Basic Obstetric POCUS

Chapter 3. Machines and Image Creation

This chapter reviews types of ultrasound machines, comparing them by size, function, and uses for different types of scanning. All machines are meant to essentially do the same thing – image the body in a way that creates an image to be used to help examine body structures. With these images we can learn that a pregnancy is progressing normally, or we can detect possible complications.

We will focus on the use of handheld ultrasound machines (in particular, the Lumify machine), which will be the primary type of machine used by the midwife in Gulu for observing pregnancy.

Objectives:

By the completion of this Section the participant will:

Understand the interaction of the various main components of the ultrasound machine in how it produces an image

Begin to navigate basic functions for point of care ultrasound on a handheld to achieve an optimal image

Machine Essentials

Simply put, the ultrasound machine is a specialized computer that links a device (transducer) that sends sound into the body, interprets the returning echo, and provides an image on a display that represents the structure of the body it encounters. These images are then interpreted by the person doing the scan, either in real time, or in review of still images. Image recognition enables users to discern normalcy versus pathology.

An analogy sometimes used with ultrasound machines is comparing them to motor vehicles. They all operate in similar ways- they all have engines and wheels, for example, and are used to transport people and things.

But there are differences in how some are driven. For example, on the dashboard of a car you can have either simple or complex set of controls.

Figure 1. Auto dashboard differences.

This analogy can be extrapolated to ultrasound machines. All ultrasound machines have the same essential functions: sending sound waves into the body and translating the returning echoes into meaningful signals. But they come in various sizes and complexity based on their specific uses. The controls or buttons may be in different locations (such as a steering wheel being on the left instead of the right), but the functions are the same. This course will teach you the basics of ultrasound, but you must learn how to operate your own machine.

Figure 2. Differences in ultrasound machine types.

Here is how the ultrasound machine works:

Figure 3. Ultrasound creation process.

1. Computer tells the pulse transmitter how to shape the beam to adjust the beam width and focus. It does this because you have entered the preset on the machine to do a specific type of scan.

2. Pulse transmitter – Generates the electrical pulses sent to the transducer

3. Transducer – transfers the electric charges to the crystals on the end, which vibrate based on the voltage that was applied to them. Between pulses sent into the body, it stops to receive and amplify returning echoes. These vibrations of the crystals receiving the returning signal are then sent to the receiver as electrical signals.

4. Receiver – accepts electrical signals from the transducer and prepares signal for display. It amplifies the returning signal many times and sends it to the image processor.

5. Image Processor – The processor makes the images by generating pixels (like a computer) allowing for 256 shades of grey per pixel (more detail than what the human eye can distinguish, since the human eye can only process about 100 shades of grey).

Screen or Display – Signals coming from the image processor create the display from the pixels, creating both still and video images

MACHINE TYPES

These are presented here to familiarize you with the types of machines you may see in some clinical settings. You may never have the opportunity to use one of these, but you can use any of these machines to perform point of care assessment.

Figure 4. Full console machine

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Figure 4. is a photo of what is termed a “full console” machine. It has many capabilities for enhancing and storing images, with features that are used primarily for more sophisticated diagnostic uses like those provided in radiology clinics and hospital radiology units. They have many options for creating sophisticated images used by sonographers and radiologists. Despite their complexity, they can still be used to perform POCUS exams.

A smaller version is the portable laptop machine (Figure 5). This unit has the same basic features as the full console, but in a smaller, more portable machine. Some laptops have only one port for probes, so any time a probe is changed, it must be done manually. Screen size differs based on the size of the laptop, just as with laptop computers in general.

Figure 5. Portable laptop ultrasound machine.

Figure 6. Types of handheld machines used for POCUS

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MACHINE COMPONENTS AND FUNCTIONS

All ultrasound machines have the following basic parts and capabilities:

transducer (probe) - probe that sends and receives the sound waves
central processing unit (computer) - computes the calculations and contains the electrical power supplies for itself and the transducer (probe)
transducer pulse controls - changes the amplitude, frequency and duration of the pulses emitted from the transducer probe
display - displays the image from the ultrasound data processed by the CPU
keyboard/cursor - inputs data and takes measurements from the display
knobs - these allow the scanner to adjust images as they are made
Method for storing (archiving) images. All ultrsound machines have internal storage for images. They are stored by the patient’s name. Most machines have the ability to send images elsewhere - most have USB ports that allow you to download the images.

The Lumify is connected to a tablet, so the images you save will be saved on the tablet itself. Then you are able to send the images from the tablet. You will have specific instructions on how to do that later.

Figure 7. Ultrasound machine components

Figure 8. (below) shows the usual locations for the basic machine components on a full-console machine: the screen, computer, keyboard, transducer, and the control buttons or knobs used to adjust and improve your image. This is just for your general information.

Figure 8. Machine components on a large full-console machine

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Figure 9. The same essential functions on a tablet (handheld) machine - using touchscreen instead of knobs

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Figure 10. Using a handheld with tablet, showing adjustments on the tablet screen

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Figure 11. The Lumify machine placed on a stand.

Format for storing images

Just as when an image is captured on a digital camera or cell phone, an image is formatted for storage. We are most familiar with JPEG and PNG, for example, as image formats. Many machines will let you save your ultrasound images in those formats, but there are some security risks with doing that.

Most newer ultrasound machines have the capability for archiving images using a format called DICOM. This is more secure and allows remote reviewers to manipulate the images on the receiving side for better viewing and annotation.

Figure 12. Digital storing and archiving formats

--DICOM (Digital Imaging & Communication in Medicine): used primarily in hospital or large inter-hospital groups to send images to a central office for review by radiologists or technicians.

--PACS (Picture Archiving & Communication Systems) used in some hospitals to configure images to be reviewed remotely. You will likely not be using this in your setting.

The reason we are discussing these terms is that you will have the option in the Lumify to choose how you want to save your images, including the option of DICOM. This becomes important because this format may be required by your healthcare system.

Machine protection and care

It is important to protect the machine and its parts, since these machines are very costly. They are built to be sturdy under normal conditions, but some parts are easily damaged and not easily repaired or replaced.

It is important for EACH person to take extra care in maintaining the probe. The transducer is the most fragile and most expensive partto replace, so it is critical that they are protected.

Avoid laying the probe on a bed or table since it is too easy for it to fall off. When a transducer is dropped the delicate crystals in the head can be broken, making the transducer non-functional.

And it is also very important for the transducer cords and the power cord to be kept OFF THE FLOOR.

Cleaning the machine and its parts will be reviewed in the section on Safety.

Image Creation: How to operate the ultrasound

“KNOBOLOGY”

This next section reviews the functions of the ultrasound machine that enable the scanner to adjust and improve acquiring and viewing an image.

NOTE:

The ultrasound machine does not know what type of scan you are trying to perform. You must tell it what you are looking for (enter information) so that it will construct the settings you need to capture the most accurate image.

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.

Modes are types of ultrasound used for specific purposes, using different ways of sending the sound beams. Of the 3 basic modes, one is selected to be used before starting an 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.

Figure 13. Ultrasound modes.

The 3 types of mode you will use when performing POCUS in obstetrics are:

—2-D mode (B-mode, or greyscale)

—M-Mode

—Doppler Color mode (or color Doppler)

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 (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.

This mode is the most often used, particularly in point of care ultrasound.

Figure 14. Image of early gestational sac using 2D or B-mode.

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.) It is considered safe at any gestation of pregnancy since it does not use increased power to the fetus..

Figure 15. M-mode image.

Figure 15 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. The change in frequency is displayed as a moving line as the target object (heart activity) moves.

M-mode is used for any heartbeat (adult, child, or fetal)

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.

Doppler Mode

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 ultrasound is primarily used to detect and measure the presence, direction and rate of blood flow. Doppler mode in general is not recommended for use in first trimester due to a theoretical risk of harming delicate fetal tissue. The reason for concern is that overall intensity of Doppler: it transmits more energy than 2D mode. Sometimes it becomes necessary to use color Doppler, even in first trimester, but those indications will be reviewed in the First Trimester chapter.

Figure 16. Illustration of how frequency changes as an object moves closer to the transducer

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Do not worry if you can’t understand this concept since it is a bit difficult to understand when you are beginning. Just understand that it is used to detect blood flow in blood vessels and tissue.

Figure 17. Explanation of interpreting color differences in viewing color flow images

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The Doppler Effect is discussed here in very basic terms so you will have an understand of its use. It illuminates blood flow both in blood vessels (fetal cord) and in tissue (pelvic cysts, placental flow). It is also useful in assessing fetal viability, ruling out an ectopic pregnancy, and observation of some simple ovarian cysts.

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.

Although Doppler ultrasound for vascular abnormality diagnosis is not taught in this course, it is important to understand the principles and be able to interpret written reports. Simple color flow Doppler WILL be taught for assessing fetal viability, ruling out an ectopic pregnancy, identifying umbilical cord, and observation of some ovarian cysts. It is simple to apply.

The presence of color flow in a vessel or tissue indicates a living, growing structure with active blood supply (such as early gestation, an ovarian cyst, or even a malignancy). 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 chapters.

Figure 18. Images showing how color is used to identify the umbilical cord and to see blood flow into the placenta (on the right).

The image below ( Figure 19) is an example of how color flow is used in early detection of an embryonic demise. There is color flow around the gestational area (red and blue), but no color inside the tiny embryo inside. This helps us determine that the embryo has no circulation and is therefore not living.

Figure 19. Color flow seen in chorion, but not inside the embryo, indicating early embryonic demise

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Cord blood flow Doppler is used to measure the direction of movement and velocity of movement of red blood cells through fetal vessels (usually an umbilical artery) in order to determine presence or adequacy of circulation. This is done by sonographers and radiologists, but not part of a POCUS exam. It is described here because you may see this on an ultrasound report from another facility.

Image Creation: How to understand what is on the screen

IMAGE CHARACTERISTICS

Now that we know how the ultrasound operates, we need to learn to discern what is seen on the screen. The display screen looks like a patterns of grey, black and white. This is a ‘language’ that we need to learn to interpret what these patterns mean.

This is also why when we teach POCUS, we include many images to familiarize you with how pelvic and fetal structures appear on ultrasound. This is called pattern or image recognition. If you become familiar with what normal and abnormal structures look like, it will make scanning more efficient and accurate. You will understand what you are seeing.

First, let’s review common terms used to describe these patterns.

ULTRASOUND DESCRIPTIVE TERMS

The following terms are used universally to describe the characteristics of what is seen on an ultrasound screen. They help categorize the types of tissue that we “see” on the screen, making interpretation more accurate.

The picture we see on the screen has shades of gray - from white to black.

Brighter echoes indicate a more dense or solid tissue like bone. Air also returns echoes from within the body since sound cannot penetrate through air.

Darker echoes represent tissue that has moderate density. Black echoes indicate there are no particles or tissue to reflect sound back to the transducer. This is typical of any fluid-filled structure such as bladder or amniotic fluid. An structure seen as dark on ultrasound usually represents tissue with higher fluid content.

These variations in shades of gray help us to know whether what we are seeing is dense (like bone) or fluid-filled (like a bladder or a cyst).

Figure 20. Shades of echogenicity shown on ultrasound display by tissue type

Figure 21. Descriptive Terms: Echogenic, Hyperechoic

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Echogenic

A portion of the structure has at least a few echoes.

Hyperechoic

If a tissue contains more echogenic material than the surrounding tissue (more echoes , so looks brighter) Example, bone or dense masses.

Figure 22. Descriptive Ultrasound Terms: Hypoechoic, Anechoic or Sonolucent, Isoechoic

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Hypoechoic

If a tissue contains less echogenic material than the surrounding tissue (fewer echoes , so looks darker) Example, fluid filled cysts, bladder, amniotic fluid

Anechoic or Sonolucent

Area containing no echoes at all – transmits the sound through entirely

Isoechoic

A description of a structure that has a similar echogenicity as the surrounding area

Homogeneous tissue

Tissue that has smooth texture – the entire structure has the same brightness and evenness of echoes

Mixed Echogenicity

Tissue that has a mixture of textures – dark and bright. Sometimes appears splotchy or speckled.

Learn these descriptions - you will be using them every time you report or document a POCUS. Since these terms are universal, the person you are reporting to will understand the image characteristics.

ARTIFACTS

Artifacts are a result of several factors: properties of sound physics, operator error, inaccurate assumptions about where the structure is located, or even possibly machine error. It represents something seen on the screen that is not what is actually in the body. It is essential that we recognize artifacts so the image allows us to make the correct clinical assessment.

Figure 23. Artifacts: definition

Why is this important?

Artifacts can easily be produced or corrected by changing the angle of insonation, the angle at which the ultrasound beam is sent to the structure. These are all things that the person scanning can do to correct for these.

It is critical to avoid misinterpreting what is seen on image. This could happen when “seeing” a structure that is not really there, or the opposite: not seeing a structure correctly due to distortion or obscuring from an artifact. Artifacts can either enhance an image or distort it.

This is why artifacts are described here so you can detect them and adjust accordingly.

Images can be improved by moving away from the artifact by changing positions or scanning angle. It will become more evident as each artifact is described as to whether it is a hindrance or help.

Figure 24. Artifacts analogous to mirage.

In the image above (Figure 24) there appears to be a lake at the base of the mountains. This lake is not real – it is an illusion caused by the bending of light waves bouncing off warm ground up to air which is cooler. This “bends” the light waves and refracts light from the sky (and acts like a mirror of the sky).

Sound waves act similarly to light; they can also “bend” by passing through different tissues, causing the images to distort.

These image distortions are caused by attenuation. As explained in the last chapter, attenuation is what slows, hinders, or distorts the sound from propagating or moving forward. This affects loss of signal strength or sound deflection as it passes through various tissue densities before reaching the desired structure.

Remember: attenuation is the reduction of signal strength as it passes through various tissue densities before reaching the desired structure.

Example: Bone attenuates the sound and does not allow sound waves to pass through it to the structures deeper to it and causes a shadow

Example: A fluid filled structure like bladder or cyst may appear to enhance the sound waves, increasing the brightness of the field beyond it, either making it easier or more difficult to clearly see that region.

The most common artifacts seen when performing ultrasound include:

Figure 25. Most common artifacts seen in POCUS

Transducer artifacts

One of the most common artifacts causing poor resolution or visualization is due to having insufficient amount of gel on the transducer. If there is no gel between the transducer and the skin, the air between these will attenuate or stop the sound signal. Air is a poor conductor of ultrasound. Gel eliminates the air between the vibrating crystals of the transducer and the skin. Using enough gel enhances direct sound transmission through the skin into the body. This gel is sometimes called “coupling” or “transmission” gel. Ultrasound gel is specially formulated to give the best results for connection to the skin. However, other gel-like products can be used if this is not available. Personal lubricant gel can also be used, or any gel that is medium viscosity and bacteriostatic. One important consideration in choosing a gel is whether it has alcohol or any other chemical that could be damaging to the transducer membrane. Each manufacturer will have specific instructions on an appropriate gel to use.

Figure 26. Artifact of poor image due to insufficient ultrasound gel

Figure 27. Transducer artifact of ‘drop-off’ where the transducer loses direct contact with the skin.

In Figure 27. it appears that part of the image on the right is darkened. It could be that something is causing a large shadow (but not likely). This darkening inhibits the view of the maternal and fetal anatomy. By looking closer at the position of the transducer on the abdomen, it is evident that the transducer is not entirely in contact with the woman’s skin, causing there to be no transmission of sound. When this type of shadow is seen, look down at the transducer and be sure that it is in contact with the skin. Towels or drapes on the abdomen can also inhibit transmission of sound. While scanning it is easy to be unaware of the exact position of the transducer.

Remember: air stops the transmission of ultrasound, so if there is air between the transducer and the skin, there will be no image. Gel eliminates the presence of air.

Shadowing

A shadow is created when light cannot pass through a dense object. Similarly, sound can create accoustic” shadows” that occur where sound cannot pass through a dense structure, and the tissue beyond that structure is darker or not seen. Shadowing is a reduction of echo amplitude directly behind a stiff or highly reflected object being scanned, preventing the sound wave from penetrating through it.

Figure 28. Shadowing in ultrasound

The sonographic characteristic of shadowing is the appearance of hypo or anechoic background beneath the object being scanned.

Figures 29 and 30 (below) demonstrate how dense structures (like fetal bone) absorb the sound so that the waves do not penetrate well beyond the structure, casting a “shadow”.

Figure 29. Shadow cast by fetal vertebrae attenuating the sound beneath it.

Figure 30. Image of a dense (bright) fetal femur casting an acoustic shadow behind it (indicated by the blue arrow).

Edge shadow

An edge shadow is a type of shadow that is seen coming from the edges of a very round, smooth object (Figure 31). This happens because of bending or distortion of the sound waves. This artifact identifies the object being viewed is a very round structure, aiding in the assessment or diagnosis. A very round calcification (gallstone, for example) casts edge shadows rather than a simple shadow. In ob/gyn POCUS, edge shadows are commonly seen beyond a round structure such as a fetal head or abdomen. Edge shadows are usually helpful in identifying very round structures especially when the borders of that structure are not clearly seen.

Figure 31. Illustration and image example of edge shadow

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Figure 32. Round fetal head casting edge shadows (indicated by the blue arrows

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Acoustic Enhancement (“through-transmission”)

Instead of shadowing, an opposite phenomenon occurs when sound moves through fluid. Instead of attenuating the sound beam, it allows sound to pass directly through it. Since clear fluid contains no particles to reflect sound back, the sound passes through until it encounters denser tissue on the back side of the structure. Fluid has a lower attenuation (absorption) of the sound than the tissue surrounding it, which creates the artifact of enhancing or brightening the tissue beyond it, as well as increasing the resolution of the image behind the fluid.

The tissue beyond the fluid is not actually more reflective; it just appears that way due to the compensation created by the ultrasound processor that accounts for higher speed of the sound wave going through the fluid.

Figure 33. Illustration and image demonstrating coustic enhancement

Acoustic enhancement is also known as “through-transmission” and is considered useful in ultrasound since it can be employed as a “window” through which to improve visualization of a deeper structure. Through-transmission is also helpful in that it can help differentiate whether a structure is more fluid or solid. Sonographers typically try to scan through a fluid-filled structure to image what is behind it, finding this creates a better view of the organ of interest.

Figure 34. Image showing acoustic enhancement (brightening) of the section of placenta directly behid the pocket of amniotic fluid

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An example would be when one is scanning a cystic structure– it is more reassuring to see that a cyst has through transmission (indicating clear fluid inside of it) rather than NOT having through enhancement (indicating it may be more solid, causing suspicion for an abnormality.

Acoustic enhancement, also known as “through-transmission” is useful in ultrasound since it can be employed as a “window” through which to improve visualization of a deeper structure.

Reverberation.

Reverberation is a very common artifact, and essential to recognize it when measuring amniotic fluid pockets.

Reverberation happens when sound bounces off a highly reflective surface. As described in the last chapter, this is more likely to happen with a stiff structure). The wave is bounced back and forth between it and the surface of the skin or reflective structure), sometimes a number of times. It is similar, in a way, to what happens when a ping-pong ball is bounced between the table and a paddle – it can bounce back and forth several times to the paddle before it loses energy and drops.

Figure 35. Explanation and image of reverberation seen in the bladder

In ultrasound, each “bouncing” sound wave takes successively longer to reach the transducer and be recorded, so the machine creates parallel lines on the screen based on these differing return times. In other words, two round trips take twice as long, so the machine then “creates” a line that looks like it is twice as far away as the first line. Then the same thing happens for the third and fourth bounces, where the machine creates additional image lines based on the time it takes for those signals to be processed. Reverberation is distinguished by straight and evenly spaced parallel lines. It can sometimes be subtle and appear as a “blur”, but otherwise it can appear as a venetian blind.

Figure 36. Image of a bladder showing lines of reverberation (blue arrow)

The bladder image in Figure 36 (above) clearly demonstrates the parallel lines created by the bouncing of the sound waves back and forth from the highly reflective anterior bladder surface.

WHY IS THIS IMPORTANT?

When measuring a fluid filled structure (cyst, bladder, or amiotic flud pocket) it can be confusing as to where to place the measuring caliper. You need to find the actual structure wall rather than start from where you see reverberation. Starting a measurement from the reverberation area will make the measurement seem smaller than it really is. This will be of great importance when performing amniotic fluid measurements since reverberation is very often seen in these images.

Ring down artifact - a form of reverberation

Ring down is an artifact that often looks like a moving bright streak, hindering view of the structures around it. It is often a result of gas in the intestines, moving with peristalsis. If this is seen, then changing the transducer plane or angle can be helpful. Sometimes waiting until later to scan is the best option, allowing time for peristalsis to dissipate

Figure 37. Image of ring down artifact

Color flash

A color flash artifact is simply a momentary “flash of color” seen when performing color flow scanning to look for blood flow. It does not represent motion of blood through a vessel. When color flow mode is activated, it will detect movement from most anything that is moving. In obstetric ultrasound you are most likely to see color flash when you are measuring amniotic fluid that is moving or “sloshing”

FIgure 38. Image of color flash

Figure 39. Image showing several artifacts.

In Figure 39, there are several artifacts within the same view. On the left side of the image is a darkening beyond the abdominal wall caused by acoustic shadowing. Across the top near the skin are bright parallel lines caused by reverberation. These lines are too smooth and straight to be physiologic. On the right side of the image there is a brightening of the area beyond the dark fetal stomach toward the top. The fluid in the stomach created an “acoustic window”, allowing enhancement of the structure beyond it (through enhancement).

Artifacts - useful or detrimental?.

In summary, artifacts are either useful or can make scanning more difficult. Recognizing them can either help to avoid them or use them as an advantage.

Figure 36. Description of artifact benefits and hindrances

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 3. Machines and Image Creation