Echocardiography is the most widely used noninvasive imaging modality for the evaluation and diagnosis of cardiac pathology. However, because of the physical properties of ultrasound waves, cardiologists are often confronted with specific image artifacts. It is of particular importance to adequately recognize such artifacts and avoid misdiagnosis arising from their presence. A typical artifact seen in clinical practice is a “mirror artifact,” caused by the reflection of ultrasound waves on a strong reflector (pericardium, aortic wall, pleura, diaphragm). These artifacts are usually easy to identify in two-dimensional images, as they present below a strong reflector as copies of objects that are located above the reflector and move in the opposite direction.
We present a mirror artifact in a transthoracic parasternal short-axis window in which the mirrored objects (the mitral valve leaflets) are not present in the two-dimensional image above the reflector but are located in the third dimension, outside the two-dimensional scanning plane ( Figure 1 B). A key issue is the three-dimensional morphology of the strong reflector (the pericardium) and the fact that the short-axis scanning direction is not truly perpendicular to the long-axis direction of the heart (and thus the pericardium). As a result of this, ultrasound waves in the short-axis imaging direction are reflected outside of the imaging plane toward the mitral valve leaflets (reflection angle α; Figure 1 C), where they are reflected back into the imaging plane at the same angle and reach the transducer after some time period. Because of the assumption of wave propagation, these valve leaflets are thought to be located in the imaging plane below the reflector and move in the opposite direction as the actual valve leaflets. Some might misdiagnose these findings as a loculated pleural effusion. Cross-correlating the images with the long-axis view, or using the simultaneous biplanar imaging mode of a three-dimensional probe, certainly helps in identifying the findings as a normal imaging artifact ( Figures 1D and 1E ).
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