Imaging description

The thymus is the organ of T-cell maturation. The retrosternal gland increases in weight from birth to the age of about 12 years and subsequently involutes with gradual fatty replacement of cellular components. During infancy the ratio of thymus weight to body weight is highest, which can lead to its prominence on chest radiographs of small children. Gradual fat replacement of thymus tissue starts at puberty, which is why the residual thymus is typically detected on CT scans of young adults. MR data suggest that the thymus thickness itself does not actually change much with increasing age [1]. The thymic density on CT is highest in infancy where it measures about 80 Hounsfield Units (HU) on non-contrast CT of the chest [2], which is similar to the attenuation of acute hematoma. The thymus density in teenagers and young adults usually approximates muscle tissue. Above the age of 50 years, residual thymic tissue is uncommonly separated from surrounding mediastinal fat on CT. “Thymic rebound” occurs in some adults who have undergone chemotherapy [3].

Normally, the thymus fills the mediastinal perivascular space up to the age of 20 years [4]. The thymic borders are initially convex but become straight or concave as a child grows, assuming a triangular shape [4]. On radiographs, the thymic sail or notch sign, a triangular margin of the upper mediastinum, is specific for the normal thymus when it is present and should not be confused with the spinnaker sail sign, which is seen with pneumomediastinum [4]. More specific findings for aortic injury include abnormalities of the aortic arch or loss of concave margin seen normally at the aortopulmonary window [5]. One recent review of pediatric thoracic injuries found an indistinct aortic knob to be the most specific sign of blunt thoracic aortic injuries (BTAI) on chest radiographs [6]. Rightward tracheal or esophageal deviation, left mainstem bronchus depression, and a left apical cap are other corroborative radiographic signs of BTAI [6]. Imaging of BTAI is described in detail in Cases 39 to 41.