The embryologic footprint of the thymus: study by imaging.

The thymus is one of the primary immune organs in childhood. During embryonic development, the migration of the thymus from the cervical region to the mediastinum coincides with the formation of other structures in the anterior cervical region, which can result in anomalous thymic locations. Among these, the most frequent are cervical remnants of accessory thymic tissue along the normal migration path and thymic extensions that project beyond their usual location. Ectopic locations, which deviate from the normal embryological course, are less common. Radiologists must recognize these variants and their imaging manifestations to avoid misdiagnosis and incorrect interventions. This article aims to review and illustrate, using imaging, the embryology, normal development, anatomy, and the most common locations of thymic tissue outside its normal position in the anterior mediastinum in children.