Canis ISSN: 2398-2942
Contributor(s): Alison Dickie
- Reflection of sound waves at a strong reflective interface can result in the appearance of a mirror image of the object under examination on the far side of the interface.
- The classical example of this is the mirror image artifact of the liver seen on the far side of the diaphragm.
This should not be mistaken for a ruptured diaphragm.
- The amount of the sound beam reflected at an interface depends on the difference in acoustic impedence between the two tissues and also the size and shape of the surface.
- If there is a large difference in acoustic impedence between the 2 tissues most of the sound will be reflected and therefore cannot penetrate deeper tissues.
- Distal acoustic shadowing occurs where sound is totally reflected or scattered so that no sound penetrates deep to the reflector resulting in an anechoic area on the ultrasound image.
- In the body this generally occurs at interfaces of soft tissue or fluid with air, metal or mineralized tissue.
- This is commonly seen with mineralized structures, eg ribs, and calculi.
- It may also be seen in the kidney where dense tissue in the renal calices may produce slight shadowing.
- Interfaces with air usually also produce reverberation artifacts and metal may also demonstrate ' comet tail' artifacts.
- As the sound beam travels through tissue it is attenuated (loses energy) through absorption or scattering.
- There is less attenuation of sound waves passing through a fluid filled structure than through soft tissue ’ more energy entering tissue at the far side of the fluid compared with the adjacent portion of the beam which has passed through soft tissue.
- As a result the region of the image beyond a fluid filled structure will appear brighter than adjacent tissues.
- This is known asdistal acoustic enhancement.
- The best example of this is in the liver where the region of the liver beyond the gall bladder will appear more echogenic (brighter) than adjacent parenchyma.
- Distal acoustic enhancement is a useful artifact in that it enables fluid filled structures, eg cysts to be differentiated from hypoechoic masses.
Occasionally soft tissue may produce acoustic enhancement, eg enlarged lymph nodes, although this is not as pronounced as that associated with a fluid filled structure.
Slice width artifact
- The ultrasound beam has a finite thickness (typically 1.5-3 mm). This depends upon transducer construction but is usually thinnest at the focal zone.
- Echos produced by objects adjacent to the slice being interogated may appear within the slice.
- Slice width artifact is most commonly encountered when imaging curved fluid filled structures, eg blood vessels and gall bladder.
- The resulting echoes appearing within the lumen are termed 'pseudosludge' and can be differentiated from true sediment - see side lobe artifacts.
- Reverberation artifact is produced when sound reflected from a highly reflective interface hits the transducer face and is reflected back into the tissue. The beam then becomes trapped bouncing back and forth between the transducer and the interface.
- This results in the appearance of multiple parallel echoes regularly spaced beyond the interface.
Side lobe artifact
- The ultrasound beam is composed of a main sound beam and a number of weaker beams directed at different angles from the main beam. Side lobes are undesirable and manufacturers try to minimize them. The weak side lobes may be reflected off objects and be detected by the transducer.
- The weak returning echoes are misregistered by the computer. This is most commonly seen as erroneous echoes within cystic or fluid filled structures, eg bladder.
- In the bladder this is seen as 'pseudosludge' which may be mistaken for sediment.
- Pseudosludge may be differentiated from sediment in that it has a curved interface and does not move if the position of the patient or transducer is altered.