Basic Obstetric POCUS

Chapter 2. Understanding Sound and Ultrasound Basics

Before you can understand how to use ultrasound there are a few basic principles to learn about how sound works and is used to create ultrasound images.

A registered diagnostic sonographer must study comprehensive physics and instrumentation before becoming certified. This is not the case for the POCUS provider.

However, there are concepts that can be described in simpler terms to enable you capture and improve an image. These concepts will also explain safety parameters for scanning women and babies.

This chapter will review the most basic principles of sound and how it interacts with tissue to improve getting a meaningful ultrasound image. 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, sound is energy. It is created by a vibration that generates a wave form. That wave is propelled forward by the energy that initiated it.

Figure 1. Properties of sound

As the wave moves forward, it encounters a surface or substance, which can reflect it back to the source. How the sound travels, and the time it takes to return to the source, allows us to interpret the characteristics of what the sound encounters. It is essentially the “echo” back of the reflected sound wave that allows the ultrasound machine to calculate both the distance to the encountered substance, and the relative density of the substance.

Figure 2. 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, which is an innate sense used by animals, and to some extent by humans.

Figure 3. Animals that use echolocation

Ultrasound is used by dolphins and whales to locate food and predators. It is used by bats to locate insects and other objects. Sound waves are emitted from the animal, and when 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 the opposite - it is 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.

In ultrasound, the following events happen:

Sound waves are produced by a source at a frequency higher than the range heard with the human ear. A single wave is defined as one Hertz (Hz) or cycle per second. Ultrasound is defined as acoustic oscillation or sound wave greater than 20,000 Hertz (Hz) or 2 kilohertz (kHz) per second.

Ultrasound is used in a variety of fields of science, ships and submarines to detect seabed structures and other vessels under water.

How sound travels through the body

Figure 4. Illustration of sound reflecting from tissue layer back to the transducer

The first principle we will describe 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 wave is slowed or interrupted (called attenuation), 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 either transmitted through the tissue, or reflected back.

These concepts are important to know because they help interpret the image that is ultimately created on the screen of the ultrasound machine. Tissue characteristics have definite patterns seen on the ultrasound screen based on how they reflect back the sound to the ultrasound transducer. We will look at this in the coming sections.

Figure 5. Properties of how sound travels in the body

Properties of sound

Let’s stop and take a closer look at the properties of sound in general.

First, sound has to be generated, emitted, travel through time and a medium (air or other substance), and be reflected or deflected when encountering surfaces.

Figure 6. Sound wave reflected

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.

In some ways the properties of sound are similar to light, but the difference is the form of energy. Light is considered an electromagnetic wave and does not require any medium to be able to propagate. Sound is considered a mechanical wave since it actually moves something in its path.

Figure 7. Anatomy of a sound wave

Amplitude: A soundwave's amplitude, force or strength, is how high the waveform rises Amplitude will be discussed later in this chapter.

Speed of sound

There are principles of how the speed of sound is affected by transmission through various tissues. This concept is not necessary to memorize. All that is needed for you to understand is that the computer in the ultrasound machine compensates for these differences depending on the time it takes for sound waves to return to the transducer.

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

Frequency

Frequency is one of the important concepts for you to understand because it determines how far the sound wave can penetrate into the tissue. Various ultrasound probes use different frequency ranges for different purposes. For now, you will only be using one probe the one that comes with the Lumify machine. It is called a curvilinear probe. This will be described in more detail in the next chapter.

Let’s explain frequency.

Frequency means, simply, how many times something happens in a given period of time. In real life, the number of times we go to the store for food in a month is the ‘frequency’ of trips made in a month.

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”.

Figure 8. Illustration of sound wave frequency: 5 cycles per second.

By comparison, the image below (Figure 9) 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 9. Frequency of 10 cycles per second

Why do we need to know that? Frequency affects how deep the sound penetrates into the body, and how much detail it can reflect back.

Figure 10. How frequency affects sound travel.

F

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 a different length, and vibrates 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 opposite is true: longer strings create slower vibrations and lower notes.

Frequency has to do with the “pitch” of the sound. In the case of ultrasound we are not able to hear the sound since it is to high to be undetectable by human ears. But if we could hear the sound waves, frequency would determine the sound’s “pitch” (or a very high musical note).

Since sound occurs more often in much higher frequencies than 1 or 2 cycles per second, we have terms that describe these higher frequencies that are most applicable to human hearing and for use in medical ultrasound.

Figure 11. Description of terms used to describe ultrasound frequencies.

--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)

Ultrasound used in medical imaging is typically characterized by the KHz or MHz values. These values correlate with how well the sound penetrates certain tissues in the body. You will not need to memorize these; just understand what these terms indicate.

Figure 12. Comparison of frequencies between infrasound, human hearing, and ultrasound

WHY DOES THIS MATTER?

When using ultrasound, sound waves are generated and transmitted into the body. How that sound interacts with the tissue and returns signals all are affected by the following characteristics.

Frequency affects the depth of penetration of the ultrasound, as well as the image clarity or resolution. To get a clearer image of what we are trying to see in the body, we must understand how to use appropriate frequencies in the equipment to accomplish this.

As sound travels through the body, it encounters many bits of information along its path that are reflected back to the transducer. The greater number of waves in higher frequency causes greater detail to be seen on the screen. This is known as resolultion.

The disadvantage of higher frequency, though, is that it does not travel as far.

The higher the frequency, the shallower the penetration. The waves do not propagate very far since the energy required to sustain so many oscillations uses up the wave’s initial energy, allowing it to dissipate more quickly.

Figure 13. Review of low frequency versus high frequency.

What does this mean for an image? Using higher frequency gives us a clearer image (better resolution). But a higher frequency wave doesn’t travel very deep into the body, so we don’t visualize deeper structures well.

Lower frequency can penetrate deeper into the body or structure, but it creates wider spaces between its oscillations, so some of the tissue particles are not encountered/reflected as it travels. This means less clarity or resolution. But 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.

Low and High Frequencies

The illustration above 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 illustration 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.

Figure 20 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.

In summary, the reason for understanding frequency 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.

This becomes relevant when you are imaging and you cannot see deeper into the body, or when you are trying to increased the resolution or clarity of the image. We will discuss ways that the ultrasound machine can help with this.

This information also helps you to explain that the baby cannot “hear” ultrasound when it is performed; a common question that women have.

Knowing the frequency range of your transducer and machine will help you know if you are able to get an acceptable image, or send the woman to a provider that has different frequency probes.

Figure 14. Summary table of high frequency and low frequency

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.

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.

SOUND-TISSUE INTERACTION

The following concepts help describe how sound affects tissue and how tissue affects sound.

Sound Characteristics

We have already described frequency as the number of waves being generated. Now we will discuss other characteristics of sound that affect how it travels through the body.

Figure 15 . More sound wave properties.

Amplitude is an important concept to learn.

Represented in an illustration in Figure 24, amplitude is shown as the displacement or distance moved upward by a point measured from its resting position to the top of the wave (and below the resting place).

An energy source creates a wave with pressure, and that energy is transferred as the forward motion of the wave. In other words, it has to do with the strength of the force or energy that pushed it. That amount of initial pressure determines the height of that wave (or its amplitude). A low amplitude wave is created by a low energy source, and its forward energy continues to be low, dissipating faster. A high amplitude wave is created by higher energy. It perpetuates as higher energy, and if unobstructed, it travels a farther distance.

Figure 16. Low versus high amplitude energy

An example would be the description of ocean waves. The amplitude is a measure of the strength (or height) of the wave.

Figure 17. Amplitude of ocean waves.

Large ocean breakers churn up the shore more than small ones. Smaller waves occur, as well, but their lower amplitude carry less energy, and therefore do not move much sand on the beach. Undersea earthquakes produce large ground displacements pushing water upward and forward into large waves (known as a tsunami) that cause tremendous damage.

Just like ocean waves, sound waves have different amplitudes or strength. Loud sounds have high-pressure amplitudes and come from larger-amplitude source vibrations than soft sounds. Because they transfer more power or energy, they have increased ability to disrupt material in their path. This has relevance to ultrasound since greater power can cause increased risk of tissue disruption.

There is an important relationship between frequency and amplitude, but THEY ARE NOT THE SAME. In a sound wave, frequency is analogous to the pitch of the sound, where amplitude determines the energy (or loudness) of the sound. Pitch has to do with the frequency of vibrations of the sound waves (more vibrations, higher pitch, fewer vibrations, lower pitch).

Figure 18. Amplitude represents loudness.

When excessively loud music is encountered by the human ear, it can be painful due to the excessive mechanical vibrations of the eardrum caused by the sound waves. This tends to be caused by the volume of the sound and not the pitch.

Ultrasound amplitude describes how much energy or pressure was put into generating the sound wave. This has to do with how hard the transducer crystal is “struck” by the electric signal sent to it from the machine. Most ultrasound systems have a control that changes the size of the electrical pulse, which determines the amplitude or power

WHY IS THIS IMPORTANT?

The energy of the increased waveform size (greater amplitude) has the potential to create more alterations in the tissue it encounters. Most of these changes are not harmful, but in general, increased strength or energy sent into the body has a potential for creating:

—increased heat, called the thermal effect

—vibrations, called the mechanical effect that could affect tiny fetal tissue in the first trimester.

Both of these create potential safety concerns for fetal tissue

POWER

Power is a measure of the overall strength of the force that generated the sound wave, while amplitude reflects how much that power is transmitted forward in the waves. This is important to know because of how it influences safety. In other words, when delicate body tissues are encountered by ultrasound waves, we use only enough power needed to create a readable image.

Power is an important aspect of the use of sound and is defined as the rate of energy transferred along the wave, expressed as Watts (a unit of energy).

Example: The sun generates tremendous power. Rays from the sun dispersed through the air in an open field, for example, have strength to warm the whole area. No one area is overheated since it is dispersed over a very large area.

Figure 19. Difference between overall power (on the left) and intensity (concentrated power)

INTENSITY

Intensity is different than power. Intensity is where power is focused on a particular area. This is sometimes done in ultrasound to achieve a better image (when we select “focus”). It has to do with the specific area to which the beam is directed. By directing the sound beam to a smaller narrower area, energy exposure is concentrated (intensified) at the focus of the beam. The smaller the area of focus, the higher the intensity.

An example is the illustration above. If you take a magnifying glass and focus the beam of sunlight directly on an object, it can heat more significantly, causing tissue damage if aimed at a living thing. This shows that by increasing the beam focus area, the intensity is DECREASED (energy is spread out). By narrowing the beam focus there is a greater concentration of energy directed toward the focal point (INCREASED intensity).

Figure 20. Beam focus related to intensity

NOTE: Most ultrasound systems have presets determining power based on the type of scan intended. These ranges are well below the potential for causing damage to tissue, but the concept. Increasing power in the machine can increase the depth to which a sound beam can go, helping us to see deeper into the body. However, for POCUS, you should never need to change a power setting in the ultrasound machine. The Lumify has a feature that allows a user to do this, but it is not to be used routinely in POCUS for pregnancy.

WHY IS THIS IMPORTANT?

The energy of the increased waveform size (greater amplitude) has the potential to create more alterations in the tissue it encounters (heat and mechanical vibrations).

When you select focus to concentrate the ultrasound beam on a certain area, the energy of that sound beam is concentrated in that area. Generally this is not a risk to fetuses, but if the beam is focused in the same place for a prolonged period, it can significantly increase the temperature of the tissue being scanned.

We should be aware of the amount of potential power we are generating with ultrasound, and how much of that power or intensity is being directed to the tissues. This will be reviewed in more detail in the upcoming chapter on ultrasound safety.

The primary thing to remember here is that the depth of sound wave penetration is dependent on the frequency AND the amplitude or power of the signal. The person scanning has to balance use of increased power for better imaging with the potential for increasing risk to tissue.

Tissue characteristics affecting sound travel and reflection

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 either attenuated (slowed) or returned back as an echo are:

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

2) Rigidity (stiffness or elasticity)

Both affect the returning speed of sound (echo) as it is transmitted through the tissue and then back to the transducer.

How tissue affects sound speed and direction

Attenuation

The term attenuation is used to describe how a substance slows or diminishes sound transmission. The following are the ways this happens.

Reflection

Refraction

Scattering

Absorption

Attenuation is a general concept of what happens to sound as it encounters a substance or tissue. It means the loss or weakening of sound energy that is changed or inhibited as it propagates through the body.

Simply put, it describes how tissue “slows or stops” the sound wave going through tissue and subsequently the signal reflected back to the transducer.

A sound wave that is unobstructed as it passes through the body would yield a perfect image of an underlying structure. However, the body has many layers of tissue, and the sound has to penetrate those layers. Each layer and structure creates some form of attenuation as the wave passes through it.

Because sound energy dissipates as it travels further, there is not as much concern about power of an ultrasound beam persisting throughout all the tissue layers, since it diminishes.

Figure 21. Energy dissipation from attenuation.

This illustration depicts visually how sound interacts with layers of tissue. As sound is sent through various types of tissue, each layer reflects some of the sound, absorbs some of the sound, and some sound gets transferred through the layer to be propagated to the next layer. As the sound wave hits the next surface of tissue it repeats this cycle, but at lower intensity since some of the energy was dissipated by the first layer. This process happens over and over, with each successive layer absorbing more energy, until the sound has no more energy to reflect a wave back to the transducer.

Figure 22. How sound diminishes in reflection with successive layers

.

Ideally, the sound signal would travel indefinitely. But as it loses strength over time, it diminishes the sound reflection that returns to the ultrasound transducer. This must be compensated for by the ultrasound computer by augmenting the incoming returning signal.

Only about 1% of sound is reflected back to the transducer – the remainder is either attenuated, absorbed, or deflected.

Figure 23. Attenuation and what is seen on the screen.

Brightness or darkness on the ultrasound screen correlates with the strength of the returning signal (echo). If the returning signal from the reflection is strong, it stimulates the transducer crystal to create a strong electric signal, creating a bright pixel. Brighter images indicate greater reflection of sound waves as bounced back from a denser or more rigid structure. The opposite is also true: darker images on the screen mean that there is sound was slowed or “absorbed”. resulting in a weaker reflected signal.

So why does all this matter?

Attenuation limits imaging depth and returning signal strength, both of which can dramatically affect image quality. As this next diagram depicts, the deeper (or further) the sound travels, the less energy or amplitude it retains. The ultrasound machine is capable of compensating for the energy signal change due to attenuation, and factors that into the calculations of distance as it creates the image on the screen..

Review of the process:

Sound is created, it is sent or propagated, it encounters a substance, and that sound is returned as an echo. Some of the sound is re-directed, scattered, deflected, or absorbed, and do not return as direct echoes, as was previously described.

Ultrasound is generated by the transducer and transmitted into the body.

Sound waves encounter various tissue types. The type of tissue either absorbs the energy (where little is passed through the structure), or reflects it. The reflection can either be directly back to the transducer, or it can be diverted or scattered (refracted). These all affect the quality of the image being capture.

Figure 24. Process of transducer reception and creation of image on the screen

.

Reflections are responsible for the images created by the ultrasound computer. What we see on the screen is affected by how efficiently the signals return to the transducer from the body structure we are observing.

Reflection is the “echo back” of the sound wave from a surface, but perhaps at a slightly different angle (refraction).

Figure 25. Reflection is known as an ECHO

.

Reflection is often described as “echo “It is generally understood that an echo is the delayed “bounce back” of sound reflecting off a distant wall and returning to our ears to hear.

Reflection is the mainstay of ultrasound imaging. Reflected sound is what the computer translates into a visual image. Smooth tissue tends to reflect sound straight back to the transducer, creating a bright image. A rough-textured tissue interface, though, scatters the sound and a less distinct image appears.

—The angle of incidence (or angle of insonation) is the angle at which a transducer is held in relation to the surface of the tissue you are trying to image. Ideally the sound beam should be directed at a right angle to the structure you want to scan - not necessarily to the woman’s torso. This right angle reflection creates the best image. Angling the sound beam tangentially causes some of the sound beam to deflect in a different direction, worsening the image.

Figure 26. Angle of insonation.

Figure 27. Angle of insonation or incidence, showing deflection of sound that is not at right angles to the object

.

Refraction is the apparent bending of sound as it encounters certain materials or tissues. This simply means that the sound appears to bend in a different direction as it travels through differing media. It is similar to looking at an image above and below water (appears to be in a different plane or “bend”). So when the signal is returned, it may not represent the exact location of the strucure we are imaging.

Figure 28. Refraction or bending of sound waves.

This occurs because the speed of the sound is altered at the interface of 2 types of media, causing the image to reflect at an oblique angle.

Figure 11. Illustration of sound deflection causing refraction.

Scattering is reflection off an irregular or circular surface that causes waves to reflect at various angles and not directly back to the transducer. It often creates a “fuzzy” image instead of a clean white line. Smooth tissue tends to reflect sound straight back to the transducer, creating a bright image. A rough-textured tissue interface, though, scatters the sound and a less distinct image appears.3 Figure 10.

Figure 29 . Scattering of returning waves

.

Beam divergence is how beams are sent out from a curved transducer – the beams separate out as they go deeper, so resolution worsens.

Figure 30. Beam divergence with a curvilinear probe.

Absorption means that the energy is absorbed by the tissue, and converted to heat.When the sound waves encounter the medium or object, friction of the vibrating molecules produced by the sound wave energy absorb the energy.

Because of attenuation, only 1% of sound energy is reflected back to the transducer. This means that 99% of sound waves do not return to the transducer..

Figure 31. Conversion of heat by absorption of sound energy

.

.

From this we learn that the denser tissues like abdominal organs and bone, have the greatest absorption, and therefore the greatest potential for heat begin generated from the sound waves. The greatest danger is to send ultrasound waves for an extended period of time directed toward fetal bone (especially the fetal skull), since bone converts more energy to heat.

The illusration below (Figure 32) depicts how different body tissues absorb ultrasound, converting it to heat.

Figure 32. Comparison of heat absorption by body tissue type

.

This is an important principle to understand as it applies to how long you hold a transducer on a woman’s abdomen. Prolonged transmission of sound in the same position above fetal tissue can generate heat.

Figure 33. Summary of primary ways that sound is affected by the surface it encounters: absorption, reflection, and transmission.

Resolution – describes clarity of detail in the image

The goal of performing ultrasound is to obtain a clear and readable image of the body structure of interest so that appropriate clinical decisions can be made from what we see.

Resolution is simply the term that describes image clarity and how much detail can be seen. In ultrasound, it is determined by the ability to view two reflectors as separate from each other, and accurately display them. In ultrasound, resolution has to do with how well we can discriminate tissue details and differences while looking at an image. Just as with a higher-pixel camera, the clarity of a picture means that there is greater distinction between tiny particles. In the top image of this diagram, you can see clearly the details and boundaries of six metal balls. In the lower image, there is less clarity, and it appears that there are 2 objects – one small round, and another elongated structure.

Figure 33. Resolution

.

The ultrasound beam is made up of multiple smaller beams that fire in sequence. This happens with such rapidity that it is undetectable. The closeness of spacing between the beams and between the pulses determines how much detail the beam picks up along the way, affecting image clarity or detail.

Figure 34. High versus low frequency and resolution.

YOU MAY NOW TAKE THE QUIZ AND MOVE TO THE NEXT CHAPTER. YOU WILL NOT BE ABLE TO OPEN THE NEXT CHAPTER UNTIL YOU COMPLETE AND PASS THE QUIZ. YOU WILL HAVE AS MANY TRIES AS YOU NEED TO GET ALL THE ANSWERS CORRECT!

GULU Midwives Basic Obstetric POCUS

Chapter 2. Understanding Sound and Ultrasound Basics