What causes Thymic sail sign in thymus on chest X-ray?
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Answer:
Normal prominent thymus in infants, appearing as a triangular soft tissue shadow (the Thymic sail sign) with convex lateral border along the right or left mediastinal border on chest radiograph. This is a physiological finding in approximately 8.8% of children, reflecting the normal size and position of the thymus in the anterior superior mediastinum, and is not associated with pathology.
Why is it called so?:
Named for its resemblance to a sail due to the triangular shape of the thymic shadow with a convex lateral border projecting from the mediastinum, evoking the image of a ship’s sail billowing outward. This visual appearance is commonly referred to as the Thymic sail sign on pediatric chest X-ray.
Pathophysiology:
In infants, the thymus is relatively large, occupying the anterior superior mediastinum behind the sternum and extending superiorly toward the neck and inferiorly over the heart. On frontal chest X-ray, the thymus forms a triangular soft tissue density with convex lateral margins due to its natural contour, bounded inferiorly by the major fissure and continuous with the mediastinum without intervening air. This anatomy produces the characteristic Thymic sail sign on imaging.
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