Women’s Point of Care Ultrasound

Essential Concepts Made Clear

Chapter 3. USD Principles II: How Sound Interacts with Tissue

This chapter will continue to build on the concepts just learned about sound and its properties. It is divided into three sections having to do with three different properties on how sound interacts with tissue.

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

Define propagation, attenuation, and impedance, describing how these all affect the ability of ultrasound to travel through the body
Describe the types of attenuation that commonly affect the use of ultrasound: reflection, absorption, refraction
Describe resolution and how ultrasound frequency affects it
Understand the basic concept of Doppler effect as it applies to Doppler mode in ultrasound

How sound travels relies on the properties of sound. All of these are important concepts to learn since they affect how ultrasound is performed. Concepts included in this Chapter:

1. Amplitude describes the strength of the source creating the wave energy.

2. Power describes the energy used to create wave and how that energy is received by the medium

3. Intensity describes how the sound impacts tissue at its area of focus.

Propagation speed describes how fast the sound travels through the tissue, which is affected by the tissue it encounters and how it is slowed or attenuated.

Amplitude

Next to frequency, the second most important characteristic of sound to understand is amplitude and what it represents.

Amplitude describes the height (or strength) of the wave and is a result of the amount of energy that initiated the wave. It describes the “bigness” of the wave.¹

Sidebar 1. Amplitude compared to ocean waves.¹

An energy source creates the wave with pressure, and that energy is transferred with the forward motion of the wave. That amount of pressure determines the height of that wave (or its amplitude).

Represented on a graph, 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). Amplitude (or strength of the power of the wave), can be changed or manipulated by changing the electrical signal strength sent to the transducer crystals by the ultrasound machine. An energy source creates the wave with pressure, and that energy is transferred as the forward motion of the wave. That amount of pressure difference 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.³

Sidebar 2. Amplitude.¹

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

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.

Figure 1. Photos of amplitude of ocean waves

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.

In ultrasound, amplitude describes how much energy was released to vibrate the transducer crystals, creating the sound signal that is then transmitted into the body.

Most ultrasound systems have a control that determines the size of the electrical pulse, which defines the amplitude or power.

Figure 2. Explanation of Amplitude

There is an important relationship between frequency and amplitude. 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). amplitude loudness frequency “pitch”.

When loud music is encountered by the human ear, it can be painful due to the excessive 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.

This diagram (Figure 2) illustrates how the two concepts of frequency and amplitude affect both the sound quality, and how far it will travel if unimpeded.

In addition to Amplitude, two additional concepts that impact the STRENGTH of the sound wave

Power: Total amount of energy that impacts the tissue with force over a certain timeframe
Intensity: The amount of energy that is directed toward (focused) and concentrated at a specific area.

Safety concerns related to Amplitude

It is important to 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 module on ultrasound safety.

On the other hand, ultrasound pulses lose energy as they are traveling through tissue due to attenuation, which will also be described in detail later. Several factors contribute to this reduction in energy. One of the most significant is the absorption of the ultrasound energy by the material and its conversion into heat. Ultrasound pulses lose energy continuously as they move through matter.

The rate at which an ultrasound pulse is absorbed generally depends on 3 factors: (1) the material through which it is passing, (2) the frequency of the ultrasound, and 3) the intensity. The primary thing to remember here is that depth of sound wave penetration is dependent on the frequency AND the amplitude or power of the signal.

SUMMARY: Amplitude

The 3 concepts presented impact the STRENGTH of the sound wave

1. Amplitude: Height of the wave representing the amount of energy that is in the wave, displacing material in its path

2. Power: Total amount of energy that impacts the tissue by the force of the wave over a certain timeframe

Note: in addition to amplitude, 2 additional properties impact sound strength: Power, and Intensity.

POWER

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).

Power is also described as a rate of energy transfer or the rate at which work is performed. It is the total energy encountered by tissue in a specified time.⁷

It is a measure of the overall strength of the force that generated the sound wave. 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 the total energy affecting tissue in a specified time frame.

Example: rays from the sun dispersed through the air in an open field have strength to warm all areas generally. No one area is overheated since it is dispersed over a very large area.

Most ultrasound systems have presets determining power based on the type of scan intended. To put this in perspective, stereo speakers may be as high as 500 Watts, where ultrasound is usually less than 0.5 Watts.⁸ Another thing to know is that this power dissipates exponentially as the sound goes deeper into the tissue. This mediates most safety concerns. Also, most ultrasound systems have a control that selects the size of the electrical pulse, depending on the transducer and type of exam, which determines the amplitude or power.

INTENSITY

Intensity is different than power in that the power is focused in a small or narrow area, increasing the energy exposure at the focus of the beam. It has to do with the specific area to which the beam is directed. The intensity of any form of energy has to do with how concentrated that energy is at the focus. The smaller the area of focus, the higher the intensity. The typical intensity value in clinical imaging is in the rage of 0.01 to 300W/cm2. ¹

Figure 3. Difference between power and 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).

Sidebar 3. Beam focus related to intensity

How that tissue is affected by the power is termed intensity. It is calculated by the incoming directed power and the size and density of the object.

Why does this matter? Ultrasound images can be improved by the use of focus toward an area of interest. However, by focusing, greater power is directed to that area, potentially increasing risk of bioeffects. For this reason, the use of focus should be minimized to prevent tissue heating.

Power can affect ultrasound safety – in other words, when delicate body tissues are struck with ultrasound waves, it is prudent to use only enough power needed to create a readable image.

Why does Amplitude matter?

Safety concerns

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.

On the other hand, ultrasound pulses lose energy as they are traveling through tissue due to attenuation, which will also be described in detail later in this chapter. Several factors contribute to this reduction in energy. One of the most significant is the absorption of the ultrasound energy by tissue and its conversion into heat. Ultrasound pulses lose energy continuously as they move through matter.

The rate at which an ultrasound pulse is absorbed generally depends on 3 factors: (1) the material through which it is passing, (2) the frequency of the ultrasound, and 3) the amplitude of the signal. 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. Tissue factors that impede sound travel will be discussed later in this chapter.

SUMMARY: Amplitude

The 3 concepts that impact the STRENGTH of the sound wave

Amplitude: Height of the wave representing the amount of energy that is in the wave, displacing material in its path
Power: Total amount of energy that impacts the tissue by the force of the wave over a certain timeframe
Intensity: The amount of energy that is directed toward (focused) and concentrated at a specific area

Principles of how sound travels through the body

Propagation

Propagation simply means the transmission of ultrasound energy from one place to another - movement from one area to another.

Characteristics that affect this transmission are:

Properties of the sound itself (wavelength, frequency. Amplitude
Type of tissue encountered as the wave progresses

Propagation Speed

Although the speed of sound is relatively constant, sound can be affected by a number of factors. Propagation speed is dependent on the substance through which the sound is transmitted – some materials are dense, and others are less dense. Propagation is the effect of the medium upon the sound wave.^{1, 11}

Sound waves propagate through matter by causing vibration of molecules along its path.

The stiffness and density of the matter indicates that the molecules are very densely packed together and affect how the signals move by each wave causing molecules to vibrate close to one another, perpetuating the movement of the wave.

Note: Propagation speed is highest in solids and lowest in gasses (air)³Remember this fact when scanning in the abdomen - bowel gas has air that prevents adequate transmission of sound.

Density

Density has to do with the concentration of mass or molecules in the substance, which most often affects its weight. Density is technically defined as gm/cm3¹.

An example of this is comparison between a concrete block and wooden block, both of the same dimension. The concrete, however, is denser than the wood.

Using that example, sound would travel faster through the wood than it would through the concrete. The concrete slows the speed of the sound, or “attenuates” (slows) it.

Figure 4. Size versus density - comparison between concrete and wood with same dimensions

Stiffness

Stiffness has to do with the compressibility of the object or tissue. A stiff tissue retains its shape when a force is applied to it. There is a stronger reflection from stiff objects than from flexible objects.

Example: Metal pipes are stiffer than Styrofoam pipes. If a metal pipe is struck with a hard mallet, a loud crisp sound would be reflected. If the Styrofoam tube is struck with the same mallet, the sound would be relatively muted (absorbing the energy rather than reflecting or transmitting it).

Examples in the body: Bone is both dense and stiff, in contrast to, say, liver tissue. So bone reflects back more energy than the liver, translating into a brighter image on the display due to stronger signal returning.

Note: Stiffness increases sound speed, and density DECREASES sound speed.¹

Figure 5. Stiffness examples: Styrofoam pipes versus metal pipes.

^Attenuation

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.³

Simply put, it describes how tissue “slows or stops” the sound wave going through tissue and subsequently the signal reflected back to the transducer. It is the loss of energy as sound travels through tissue.

Power is attenuated exponentially as sound travels through tissue, even as shallow as 1 cm.⁸

Because of this, there is not as much concern about power of an ultrasound beam, since it diminishes fairly fast as it travels into the body.

As sound is sent through various types of tissue, each layer reflects some of the sound, absorbs some of the sound, and then allows some sound to be transferred through the next layer. As the sound wave hits the next surface of tissue it repeats this cycle, but with lower energy since much of the energy was absorbed or reflected by the first layer. This process happens with each successive layer absorbing more energy, and reflecting less, until the sound has no more energy to reflect a wave back to the transducer.

Figure 6. Reduction of amplitude over time, while frequency remains the same.

The illustration in Figure 6 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.

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. The adjustment in augmenting the sound, as well as calculating the time of sound return, helps the computer to know the distance and properties of the tissue that was encountered.

Only about 1% of sound is reflected back to the transducer – the remainder is either attenuated, absorbed, or deflected. As stated earlier, the brightness or darkness on the ultrasound screen correlates with attenuation. 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 greater attenuation of the sound (the tissue stops the sound wave and “absorbs” it rather than reflecting it back).

As described in Chapter 2, high frequency (pitch) sound does not last long – the energy dissipates more quickly. If there is greater energy applied to it (increased amplitude), not only will the sound be perceived as stronger (or louder), but it will also travel farther because of the increased energy, A low pitch or low frequency sound travels further due to slower dissipation as it travels. If the energy behind it is low, it may travel farther, but it is still perceived as soft. However, if high energy is applied, increasing its amplitude, it will be perceived as a loud sound, and will travel the farthest because of the low wave span and the increased energy perpetuating it.

Amplitude must be sufficient to overcome attenuation as it travels through tissue. Sound is prevented from penetrating or returning if there is insufficient energy to keep it moving. Since most of sound is absorbed in the body, there must be adequate strength for the signal to penetrate, then reflect and then return to the transducer.

So why does all this matter?

Attenuation limits imaging depth and how strong it will be in its ability to return a detectable signal for interpretation. 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.'

Figure 7. Frequency related to depth of penetration.

Safety concerns related to Amplitude

One should always 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 module on ultrasound safety.

On the other hand, ultrasound pulses lose energy as they are traveling through tissue due to attenuation, which will also be described in detail later. Several factors contribute to this reduction in energy. One of the most significant is the absorption of the ultrasound energy by the material and its conversion into heat. Ultrasound pulses lose energy continuously as they move through matter.

The rate at which an ultrasound pulse is absorbed generally depends on 3 factors: (1) the tissue through which it passes, (2) the frequency of the ultrasound, and 3) intensity. 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.

Note: Attenuation is the loss of energy as sound travels through tissue.

The higher the amplitude, or the more energy the wave has, the longer it will last. As the ultrasound pulse moves through matter, it continuously loses energy. This is generally referred to as attenuation. Attenuation happens as the strength of the wave itself loses energy as it travels. But it also loses strength as it encounters tissue that slows or stops the energy waves.

Several factors contribute to this reduction in energy. One of the most significant is the absorption of the ultrasound energy by body tissue and its conversion into heat. Ultrasound pulses lose energy continuously as they move through matter.

Figure 8. Reflection decreasing with increased depth of penetration.

This diagram describes how frequency CAN affect attenuation. As we described earlier, frequency affects how deep the sound wave will travel. Higher frequency sounds lose energy more quickly anyway. Attenuation is simply the slowing of sound because of the diminishing energy due to its frequent vibration and rapid expenditure of energy. And with lower frequency there is less attenuation since the energy persists longer.

Impedance

Acoustic impedance is another property of tissue that impacts the sound propagation, and a term you will see used in sonography Acoustic impedance is a term used in sonography describing the resistance to the propagation or movement of ultrasound waves through tissues. Acoustic impedance is another way of describing the resistance created by dense objects that slow the sound speed.

Acoustic impedance is the product of the density and speed of sound in the tissue. It is the main cause of attenuation.

Impedance in ultrasound is important to know because the greater the DIFFERENCE in acoustic impedance between 2 adjacent tissues, the stronger the reflection back to the transducer, making it appear on the image to be a bright line or membrane. Acoustic impedance is sometimes referred to as “acoustic mismatch”.

Impedance determines how much of an incident sound wave is reflected back through tissue layers. Impedance increases as density increases.²

Sidebar 4. Acoustic impedance

Other Attenuation Factors

Attenuation occurs in the following ways:

Reflection is the “echo back” of the sound wave from a surface, but perhaps at a slightly different angle (refraction)
Refraction is the bending of sound as it encounters certain materials or tissues
Scattering is reflection off an irregular or circular surface that causes waves to reflect at various angles and not directly back to the transducer.
Beam divergence is how beams are sent out from a curved transducer – the beams separate out as they go deeper
Absorption means that the energy is absorbed by the tissue, converting it to heat

Reflection

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.

Ultrasound operates in the same manner. 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.

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 Pulse-Echo process is the term used to describe how the ultrasound machine computes the entire process of sending the sound, receiving the incoming echo, and creating a visual scan line on a screen to depict what is “seen” on the computer display screen. ¹

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 can also cause the sound wave to angle away from the transducer, causing loss of the signal returning to the transducer and an “incomplete” image of the structure. (Figure 9.) 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 cause direct reflection back to the transducer.

Figure 9. Reflection of sound off smooth versus rough texture

The next way that sound is attenuated has to do with a scattering of the reflected waves. This diagram depicts what happens to sound as it encounters some types of tissues. 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.³Figure 10.

FIgure 10. How sound waves change based on angle of incidence and tissue interaction.

Another type of sound attenuation is called refraction. 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”) This occurs because the speed of the sound is altered in the change between the 2 mediums, causing the image to reflect at an oblique angle.²

Because of the characteristic 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.³

Refraction

Another type of sound attenuation is called refraction. 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”).

Figure 11. Illustration of sound deflection causing refraction.

This occurs because the speed of the sound is altered in the change between the two substances, causing the image to reflect at an oblique angle.

Absorption

Absorption is the last attenuation property described in this chapter. It is sound energy converted to HEAT in the medium being scanned. When the sound waves encounter the medium or object, friction of the vibrating molecules produced by the sound wave energy absorb the energy. This energy is typically converted to heat. This conversion is proportional to the frequency of the sound (the higher the more the molecules vibrate), and the density of the medium (the denser, the more heat will be generated).

The diagram above depicts how different body tissues absorb ultrasound, converting it to heat. 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, since bone converts more energy to heat.

This is an important principle to understand as it applies to how long you can safely hold a transducer directed toward a fetus. Prolonged transmission of sound in the same location over fetal tissue can generate heat. The greatest danger is to send higher frequency ultrasound waves for an extended period toward fetal bone, since bone converts more energy to heat than other fetal structures.

Example: bone heats faster than fluid-filled structures. 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.

Impedance is a characteristic of tissue or material that affects sound reflection and propagation

Sound interaction with tissue can cause any of these things to occur:

Reflection, or echoing back to its source when it encounters tissue differences
Reflection or angling away from the tissue if at an oblique angle
Refraction
Scattering
Absorption by the tissue in the form of heat energy

Properties of how sound travels through the body

Resolution

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.

Sidebar 5. 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.

How Resolution Affects Image Clarity

Figure 12. How resolution affects image clarity

Resolution is affected by the depth of penetration since the beams diverge and create more space between them as they penetrate deeper into the body.

Sidebar 6. Resolution - Ultrasound beam.

There are two types of resolution – one has to do with how much detail can be picked up longitudinally (along the longitudinal wave of the sound beam) and the other has to do with the detail that can be picked up laterally, or side to side.

Longitudinal resolution is called axial (along the axis – or longitudinal) and side-to side resolution is described as lateral (perpendicular to the beam axis) resolution.

Figure 13. Axial versus lateral resolution.

For reference, the trapezoid is the shape of the display on the ultrasound screen. Toward the top of the display is what is closest to the transducer (body surface). Toward the bottom is deeper into the body.

Figure 14. Axial resolution diagram.

To improve an image, frequency could be increased, making the sound waves closer together, interacting with more tissue detail. Increasing frequency can be accomplished by changing to a different transducer or by machine adjustment. The compromise, however, would be a shortened range of depth. These adjustments will be reviewed in more detail in the chapter on knobology.

Figure 15. Divergence affecting resolution.

Lateral – or side to side – resolution has to do with the ability to pick up detail from the structure along a horizontal axis. It is determined by how close the structure is to where the beams are closer together. Particles farthest from the beam are more likely to be missed between the width of the curved beam as it diverges. To improve, narrow the beam which brings the individual "beams” closer together.

The ability to distinguish two separate objects that are close together and is itself divided into axial (along the axis – or longitudinal) and lateral (perpendicular to the beam axis) resolution. Closer to the surface the ultrasound beam is the strongest and can discriminate detail well (axial). Deeper into the body, the sound waves begin to diverge, and there is greater lateral space between the waves. The lateral resolution is then affected by imaging deeper into the body.

Analysis of an Ultrasound Beam: Pulsed Waves

Prior to this section, the concept of a sound wave being that a wave is a single continuously directed beam. However, for the ultrasound machine to detect returning echoes, it must have time to listen to sound being returned to it. So the transducer produces pulses of sound waves, separated by longer time intervals of listening or receiving incoming or returning echoes. A pulse consists of a few cycles of sound waves.

This creates a new understanding of the term “frequency”. Frequency is NOW defined as pulse frequency (how many pulses of waves are sent) instead of wave frequency.

The ultrasound transmitter creates brief bursts of acoustic energy, precisely timed, and sent to the transducer.

Pulses must be timed so that the returning echo has time to reach the transducer before the next pulse is sent. Each pulse sent will continue to vibrate or “ring” for a short time, so the pulse will be several cycles long. When returning echoes strike the transducer face, tiny voltages are produced by the piezoelectric elements which are sent to the amplifier to be enhanced.

The frequency of how often the pulses occur (pulse frequency, or how many pulses of waves are sent) is a more accurate definition instead of just considering wave frequency.

The ultrasound transmitter creates brief bursts of acoustic energy, precisely timed, and sent to the transducer. Pulses must be timed so that the returning echo has time to reach the transducer before the next pulse is sent. Each pulse sent will continue to vibrate or “ring” for a short time, so the pulse will be several cycles long. When returning echoes strike the transducer face, tiny voltages are produced by the piezoelectric elements which are sent to the amplifier to be enhanced.

In pulsed ultrasound, we count the number of pulses in a timeframe (such as one second). This is called the Pulse Repetition Frequency (PRF). Diagnostic ultrasound uses a few thousand pulses per second. Pulses must be timed so that the returning echo has time to reach the transducer before the next pulse is sent.

Insert slide: analysis of an ultrasound beam (slide with all text - still want this?)

Figure 16. Pulse repetition frequency.

This is a diagram depicting what pulse repetition frequency looks like. Since there is so much “listening” time, and little wave time in comparison, the cumulative energy that is sent into the body is much less. Actually, the amount of time spent in sending pulses is roughly 0.5% of the time – over 99% of the time is “listening” time. When a transducer frequency is described as 5 MHz (or 5 million hz per sec), the number of waves actually being directed into the body is roughly 50,000 instead of 5 million listening/receiving pulses. Only the few waves in each pulse are counted as waves “sent” to the body.^4,5

Why is this important?

This means that fewer cumulative sound waves encounter the tissue being scanned (by only sending small bursts rather than a continuous wave). Exposure to small bits of sound results in much lower overall exposure for a given time frame, reducing the risk of potential bioeffects.

True resolution based on how pulses interact with tissue.

Figure 17. True resolution based on PRF.

If the pulse frequency is higher (more pulses per second), then more particles will be encountered, and the image will have more detail. So now resolution is determined not only by the sound wave frequency itself, but by the frequency of the PULSES of sound waves. Since the ultrasound beam is pulsed, resolution now has to do with what is missed or encountered between the pulses as well as between the ultrasound waves. If pulses are spaced wider apart, then particles are missed, and the image has lower resolution.

Figure 18. Pulse repetition frequency and higher pulse frequency.

Resolution Summary

Resolution has to do with image clarity and detail
Resolution in ultrasound is determined by the sound wave frequency, pulse frequency, and spacing between waves that are radial in nature
Resolution can be enhanced by increasing frequency or by adjusting the machine to manipulate frequency and beam focus.

Figure 19 Process of sound reflection translating to image on ultrasound machine.

Ultrasound Modes

The last concept in this chapter describes ultrasound modes. The ultrasound transducer sends out sound waves, but there are different types or configurations of sound waves (and how the waves are interpreted when they return to the machine).

Safety can be impacted by incorrect use of modes or applications without regard for potential bioeffects. This is why modes are important to know.

A mode is selected on the ultrasound machine before starting a POCUS exam. The computer in will then alter the settings in the machine to align with the type of ultrasound beam to be used. This ensures safety in providing the lowest possible exposure to ultrasound energy.

insert ultrasound modes diagram (all text slide - still want this???)

Basic 2D mode

Basic 2D mode is the mode performed during most scans, and particularly in point of care ultrasound. It is also called B-mode, which stands for brightness mode. Pixels are generated that are brighter or darker, depending on the strength of the echo that returns to the machine. mode performed during most scans, and particularly in point of care ultrasound. It is also called "B-mode," which stands for brightness mode, or sometimes "grayscale." Pixels are generated that are brighter or darker, depending on the strength of the echo that returns to the machine. The stronger the reflected echo, the brighter the pixel.

M mode

M Mode is a form of B mode that detects motion (such as a fetal heartbeat) using 2D in real time. It is safe for viewing fetal heartbeats, especially in first trimester.)

M mode is a form of B mode that detects motion (such as a fetal heartbeat) using 2D in real time. It is safer for viewing fetal heartbeats, especially in first trimester. We will spend more time reviewing this in later Sections.

Figure 20. M-mode example.

This is a demonstration of what M-mode looks like. Real-time wave forms are created by the movement of a structure, in most cases a heart valve moving. M-mode is used not only for fetal heartbeat, but for adults as well.

The ultrasound beam or line is directed over an area that is moving (a heart valve). The bottom part of the stream moves and creates a simulation of the valve movement along a continuing timeline.

The change in frequency is displayed as a moving line as the target object moves

Doppler

Doppler Mode is the use of ultrasound to follow moving objects (like blood or blood cells) and capture their flow direction and speed. The type of doppler mode used in point of care ultrasound is color or color flow mode. Doppler mode in general is not recommended for use in first trimester due to a theoretical risk of harming delicate fetal tissue. The concern is for the overall power of doppler being in excess of what is used for 2D mode. Sometimes it becomes necessary to use color doppler, even in first trimester, but those indication s will be reviewed in the subsequent OB course.

Doppler mode is presented here since POCUS includes the use of color doppler to identify blood flow. OB/GYN POCUS does not teach applications of use of doppler ultrasound for vascular assessment. And although Doppler imaging is done in third trimester for fetal assessment of cord flow, it is not a POCUS skill. However, it is important to understand how it is done and how to interpret written reports.

Although doppler ultrasound for vascular assessment is not taught in this course, it is important to understand the principles and be able to interpret written reports. Color flow doppler WILL be taught for assessing fetal viability, ruling out an ectopic pregnancy, and observation of some simple ovarian cysts. The Doppler Effect is discussed here in very basic terms to understand its use in observing blood vessel function in some pelvic cysts, fetal cord, and placental flow. Doppler ultrasound is primarily used to detect and measure the presence, direction and rate of blood flow. It is also useful in assessing fetal viability, ruling out an ectopic pregnancy, and observation of some simple ovarian cysts.

Figure 21. Color Doppler diagram.

The Doppler Effect is discussed here in very basic terms in order to understand its use in observing blood vessel function in some pelvic cysts, fetal cord, and placental flow. Doppler ultrasound is primarily used to detect and measure the presence, direction and rate of blood flow.

General ultrasound surveys tissue that is not moving, or static. Doppler ultrasound involves imaging of moving tissue, such as blood or blood vessels, to detect abnormalities in blood flow. The Doppler Effect is the principle that helps us understand how ultrasound can capture this motion and quantify it.

Essentially, the doppler effect happens when a sound-producing object comes nearer – the sound goes higher in pitch because the frequency is increased as the source moves toward you due to sound compression. When the object moves away from you, the pitch becomes lower as the sound waves begin to space out. The same thing happens with echoes returning from objects that are moving. The sound wave source is static, but the blood cells moving in the vessels are moving, changing the echo frequency as they move, The Doppler echoes are interpreted by the computer as to the general direction the object(s) is moving (either toward the transducer or away from it). The computer assigns a color scheme to quantify the general intensity and direction of the flow.

Figure 22. Diagram of how frequency changes based on movement.

Doppler Functions³

Types of Doppler ultrasound include:

Continuous wave (or audible only) doppler – allows us to know there is blood flow but does not tell us the depth or location of the vessel, nor does it provide any indication of velocity. This is the type of doppler used in the hand-held units used for prenatal visits.
Color doppler is pulsed doppler – it sends signals to detect the motion of red blood cells and superimposes a color pattern over the 2D image.
- In medicine, and in particular, cord blood flow, doppler is used to measure the direction of movement and velocity of movement of red blood cells through fetal vessels in order to determine adequacy of circulation. The presence of color flow can indicate a growing structure, such as a gestation, or an ovarian cyst, or even a malignant tumor. All neoplasms (new growth) require increased blood supply, and so color flow helps us to identify these. There will be more discussion on the application of color flow in later modules.
Spectral doppler is also pulsed doppler, and it captures the blood cells moving through a very narrow sample or “gate” inside a blood vessel.

Spectral or Power doppler

Power doppler (or spectral doppler) is a more powerful beam and it is used to pick up very small areas of increased vascularity, say in malignant tumors. It has diagnostic purpose for this, but is too powerful to use routinely in point-of-care ob/gyn. The limitation of power doppler is that it does not indicate direction of blood flow – only the presence and intensity of the flow. It is used in diagnosing emergencies such as torsion of an ovary or testicle.

Figure 23. Illustration of Color Doppler versus Spectral Doppler: window versus gate.

Doppler Bioeffects

Use of doppler (even color doppler) does raise safety concerns for early pregnancy due to its intensity. Power doppler utilizes a specific window, or “gate”, used to both narrow and focus on the field of investigation. Color flow uses a window to view an area or structure, but it does not create as small or focused gate. Doppler ultrasound uses higher PRFs than with other imaging. Longer pulse duration and higher PRF result in higher energy exposure. Based on current evidence of potential harm to early gestation, most ultrasound societies do NOT recommend routine use of doppler prior to 10 weeks’ gestation.

Figure 24. Illustration of Power Doppler versus Spectral Doppler.

Despite some concern, color doppler causes less overall energy exposure than spectral doppler imaging. This occurs because color doppler tends to send more pulses down each scan line and may use longer pulse durations than other imaging. This means that although there is more power with color Doppler than 2D or M mode, it is still safer than spectral doppler.

The thermal effects of doppler are outlined as follows

With 2D imaging intensities are not sufficient to produce measurable heating of tissue.

With Doppler the potential for thermal effects is greater due to increased PRFs and narrow focusing of the beams

Some instruments are capable of producing temperature rises of greater than 1° C (34 ° F) at soft tissue/bone interfaces if the focal zone of the transducer is held stationary. ISUOG recommends no longer than 5-10 minutes using doppler scanning³⁶

Ultrasound Modes Summary

To summarize this section on ultrasound modes, modes are various ways that ultrasound beams are produced to capture images.

B-mode or 2D mode is static (still) imaging using sound waves to reflect off tissues to create an anatomic image.
M-mode is a form of B-mode but allows for capturing of a moving object, such as a heart valve, creating a measurable waveform.
Doppler is a way of capturing and quantifying direction and velocity of small moving objects such as blood cells.
Pulsed doppler increases energy sent to the body because it increases pulse strength and narrows the field of view, increasing intensity. Pulsed doppler increases energy sent to the body because it increases pulse strength and narrows the {eld of view, increasing intensity. For this reason it is used only as needed, and with caution in first trimester of pregnancy.
Spectral doppler has the highest level of energy and has no real purpose in POCUS.
Spectral doppler combines focused beams superimposed on 2 D imaging, and creates the highest level of energy. Spectral doppler is not used in POCUS unless performed by a radiologist, sonographer or Maternal Fetal Medicine specialist.

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 WITH 100%. YOU WILL HAVE AS MANY TRIES AS YOU NEED TO GET ALL THE ANSWERS CORRECT!

Foundations of Women's Point-of-Care Ultrasound

Chapter 3: Ultrasound Principles II: Sound and Tissue Interaction