Mirror image artefact
This is one of the commonest artefacts encountered in NephroPOCUS. It is generated by the false assumption that an echo returns to the transducer after a single reflection. In this scenario, the primary beam encounters a highly reflective interface (= mirror), the reflected echoes then encounter the “back side” of the structure in its pathway and are reflected back toward the reflective interface before being reflected to the transducer for detection. The display shows a duplicated structure (= mirror image) equidistant from but deep to the reflector. Remember, in the ultrasound machine’s mind, time = distance, which means if the echoes take longer time to reach back to the probe, that structure is displayed farther from the top of the screen and vice versa. In other words, structures closer to the probe are displayed at the top of the image.
Here is a nice animation from PMID: 26969139, illustrating mirror image formation. The pathway leading to mirror image is shown in light yellow.
In the example below, the diaphragm sitting next to the air-filled lung that is reflective, acts as a mirror, and the mirror image artefact appears as hepatic parenchyma (or spleen on the left) in the expected location of lung. However, when there is pleural effusion, this artefact usually disappears (fluid is an excellent transmitter of ultrasound waves unlike air) and you’ll see an anechoic area above the diaphragm representing the effusion. In these cases, identification of thoracic spine sign confirms the diagnosis of pleural effusion.
You can watch video 10 from the curriculum module for more ultrasound artifacts.
Mirror image artifact can also occur in the thorax at lung-pericardium interface. Here is a nice example where you see beating structure (heart) in the left lung area.