{(Z_1 + Z_2)^2}\]
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:
- If it equals zero (mediums with the same acoustic impedance), there will be no reflection.
- If this value increases, the intensity of the reflection also increases, and the image will be closer to white.
- If the ultrasound wave doesn’t come up against a medium with a different acoustic impedance, it doesn’t reflect back, and there is no echo. As a result, the image on the screen will be black.
- When the ultrasound wave comes across a medium with a different acoustic impedance, the ultrasound picture will be white or grey depending on the strength of the reflection.
Unlike X-rays or CT scans, ultrasound imaging cannot identify tissue density. Instead, it looks for sonotransmission (the passage or reflection of sound).
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.
| Purpose |
Abdominal ultrasound | Visualise abdominal tissues and organs. |
Bone sonometry | Assess bone fragility. |
Echocardiogram | View the heart. |
Doppler ultrasound | Visualise blood flow through a blood vessel and organs. |
Doppler foetal heart rate monitors | Listen to the foetal heartbeat |
Foetal ultrasound | View the foetus in pregnancy. |
Ultrasound-guided biopsies | Collect a sample of tissue. |
Ophthalmic ultrasound | Visualise ocular structures. |
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.
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