The depth at which each structure is displayed on a US image
is proportional to the amount of time it takes for a US beam
to return to the transducer from the time when it leaves the
transducer. Normally, this amount of time would be primarily
dependent upon the depth of a tissue from which the beam
reflects. This would result in an image with an anatomically
accurate depth. In mirror-image artifact, the return of sound
beams is delayed, and therefore the structures from which
these delayed beams are reflected are displayed at a greater
depth than their true anatomic depth. This delay occurs
in the presence of highly reflective interfaces, such as the
diaphragm/lung base interface on a right upper quadrant
scan. The diaphragm/lung base interface is highly reflec-
tive because gas reflects almost 100% of the sound that
hits it and is therefore the best acoustic mirror in the body.
A pulse from the main beam travels through the liver and is reflected
off the diaphragm. This reflected echo reaches the liver lesion
and reflects back to the diaphragm.14 From the diaphragm,
the echo finally reaches the transducer.
Because color Doppler scanning creates images with
marked contrast between vascular structures and soft tissues
(ie, color vs gray scale), mirror-image artifacts are particu-
larly common on color Doppler scans. As with gray-scale
imaging, color Doppler mirror images occur most frequently
around the lung. However, the increased contrast also allows
weaker acoustic interfaces, such as bone or even the back wall
of the carotid, to act as mirrors for color Doppler imaging.
Artifact due to: Propagation assumption.
Where it commonly occurs:
Diaphragm with liver lesions or the liver itself being duplicated, trachea.