Women’s Point of Care Ultrasound

Essential Concepts Made Clear

Chapter 4. USD Principles III: Artifacts

This chapter explores the concept of artifacts. Artifacts are simply apparent alterations in imaging that can make the image either easier or more difficult to interpret. The definition of an artifact is the appearance of something that does not accurately represent what is being imaged.

Objectives:

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

Describe the common artifacts seen when using ultrasound and how each affects image quality and accuracy
Recognize artifacts on demonstration images
Describe which artifacts can be detrimental or helpful in viewing a structure

TERMINOLOGY^1-4

Before discussing artifacts, it is important to be familiar with common ultrasound terminology used to describe image textures. This first section will define common terms used in image interpretation. These terms help with consistency in describing what is seen on the screen, and to correctly communicate that in documentation. It also assists in understanding what is described on an ultrasound report. Learning what is normal will help to discern artifacts and interpret their effect on the image.¹

Figure 1. Descriptive Terms: Echogenic, Hyperechoic.

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 2. Descriptive Ultrasound Terms: Hypoechoic, Anechoic or Sonolucent, Isoechoic.¹

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.

Sidebar 1. Shades of echogenicity shown on ultrasound display by tissue type.²

Angle of Insonation^2-4

Angle of insonation was described in Chapter 3. This section reviews the concept as it relates to how to maximize ability to view a structure, while minimizing artifacts that obscure it.

Sidebar 2. Angle of insonation

In addition to echotexture description, it is important to understand how images are best captured. This helps to avoid unnecessary artifacts and improve images. Angle of insonation was described in the last chapter. It refers to the optimal angle or direction for obtaining a good ultrasound reflection. The ideal angle is directing the beam at a 90-degree angle (or straight on) to the surface of the structure to be imaged so that the reflection is directed straight back to the transducer. However, not all body structures will be perpendicular to the transducer. It then becomes necessary to move or angle the probe differently to get a more direct echo back from the structure, as in the illustration. Artifacts are often produced or corrected by changing the angle of insonation.

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.

Artifacts^1-3

Artifacts are simply alterations in an image which may seem real but are created by interactions of sound with tissue, distorting what is actually there. In ultrasound, artifacts include reflections described as “not real”, not seen on the image, having an incorrect shape or size, incorrect position, or incorrect brightness of the texture.¹

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.

Why is this important?

When something is seen on the screen it is critical to avoid misinterpreting the image. This could result in “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. 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.

Sidebar 3: Artifacts analogous to mirage.

In the image above 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 have similar properties to light. Sound waves 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 loss 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:

Transducer-related
Acoustic shadowing
Acoustic enhancement
Reverberation artifact
Mirror image

Transducer artifact

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 sound. 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.

Sidebar 4. Transducer artifact.

Figure 3. Transducer artifact.

In Figure 3. 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

Sidebar 5. Shadows.

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.³

The sonographic characteristic of shadowing is the appearance of hypo or anechoic background beneath the object being scanned.^1,2

Figure 4. Acoustic Shadows.

Figure 4. demonstrates how dense structures (like bone, and particularly the fetal vertebrae in this image) absorb the sound so that the waves do not penetrate well beyond the structure, casting a “shadow”

Figure 5. Edge Shadows.

An edge shadow is a type of shadow that is seen coming from the edges of a very round, smooth object (Figure 5). 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

Figure 6. Acoustic Enhancement.

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.

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.

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)

Figure 7. Acoustic Enhancement: “Through-transmission”.

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.

Figure 8. Reverberation.

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

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.¹

Sidebar 6. Reverberation in bladder image.

The bladder image in Sidebar 6 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?

It can cause some confusion when trying to find the true border of the structure and make accurate measurements. It is important to place the measuring caliper on the actual border of the structure rather than on the reverberation band. This will be of great importance when performing amniotic fluid measurements since reverberation is very often seen in these images.

This is another image clearly demonstrating the parallel lines created by the bouncing of the sound waves back and forth from the highly reflective anterior bladder surface.

Sidebar 7. Ring down.

Ring down is an artifact that often seems to be 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.

Sidebar 8. Color flash.

A color flash artifact is simply a momentary “flash of color”. It does not represent motion of blood through a vessel. Typically, when performing color flow ultrasound, it is done to observe for continuous or pulsing color flow indicating blood vessels. But when color flow 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, In obstetric ultrasound you are most likely to see color flash when you are measuring amniotic fluid that is moving or “sloshing”

Mirror imaging⁴

Mirroring occurs when an image shows 2 duplicate structures that appear identical, but in different locations. It is caused by the bouncing off a strong reflecting surface and creating the appearance of a duplicate structure on the opposite side. Just as a duplication of an object is reflected in a glass mirror, this can happen with sound waves.

Mirror imaging is an artifact that is not seen often in women’s health POCUS. However, it is important to have awareness of this potential, especially in scanning for early first trimester pregnancy. The ultrasound may duplicate a singleton gestation adjacent to it, giving the impression that there are 2 gestational sacs. This could cause an incorrect assessment of a twin gestation. As will be discussed in greater detail in the course on ob/gyn ultrasound, it is important to view all pelvic structures in several different planes to be certain to get all angles of the structure being scanned. If something can’t be duplicated on another view, it is not likely to really be there.

Figure 8. Example of multiple artifacts.

In Figure 8, 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).

Sidebar 9. 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

SUMMARY

Artifacts are alterations in an image that do not represent an actual structure.

Artifacts can be misleading, bothersome or useful.

Commonly encountered artifacts seen in POCUS are:

Transducer or operator related
Acoustic shadowing & edge shadowing
Acoustic enhancement
Reverberation
Ring down
Color flash

Foundations of Women's Point-of-Care Ultrasound

Chapter 4 Ultrasound Principles III: Artifacts