Women’s Point of Care Ultrasound

Essential Concepts Made Clear

Chapter 2. Ultrasound Principles I: How Sound Works

This chapter will review the most basic principles of sound for the foundation of learning POCUS fundamentals. To understand how medical ultrasound works, it is important to examine principles of sound transmission in general. How sound travels and interacts with materials will be described. Sound has characteristics that are predictable, enabling us to use it in describing human tissue.

Objectives:

By the completion of this chapter you will be able to:

1. Describe properties of sound and sound transmission

2. Accurately define the terms of frequency and amplitude

3. Describe the ranges of frequency used for medical and diagnostic ultrasound

4. Apply the concept of frequency to depth of sound penetration

Simply stated, sonography is sound that begins with an energy that propels the wave forward. This sound encounters a surface or substance, and is reflected back. It is how the sound travels and is altered along the path, that allows us to interpret the characteristics of what the sound encounters.

Figure 1. Diagram of sound waves being transmitted and reflected as echo.

Ultrasound, or ultrasonography used in medical imaging, is a technique using high frequency sound waves and their echoes to locate structures and distances to those structures. The principle is known as echolocation, and it is an intuitive sense used by animals, and to some extent by humans.

Ultrasound is used by dolphins and whales to locate food and predators. It is used by bats to locate insects and other objects. Sound is emitted from the animal, and when sound is reflected back the animal’s senses can instinctively calculate the location, size and distance, based on the strength and direction of the returning signal.

Infrasound is actually the detection of very low frequency vibrations. It is what elephants employ to detect and measure the distance and scope of an approaching herd, using sensors in their footpads to detect low vibrations in the ground.

Figure 2. Animals with innate ability to use echolocation

In ultrasound, the following events happen:

Sound waves are produced by a source at a frequency above the range that can be heard with the human ear. Ultrasound is defined as acoustic oscillation or sound wave greater than 20,000 Hertz (Hz) or 2 kilohertz (kHz). The definition of Hertz (Hz) is a single wave per second.^1,2

Ultrasound is used in a variety of fields of science, including architecture, for example. It is used to help locate imbedded structures. Submarines and sea vessels have used the technique of ultrasound or sonography in detecting seabed structures and other vessels under water.

How sound travels in the body

The first principle is the speed of the sound going into and returning from the body tissues. This speed is affected by 2 things: how the sound is generated, and the type of tissue that sound encounters. Calculating this speed enables the ultrasound machine to measure distances to body structures.

The tissue type also affects the speed of sound - whether the sound is slowed, and by how much, as it travels through that structure.

The density (or compactness of molecules) of tissue, or its rigidity (stiffness or elasticity) affects how the sound is transmitted through the tissue or reflected back. These principles will be explained in more detail in the next chapters.³

Properties of sound

All waves have a beginning. They are created by a source of energy that forms the pressure wave (usually from a vibration or power surge), and its forward movement is perpetuated by the energy transferred through the substance it encounters; the waves hit molecules encountered along the wave’s path. For example, when we speak, our vocal cords vibrate and create a sound wave that is sent into the air. The sound is transmitted or propagated through the air, to be received by our own or another person’s eardrum, or through the bones in our own skull, which helps us to hear our own voice.

Sound requires a medium through which to travel – in other words, sound cannot be heard in a true vacuum. If there are no molecules to vibrate from the wave, then the sound does not continue to propagate. In the absence of molecules, such as the vacuum in outer space, hearing is not possible. Although sound principles are similar to light, the difference is the form of energy of a light wave. Light is considered an electromagnetic wave, while sound is considered a mechanical wave – since it actually moves something.³

How sound travels in the body depends on the characteristic of the sound, affected by how the sound is generated. How the sound travels also depends on the type of tissue that sound encounters.³ This is reviewed in more detail in the next chapter.

Anatomy of a sound wave

Sound travels, but it takes time to do so. These terms are used to describe the characteristics of sound travel.¹ The term used to describe the travel of sound is propogation.

Basic Terms

Wavelength: Sound waves are measured in terms of wavelength. A wavelength is a unit where a single sound wave completes a full cycle of oscillation (Figure 1.)

Figure 1. Illustration of a sound wavelength.

Period: The term “period” refers to the length of time that it takes to complete one wavelength (Figure 2.)

Figure 2. Illustration of a sound wave period and amplitude.

Amplitude: A soundwave's amplitude, or strength, is how high the wavelength rises (Figure 2.) Amplitude will be discussed in more detail in the next chapter

Speed of sound

Sound travels at different speeds depending on the type of substance it has to travel to. Some textures slow (attenuate) the speed of the sound as it travels to and from the tissue.¹ Because of variations, the ultrasound machine attributes a speed of sound by calculating an average (taking into account all the different speeds encountered in the human body). This number is 1540m/s. Knowledge of this number has more to do with application to various ultrasound physics calculations, and not necessarily a number to remember for clinical scanning.

Calculating this speed enables the ultrasound machine to measure distances to body structures.

This will be reviewed again in greater detail, but essentially the computer measures the time it takes to send a sound signal and the time it receives the returning echo. This is the “round trip” measurement. Based on the average speed of sound, the distance is then calculated and plotted visually on the screen.

The tissue type also affects the speed of sound, measuring if the sound is slowed, and by how much, as it travels through the body. The 2 most important factors in how sound is attenuated (slowed) or returned back as an echo are:

1) Density (or compactness of molecules) of tissue, and

2) Rigidity (stiffness or elasticity) also both affect the returning speed of sound (echo) as it is transmitted through the tissue and then back to the transducer.

Frequency

Frequency means, simply, how many times something happens in a given period of time. In ultrasound, frequency is defined as how many cycles (waves) occur per second, or how many times the wave oscillates per second. The following image illustrates 5 waves per second, which means a frequency of “5”.^1,2

^{Figure 3.}Illustration of sound wave frequency: 5 cycles per second.

By comparison, the image below shows 10 cycles occurring during the same time frame as the slide with 5 wavelengths. Ten cycles per second is a higher frequency than 5 cycles. For that to happen, each wavelength must be much shorter for all of them to fit into that timeframe (half the length, in this case).

Figure 4. Illustration of sound wave frequency: 10 cycles per second.

If you increase the frequency, you shorten the wavelength; the more cycles in a second, the shorter the wavelength has to be. The opposite is also true – if you reduce the frequency, the wavelength increases.

Differences in frequency

Frequency is determined by the source that generated the sound. For example, a piano string is caused to vibrate by striking it. Each string within the piano is adjusted, or tuned, to vibrate with a specific resonant frequency. This creates the “note” or “tone” that we hear. A shorter string creates a higher pitched sound (which means it vibrates more frequently). The converse is true concerning longer strings and lower notes. In the same way, on a guitar, string length is shortened or lengthened by pressure from the fingers to create a higher frequency or pitch. In the human voice, the shortening or lengthening of our vocal cords cause changes in the frequency, causing us to produce high or low sounds. Frequency has to do with the “pitch” of the sound. In the case of ultrasound we are unable to hear the sound since it is undetectable by human ears. But if we could hear the sound waves, frequency would determine the sound’s “pitch”.⁴

To repeat: Frequency is the number of vibrations that occur in a unit of time (usually a second)^1,2,4

--A single wave or vibration is termed a “cycle” – so there is one cycle per second. The technical term for this is a Hertz (Hz)

--One thousand cycles per second is called Kilohertz (KHz)

--One million cycles per second is called megahertz (MHz)

Imaging ultrasound is expressed in megahertz (MHz)

SIDEBAR: Definitions of sound frequencies³

NOTE: These concepts are necessary to remember when learning ultrasound.

Importance of the Principle of Frequency in Ultrasound

Frequency is an important concept to remember because it will be applied during most every scan that is performed. It determines how sound travels through the body, and how many bits of information it encounters along its path that are reflected back to the machine. It impacts the depth to which the sound travels. Understanding frequency will enable you to choose the correct transducer for the depth of structure you are trying to image.

The higher the frequency, the shallower the penetration.¹ The waves do not propagate very far since the energy required to produce that many oscillations per second reduces the energy of the wave, allowing it to dissipate more quickly.

What does this mean for an image? Using higher frequency gives us a clearer image (it meets more molecules along its path that reflect back to the transducer) – this is called resolution. But a higher frequency wave doesn’t travel very deep into the body.

Use of lower frequency creates wider spaces between its oscillations, so some of the pieces of tissue structures are not encountered/reflected as it travels. This means less clarity or resolution. However, the advantage is that the wave penetrates deeper into the body. The sound wave travels farther since it essentially retains more energy to keep the sound going further. These concepts will be made clearer in the next chapter.

Low and High Frequencies

Figure 4. Comparison of low frequency and high frequency sound.¹

The top image demonstrates that a low frequency is a “deeper” sound and it penetrates further into the tissue since its energy is sustained longer (less attenuation or slowing).

The bottom image demonstrates high frequency. Because there are more waves per second, the energy dissipates faster due to greater slowing (attenuation), and so the wave beam does not penetrate very deep.^1,4

Figure 5. shows a summary of the different types of sound and the ranges of frequency. This will help you understand the relative variation of ultrasound in comparison to other sound types. Compared to the range of sound that a human can hear, anything above that frequency is ultrasound, and anything below that range is infrasound.³

^{Figure 5. Uses of sound correlated with sound frequency}

What is medical ultrasound

Medical ultrasound imaging is the use of high frequency sound to “view” structures in the body. This differs from therapeutic ultrasound (which is used as a healing therapy by generating heat to promote healing of tissue). In the context of this learning series, medical ultrasound will refer only to its use as an imaging modality. The frequency range required for this penetration varies from 1 to 15 MHz. There are some transducers that use higher frequencies for specialized diagnosis (very superficial structures). Remember the principle that the higher the frequency, the shallower the penetration.

As ultrasound energy is sent into the body, the sound echoes bounce off internal structures at different speeds. Some of the sound waves are reflected back to the probe, some beams scatter, and still other beams travel deeper until they reach another boundary, and then get reflected back toward the transducer. The transducer records the differences in speed of returning echoes and translates those into a visual image. The reflected waves are sensed by the probe, translated into electrical signals, and relayed to the machine that interprets the data.

Greater detail on this process will be described in the next few chapters.

Figure 6. Ultrasound spectrum for imaging body structures.⁵

In summary, the reason for the different MHz levels is to be able to correctly penetrate to the right depth of the body structure to be viewed, while getting the clearest image.

The shorter the sound wave, the higher the frequency – and the shallower it will penetrate. However, the image will be clearer. The longer the sound wave, the lower the frequency – and the deeper it will penetrate. However, the image will NOT be as clear.

Figure 7. Illustration of how ultrasound is interpreted by an ultrasound machine.³

How is ultrasound interpreted by an ultrasound machine?

How does sound translate into a picture? How does the machine know where to place the pixels in the image? The ultrasound computer measures the time it takes for the sound wave to leave the transducer, as well as the time it takes for that sound wave to be reflected back to it (or the time it takes to travel to the next layer and then be reflected back).

It is this “time” factor that allows the computer to calculate actual distances and measurements. This process happens for every single beam coming from the transducer, taking just millionths of a second to process.

How does the computer calculate time to and from the tissue? It uses the AVERAGE of all the different speeds known to encounter various tissues (the average is 1540 m/second). It then calculates the time that it took for the echoed signal to return, knowing the speed, and this is translated into distance.

To review: when waves are reflected back to the probe, they are detected by the probe and the electric information from the probe is relayed back to the machine’s computer.

The machine calculates the distance from the probe to the tissue or organ (boundaries) by measuring how long it took for the sound to travel to and from the structure.

The machine translates those electronic signals into pixels and displays the distances and intensities of the echoes on the screen, forming a two-dimensional image combined of many thousands of pixels. ^1,2

Figure 8. Illustration of how ultrasound echo depth is interpreted by the ultrasound machine and shown on the display.

The diagram above is a simplified example of how the distance is translated by the computer into pixels on the screen demonstrating the relative distance to the several different structures

The time it took for the sound to return from the pink structure was less, so the pixels were plotted closer to the top. The time it took for sound to return from the green object was longer, so it is displayed further away from the top, and the same with the blue structure

All the combined sound waves create a composite image of all the structures that reflect the sound.

This diagram is a simplified example of how the distance is translated by the computer into pixels on the screen demonstrating the relative distance to the several different structures

The time it took for the sound to return from the pink structure was less, so the pixels were plotted closer to the top. The time it took for sound to return form the green object was longer, so it is displayed further away from the top, and the same with the blue structure

All the combined sound waves create a composite image of all the structures that reflect the sound.

Summary- Sound frequency

Sidebar: High versus low frequency summary.

Foundations of Women's Point-of-Care Ultrasound

Chapter 2: Ultrasound Principles I : How Sound Works