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ARTIFACTS

Artifacts occur when the machine interprets returning echos according to its computer algorithm and erroneously places objects where they shouldn't be or makes objects brighter or darker than they should be. This is because the US machine computer is programmed with rules that assume a uniform behaviour of US in soft tissue ie the speed of US is 1540m/s, the US wave is attenuated x amount as it passes through tissues etc. However, the body doesn't always behave with computer precision and not all tissues in the body have the same characteristics as soft tissue. Hence: artifacts occur. The following are some common artifacts to keep in mind. Some of them can be useful for diagnosis.

ARTIFACTS DUE TO ATTENUATION

SHADOWING

Shadowing is the absence of information posterior to a structure. Shadowing occurs when the US beam encounters a structure which is highly attenuating or reflective. Typically, these are calcium containing structures which absorb and reflect a lot of the US beam: highly attenuating. The structure will appear bright white but the area posterior or inferior to it will be black: shadow. 

You can use shadowing to help you diagnose things like gall stones. 

GB (long) showing multiple gall stones with shadowing

ACOUSTIC ENHANCEMENT

Enhancement is the opposite of shadowing. This occurs when the medium the US waves passes through is less attenuating than the computer expects. Normally as the US wave passes deeper into soft tissue, it is attenuated and becomes weaker. Thus the echoes returning from deeper tissues are weaker. Due to this, the computer is programmed to increase the amplitude of the echoes returning from deeper structures. However, sometimes the US wave is not as attenuated as it should be. This happens when the US wave passes through fluid filled structures. 

 So behind a fluid filled structure, the image will always be more hyperechoic (more white) than it should be. You can decrease this artefact by decreasing the far gain (ie decrease the amplitude of the returning echoes in the far field of the screen). 

Pelvis (trans): image on the left shows enhancement posterior to the full bladder making structures posterior to the bladder difficult to discern; image on the right (with far gain decreased) reveals structures posterior to the bladder

ARTIFACTS DUE TO US BEAM MECHANICS 

SLICE THICKNESS ARTEFACT

We have already spoken about lateral resolution and axial resolution affecting the machine's ability to discern two structures which as close together as seperate. This is in the two dimensional plane. 

The US beam, however is 3D. It has an elevational plane. All structures imaged in this elevational plane from one US beam will be represented on the 2D image on the screen. This is why, when you're doing IV access, the needle may look like it's in the vessel but you're actually just next to it. 

3D representation of the US beam showing the elevational plane (beam thickness)

Slice thickness artifact needs to be kept in mind when looking for dissection flaps in the aorta. Sometimes, in the ascending aorta, adjacent structures may look like a flap. Just remember that slice thickness artifacts will disappear when imaged in a different plane, whereas real structures will persist.

RANGE AMBIGUITY

The US machine is programmed to determine the depth of a structure by calculating how long the echo takes to return to the machine. The round trip time for 1cm (wave and echo) is 13 microseconds: calculated by the average speed of US in soft tissue: 1540m/s. 

If the US wave is continually bounced back and forth due to high levels of reflection, range ambiguity occurs (see reverberation artefact below). 

Range ambiguity also occurs if a structure is very deep. Remember that US waves are sent in pulses with time in between during which the machine listens for the returning echoes. If a structure is very deep, echoes reflected from this structure may come back to the machine after the next pulse has been sent. The machine will then assume that the echoes from the first pulse are actually from the second (confusing!!).  Then the machine will think wow, that echo returned really quickly: I only just sent that second pulse! and it will place the structure very superficially. 

REVERBERATION

Reverberation occurs when the US beam encounters a highly reflective structure like air or metal. The US echoes then gets bounced back and forth between the structure and the face of the US transducer. The beam never penetrates deeper than the structure into the tissues, so the machine only receives information about the tissues between the structure and the transducer face. This is the information which will appear on the screen. 

With each bounce, the machine will show the same picture: just a bit deeper and a bit less echogenic than the original. This is because the reflected echoes take longer to return (hence the machine think these echoes are coming from deeper structures) and the bounced US wave is attenuated as it passes through the tissues again and so the returning echos are weaker (hence represented as less echogenic).

Typical reverberation artifacts occur posterior to air, gas and metal (eg valve replacements). 

Reverberation artifact due to air in the alveoli

MIRROR IMAGE

 A mirror image is created when the US wave meets a specular reflector or a curved structure which acts like a mirror. A typical example of this is the curved diaphragm. At a certain angle, the diaphragm acts like a mirror and reflects the US wave in such a way that it creates a small reverberation. Just like the reverberation artefact, the machine thinks that the echoes created by the bouncing of the US is a deeper slightly less reflective image of the tissues the US has just passed through. Changing the angle of the US transducer or changing position will prevent this artefact.

Mirror artifact LUQ

REFRACTION

The machine assumes that the echoes returning to a crystal are reflected from the wave produced by that crystal. When the US beam is perpendicular to the structure, the returning echo comes directly back to the crystal which produced the wave. 

When the angle of incidence is oblique to a structure, the angle of reflection matches this angle. In some cases, the reflected echoes may never come back to the machine and the information is lost. 

Oblique angle of incidence

Refraction occurs when there is an oblique incidence AND the US wave changes direction while in the tissues. This occurs when the US hits an interface between two tissues with vastly different propagation speeds. The change in direction is not recognised by the machine and the returning echoes are assumed to lie lateral to where the structure actually is. 

solid red arrow is the initial US wave, fuzzy red arrow is the reflected wave which never comes back to the machine due to an oblique angle of incidence. 

The yellow arrow is the US wave which changes direction as it meets the interface between two tissues with different propagation speeds. The fuzzy yellow arrow is the echo returning (originally from the wave produced by the red crystal) to the yellow crystal.

"Glass" - Hamza Lafrouji - an example of refraction in real life

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