Image formation assumes:
- Sound travels in straight lines
- At a constant velocity
- With uniform attenuation
- Reflected only once from each interface
Artefacts result when the echo does not behave in this way and the system misinterprets it.
Fluid filled structures are weakly attenuating and a larger proportion and greater amplitude beam passes through to structures in the region behind. The machine interprets this as an increase in acoustic reflection and these structures show up brighter on the image.
Hard calcific substances and soft tissue-air interfaces reflect almost all of the soundwaves. No information is received from the area behind the structure.
Multiple reflections to and fro between the transducer face and a relatively strongly reflecting interface near the surface produces a series of delayed echoes. These look like stripes within a fluid filled structure.
Two types of reverberation artefact exist:
- Comet tail: from metal or calcified objects
- Ring down: from a collection of gas bubbles
Reflection / Mirror Artefact
Sound bounces off a strongly reflecting object which acts as a mirror and reflects the pulse to another tissue interface. The interpretation of the image is that the second interface is beyond the first surface, much like the reverberation artefact. This most often happens at the diaphragm wherein the liver is seen in the chest cavity due to sound waves being reflected off the diaphragm.