If you have ever had an ultrasound examination during a medical checkup, you have probably seen gel applied to the skin before the probe is placed on the body. One purpose of this gel is to help the probe move smoothly across the skin, but that is not its only role. Ultrasound gel also plays an important part in helping ultrasound waves travel into the body.
In this article, I will first look at some basic wave principles that are useful for understanding ultrasound, and then explain why gel is used during ultrasound examinations.
This article is based on my own research and personal perspective. It is provided for informational purposes only and does not guarantee the accuracy of the information. Please make your own decision when purchasing or using any products mentioned.
What Is Ultrasound?
Ultrasound is sound with a frequency above the range of human hearing. A familiar example is medical ultrasound imaging. In nature, animals such as bats and dolphins are also known to produce high-frequency sounds that humans cannot hear and use the returning echoes to understand their surroundings.
So ultrasound is not an entirely different phenomenon. It is a form of sound. From a physics perspective, sound travels as a wave, and this type of wave is called a sound wave.
However, sound waves themselves are not visible. That can make the idea that “sound travels as a wave” somewhat difficult to picture. To make the concept easier to visualize, let us begin with waves on the surface of water and use them to review some basic properties of waves.
Basic Concepts of Waves
What Is a Wave?
When people hear the word “wave,” many probably think of ocean waves or ripples on the surface of bathwater. For example, if you drop a stone into a still pond, ripples spread outward from the point where the stone enters the water.
It may look as though the water itself is moving outward with the wave. In reality, however, the water does not simply travel outward along with it. A floating leaf provides an easy way to picture this: as a wave passes, the leaf moves up and down, but it is not steadily carried outward with the ripple.
Instead, the up-and-down motion at one point on the water surface is passed to the neighboring water, then to the next area, and so on. A wave is a phenomenon in which a vibration produced at one location is transmitted progressively to the surrounding area.
What Carries a Wave?
A wave does not transport the material itself, such as the water in a pond, from one place to another. Instead, the material vibrates in place, allowing energy to be transferred from one region to the next.
For example, when a person speaks, the vocal cords vibrate. These vibrations cause the surrounding air to vibrate as well. The vibration then passes from one region of air to another until it reaches the ear, where we perceive it as sound.
A material that carries a wave in this way is called a medium. For waves on the surface of water, the medium is water. For the sounds we normally hear in air, the medium is air. Sound can also travel through liquids such as water and through solids.
However, not all waves require a material medium. Radio waves and light, for example, are forms of electromagnetic waves. Unlike sound waves, electromagnetic waves can travel through a vacuum without air.
Outer space is an easy example. Sunlight travels through space to reach Earth, and radio waves are used to communicate with satellites.
So what about ultrasound? Although ultrasound may sound highly specialized, it is simply a sound wave with a frequency above the range of human hearing. Like other sound waves, ultrasound requires a medium in order to propagate.
In medical ultrasound, tissues such as the skin, fat, and muscle act as media through which ultrasound waves travel.
Longitudinal and Transverse Waves
How, then, does the medium actually vibrate as sound is transmitted?
Waves can be classified according to the relationship between the direction in which the medium vibrates and the direction in which the wave travels. The two basic types are transverse waves and longitudinal waves.
Transverse Waves
A transverse wave is a wave in which the medium vibrates perpendicular to the direction of wave travel. For example, if you hold one end of a rope and move it up and down, the rope itself moves vertically while the wave travels horizontally along the rope.
Direction of wave travel →
Direction of medium vibration ↑↓
Longitudinal Waves
A longitudinal wave is a wave in which the medium vibrates in the same direction as the wave travels. Longitudinal waves can be a little harder to visualize than transverse waves because their motion is not as obvious from the overall shape.
A familiar example is a long, rainbow-colored spring toy.
If one end of the spring is pushed, compressed and expanded regions move along the spring. Videos showing this type of motion can make longitudinal waves easier to visualize.
Here again, the spring itself does not move from one end to the other. Instead, each section moves back and forth, passing the vibration along to neighboring sections.
Direction of wave travel →
Direction of medium vibration ↔
The sounds we normally hear are generally transmitted as longitudinal waves. In air, particles move back and forth, creating regions where the particles are more closely packed together and regions where they are farther apart.
These regions of compression and rarefaction move from one area to the next, allowing sound to travel. Because ultrasound is also a form of sound wave, it propagates through the soft tissues of the human body in essentially the same way: as a longitudinal wave.
Quantities Used to Describe Waves
To understand ultrasound, it helps to know several basic quantities used to describe waves. The main ones discussed here are frequency, wavelength, and wave speed.
- Frequency (f)
Frequency describes the number of vibrations that occur in one second. The unit is the hertz (Hz). For example, if a wave vibrates 100 times per second, its frequency is 100 Hz.
The time required for one complete vibration is called the period (T). Because the number of vibrations that can occur in one second is 1 ÷ T, frequency can be expressed as:
f = 1 / T
- Wavelength (λ)
Wavelength is the length of one complete wave cycle. For a wave such as the one shown in the diagram, it can be understood as the distance from one crest to the next, or from one trough to the next.
- Wave Speed (v)
Wave speed is the speed at which a wave travels. For sound waves, wave speed varies depending on the medium through which the sound travels. The speed of sound therefore differs in air, water, and human tissue.
Frequency, wavelength, and wave speed are related by the following equation:
v = fλ
wave speed = frequency × wavelength
When a wave travels through the same medium and its speed remains constant, a higher frequency corresponds to a shorter wavelength.
This brings us back to the meaning of the word ultrasound. In acoustics and ultrasound, this quantity is generally referred to as frequency.
The range of human hearing is generally considered to be approximately 20 Hz to 20 kHz. Sound waves with frequencies above this range are called ultrasound.
In other words, the “ultra” in ultrasound does not refer to loudness. It means that the frequency lies above the range humans can hear.
How Ultrasound Imaging Works
How Ultrasound Is Reflected
With these basic wave concepts in mind, ultrasound may now be easier to picture. Ultrasound is a type of sound wave. It travels through body tissues as its medium, primarily in the form of longitudinal waves.
Medical ultrasound examinations use high-frequency ultrasound, typically around 1 to 20 MHz, which is well above the range of human hearing. One megahertz (MHz) is equal to one million hertz. For example, a frequency of 5 MHz corresponds to five million vibrations per second.
During an ultrasound examination, these high-frequency sound waves are transmitted into the body. Information about how they travel and reflect is then used to form images of internal structures.
One of the key principles involved is wave reflection. Reflection occurs when a wave reaches a boundary between different media and part of the wave returns.
When a wave reaches a boundary between materials with different physical properties, such as density, part of the wave is reflected while the rest continues through the boundary. The returning portion is called the reflected wave, while the portion that continues is called the transmitted wave.
A familiar example is an echo heard in the mountains. An echo occurs when sound waves are reflected and return to the listener.
So how does this reflection contribute to an ultrasound image?
During an ultrasound examination, the probe sends ultrasound waves into the body. The probe is the part placed against the skin that both transmits and receives ultrasound waves.
The ultrasound travels through tissues such as the skin, fat, muscle, and internal organs. When it reaches a boundary between tissues with different properties, part of the ultrasound is reflected while the rest continues deeper into the body. The reflected ultrasound then returns to the probe, where it is received.
The system uses information such as the time taken for the ultrasound to return and the strength of the reflected signal to create an image. The return time provides information about the depth at which the reflection occurred.
The strength of the reflection also varies between tissues, and these differences appear as variations in image brightness. In other words, ultrasound imaging uses information about where ultrasound waves are reflected and how strongly they are reflected to visualize internal structures.
If there were no reflected ultrasound at all, there would be no reflected signal from which to form an image. However, stronger reflection does not necessarily mean a better image.
When a large proportion of the ultrasound is reflected at one boundary, less remains available to travel deeper into the body. For example, if only part of the ultrasound is reflected and the rest continues deeper, information can still be obtained from tissues beyond that boundary.
But if most of the ultrasound is reflected at one location, much less reaches deeper structures. This makes it more difficult to obtain information from those deeper areas.
For ultrasound imaging, then, it is important not only that reflection occurs, but also where it occurs and how much of the wave is reflected.
Acoustic Impedance
Why does ultrasound reflect strongly at some boundaries but pass through others with relatively little reflection? One important factor is acoustic impedance.
In simple terms, acoustic impedance is a numerical representation of a material’s acoustic properties in relation to sound waves. Physically, it is determined by the density of the material and the speed of sound within that material:
Acoustic impedance Z = density ρ × speed of sound c
In this context, what matters most is not the absolute acoustic impedance of each material, but the difference in acoustic impedance between two adjacent materials.
At a boundary between materials with relatively similar acoustic impedance, most of the ultrasound continues forward and only part is reflected. At a boundary where the difference in acoustic impedance is large, a greater proportion of the ultrasound is reflected and less continues beyond the boundary.
Within the body, fat, muscle, blood, organs, and bone all have different acoustic impedances. As a result, the strength of reflected ultrasound varies at the boundaries between these tissues.
These differences contribute to variations in image brightness, allowing internal structures to be visualized.
Why Is Ultrasound Gel Necessary?
This brings us back to the ultrasound gel mentioned at the beginning of the article.
Even when the probe is placed directly against the skin, the skin surface has small irregularities. As a result, the probe and skin do not make perfectly continuous contact, and small pockets of air may remain between them.
This air can pose a major problem for ultrasound imaging. There is a large difference in acoustic impedance between air and human tissue.
As discussed above, when the difference in acoustic impedance at a boundary is large, ultrasound tends to be reflected more strongly.
So in a situation such as:
Probe
↓
Air
↓
Skin
strong reflection is more likely to occur at interfaces involving the air before the ultrasound enters the body.
As a result, less ultrasound travels deeper into the body, making it more difficult to obtain sufficient information from internal structures. Reflection that occurs after ultrasound has entered the body can be used to obtain information about internal tissues.
However, if strong reflection occurs before the ultrasound enters the body, less of the wave reaches the tissues beyond that point.
This is where ultrasound gel becomes important.
Applying gel helps reduce the amount of air between the probe and the skin, creating a pathway more like:
Probe
↓
Gel
↓
Skin
↓
Inside the body
Compared with having an air layer between the probe and the skin, this can help ultrasound travel into the body more readily.
Of course, the gel also helps the probe move more smoothly across the skin, but that is not its only function. Ultrasound gel also helps reduce air gaps between the probe and the skin, which can help reduce strong reflection before the ultrasound enters the body and help ultrasound waves travel into the underlying tissues.
Conclusion
In this article, I used ultrasound gel as a starting point to explore some of the basic properties of ultrasound and waves.
When people think of ultrasound examinations, they may picture devices used mainly in hospitals by physicians or clinical laboratory technologists. In recent years, however, smaller and more portable ultrasound devices, including pocket ultrasound systems, have also become available.
Their use is expanding beyond hospital wards, and nurses are increasingly using them as part of assessment in settings such as home care and home-visit nursing.
From my perspective, the ability to visualize internal structures with ultrasound may be particularly useful in home-care settings, where assessment often relies on limited information obtained through inspection, palpation, and auscultation.
When using medical equipment, it is important not only to know how to operate the device, but also to understand the basic principles behind it. For example: Why can structures inside the body be visualized? Why is gel necessary?
As we have seen, ultrasound imaging is based on physical principles such as wave propagation, reflection, and acoustic impedance.
I believe that taking a closer look at how familiar medical devices and care products work may deepen our understanding of both nursing practice and the technologies used in care.
This article is for informational purposes only and does not guarantee the accuracy of the information. Please make your own decision when purchasing or using any products mentioned.
This blog shares information mainly about assistive products and medical supplies from the perspective of medical and long-term care settings. In particular, I look at how products are perceived in frontline care settings and the situations in which they are likely to provide value.
Drawing on my hands-on nursing experience and knowledge of patents, I provide explanations of medical supplies and care products, technical article writing, product reviews, and support related to market development.
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For inquiries, please feel free to contact me through the form on this blog.
References
Kinji Tanikoshi. Tokoton Yasashii Choonpa no Hon [A Thoroughly Easy Guide to Ultrasound]. Nikkan Kogyo Shimbun, 2004.
Practical Care Information for Frontline Use
https://www.almediaweb.jp/expert/feature/2212/
