Mirror Image Artefact

Specular reflection occurs at large, smooth interfaces. If the change of impedance is high e.g. at an air/soft tissue boundary,  the interface can act as an acoustic mirror.

A common example of this is seen above the diaphragm when the liver or spleen is imaged. A pulse of sound travels through the liver, bounces off the highly reflective lung interface, and is directed back into the liver at an equal and opposite angle.

Echoes from structures within the liver are subsequently reflected back to the diaphragm and on to the transducer. The equipment assumes a single, straight line journey, and will therefore place these echoes above the diaphragm.

Example 1

In this example of the left upper quadrant, a mirror image of the spleen is seen above the diaphragm (see arrow).

Example 2

In this image. the specular reflection in the right upper quadrant is shown. The sound is reflected off the diaphragm as shown, but the machine cannot factor this in and shows an image of the liver above the diaphragm.

Other examples:

• reflection of a liver lesion into the thorax (the commonest example)
• reflection of abdominal ascites mimicking pleural effusion
• duplication of gestational sac (either ghost twin or heterotopic pregnancy)
• duplication of the uterus