NORMAL THYMUS

• The thymus appears as a homogeneous structure in the upper mediastinum, located between both lungs, anterior to the pulmonary trunk, aortic arch and superior vena cava
• Thymic tissue is characterized by tiny echogenic foci, which facilitate its differentiation from lung. Its soft consistency allows it to move with underlying vascular pulsations.
• The thymus is composed of 2 distinct lobes, each surrounded by a collagenous capsule with septa extending into the corticomedullary junction dividing the cortex further into lobules.
• The thymus gland contains 3 major cell populations—epithelial, hemopoietic, and accessory cells (supports erythropoiesis and granulopoiesis).
• Arterial supply to the thymus varies. It could be derived from the internal mammary artery, the inferior thyroid artery, and from these 2 plus the superior thyroid artery.
• A single vein frequently leaves each side of the medial lobe. The veins join to form a short, wide vein that drains into the left brachiocephalic vein. A lateral vein drains from the right side of the gland into the superior vena cava and from the left side into the left brachiocephalic vein.
• A hypothalamic-thymic neural pathway has been suggested to explain the numerous neurological, social, psychological, and environmental factors that have been shown to influence the thymic hormones and the immune system.

NORMAL SIZE

• The marked variability in the contour of the thymus has made it difficult to visualize and determine its normal size.
• Antero-posterior diameter (APD) is measured in the midline at the sternum (1).
• 2 mm at 14 weeks.
• 20.8 mm at term.
• APD (mm) = -1.865 + 0.437 MA (weeks).   R² = 0.52.
• Zalel et al. (2) published reference ranges for thymic size with gestational age. These measurements may assist in the detection of thymic aplasia or hypoplasia in the future

ULTRASOUND

• The best plane to visualize this structure is the ventral cross-sectional plane of the 'three-vessel' view. In a more cranial plane, the gland can be demonstrated anterior to the innominate vein at the confluence of the left and the right jugular veins. Use of a high-frequency transducer and varying the gray scale can allow reliable delineation of the thymus from neighboring lung.
• There may be limitations in visualization of the thymus, particularly where there is abnormal intrathoracic anatomy; e.g. cardiomegaly, diaphragmatic hernia, cystic lung lesions, hydrothorax and pericardial effusion or at an early (before 20 weeks) gestational age. In such cases it is still unclear whether the thymus is merely shifted from its place, where its visualization is difficult, or compressed by the distorted intrathoracic structures.
• Observing the great vessels positioned directly posterior to the sternum rather than centrally within the upper thorax is useful to differentiate cases of absent or hypoplastic

Thymus at 19 weeks of gestation
Thymus at 32 weeks of gestation SVC = Superior vena cava BCV = Brachiocephalic vein

REFERENCES

1. Felker RE, Cartier MS, Emerson DS et.al. Ultrasound of the fetal thymus. J Ultrasound Med 1989;8:669-673.
2. Zalel Y, Gamzu R, Mashiach S, Achiron R. The development of the fetal thymus: an in utero sonographic evaluation. Prenat Diagn 2002; 22: 114-117