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Artefacts on musculoskeletal ultrasound

Thanks to John Leddy, MSK Sonographer for providing the bulk of the article below..

What is an artefact?

An ultrasound image is produced assuming that the returning echoes faithfully represent the underlying tissue. Certain conditions can cause significant differences to occur. These are called artefacts. There are a number of other artefacts on musculoskeletal ultrasound, several of which are listed below (this list is by no means comprehensive)

Enhancement 

A drawback of ultrasound is that each layer of tissue that is passed through reflects and absorbs the echo to some extent, reducing the strength of the signal that reaches deeper tissue.

As less sound returns from deeper tissues than more superficial ones, the image is processed to compensate for this by applying a standard correction in proportion to the depth.

Some structures however allow sound to pass through them more easily than others.  The most dramatic example is watery fluid, such as in an effusion or in a cyst. These are described as being translucent. Because only a minimal amount of energy is absorbed by the fluid, the region that lies behind will receive more sound than the processor expects for that depth.  This area will therefore appear uniformly brighter. This effect is called Enhancement. A good example is the effect of the bladder, acting as a window to deeper tissues.


Acoustic enhancement artefact, deep to a fluid filled structure
Figure 1: Acoustic enhancement artefact, deep to a fluid filled structure


 

Attenuation or Shadowing is the reverse effect, where some tissues absorb relatively more of the sound. The area of the image deep to this will appear darker.  In the extreme almost no sound is transmitted, leaving a dark shadow behind the structure.This effect is used as a diagnostic tool in identifying calculi, which if they are larger than the beam width cast a strong acoustic shadow.
Longitudinal view of calcification in the long head of biceps sheath
Figure 2: Longitudinal view of calcification in the long head of biceps sheath forming acoustic shadowing

Anisotropy (Figure 3)

This is the effect that makes a tendon appear bright when it runs at 90 degrees to the ultrasound beam, but dark when the angle is changed.

The reason for this is that at particularly smooth boundaries, the angle of reflection and incidence are the same, just as they are with a conventional mirror. Thus the probe will only receive the reflected sound if the beam strikes the surface at a right angle. This is a vital skill for a musculoskeletal sonographer to be aware of, as many MSK structures can be influenced by anisotropy. As you can see in Figure 3, scanning at an inappropriate angle, can give the impression of very different pathology!

LHBanisotropy
Figure 3: Transverse section of biceps tendon in bicipital groove. On the left tendon appears bright,
but on the right a small change in the angle of the probe makes the tendon appear dark.

 

Mirror images (Figure 4)– This is where a strong reflector at an angle to the probe causes structures that lie in front and to the side of it to appear as if they lie behind it, just as something viewed through a mirror appears to lie behind it. This effect is normally only achieved by the diaphragm.
Mirror image artefact. Percistant haematoma anterior to the tibia The lesion also appears deep to the surface of the tibia
Figure 4: Mirror image artefact. Percistant haematoma anterior to the tibia
The lesion also appears deep to the surface of the tibia

Reverberation (Figure 5) – this causes evenly spaced lines at increasing depths and is caused by sound reflecting back and forth between the surface of the probe and a strong reflector close to the surface.

Reverberation artefact from a needle
Figure 5: Reverberation artefact from a needle

Comet tail – this is the same process as reverberation, but occurs within a very small structure, with smooth highly reflective borders, such as a metal fragment. Tiny bright reverberations are seen deep to the structure slowly diminishing in size as if it had a tail.

Refraction – Sound is refracted in the same way that light is as it passes from one medium to another. Thus the direction in which it travels changes when it passes through a boundary at an angle less than 90 degrees. This can lead to subtle miss placement of structures and some degradation of image quality when the angle of incidence is particularly acute.

Ghost images – This is a dramatic example of refraction, where a structure is represented twice or more side by side.  This classically occurs deep to Rectus Abdominis which due to its shape acts as a lens and can lead to the apparent duplication of the aorta or early foetal sack.

Range distortion – Ultrasound travels at slightly different speeds through different tissues.  A rough average of 1540m/s is used, but the velocity through fat (~1460m/s) and water (~1480m/s) is somewhat slower. Structures deep to a large fluid collection can therefore appear a little further away than they actually are.

Side Lobe Artefacts – the probe cannot produce a pulse that travels purely in one direction. Pulses also travel off at specific angles.  These side lobes are relatively weak and so normally do little to degrade the image.  Their effect is only normally seen faintly superimposed in fluid filled areas which are anechoic and so do not obscure the weak side lobe reflections. The exception is when a side lobe strikes a particularly strong reflector at 90 degrees.  In this case the reflector can appear within the image.

Partial volume – the slice that makes up the ultrasound image is 3 dimensional, just as with MR. This typically means that fluid filled areas, where they are very small or adjacent to soft tissue will not appear anechoic (Black) as would be expected, but often contain low level echoes which can be mistaken for debris or even soft tissue.