What is indicated by the "figure of 3" sign in imaging studies?

The "figure of 3" sign is an important radiological finding associated with coarctation of the aorta. It is characterized by the appearance of the aorta on chest X-rays, where the upper part of the aorta seems to form a "3" shape due to constriction at the site of coarctation. This sign reflects the narrowing of the aorta, which results in a distinct contour when viewed radiographically.

In coarctation, the segment of the aorta beyond the constriction can become post-stenotic dilated, while the portion proximal to the coarctation may appear engorged and prominent due to increased blood flow and pressure. This leads to the characteristic "figure of 3" appearance, as the vessels may appear distinctively shaped due to the disparity in blood flow dynamics across the narrowed segment. Recognizing this sign is critical for diagnosis as it helps differentiate coarctation from other potential cardiovascular abnormalities.