Normal Thymus

The thymus arises from the third and fourth branchial pouches and contains elements from all three germ cell layers [15]. The gland consists of two lateral lobes placed in close contact along the midline of the upper chest [14]. The thymus increases from birth to between 4 and 8 months of age and then decreases [14]. It normally weighs about 15 gm at birth and 35 gm at puberty [14]. It is most active and largest during puberty after which it shrinks in size and activity in most people and is replaced with fat [14]. Total fatty involution of the thymus occurs around the age of 40 years when only about 5% of residual thymic tissue is retained [19]. However, the normal thymus can still be identified in up to 100% of patients under the age of 30 years, in up to 73% of patients between the ages of 30-49 years, and in up to 17% of patients older than 49 years [13]. 

On CXR in infants and young children the thymus is strikingly large- it usually has smooth borders and remains visible on radiographs through 3 years of age [20]. A scalloped or wavy contour (the thymic wave sign) occurs secondary to anterior rib impressions on the thymus [20]. The thymic sail sign - a triangular slightly convex right lobe of the thymus with a sharply demarcated base produced by the minor fissure is seen in about 5% of children [20]. On non-contrast CT the thymus appears similar in density to muscle tissue [14]. After contrast administration the thymus shows homogeneous enhancement of 20-30 HU [14]. Thymic morphology varies greatly- in young adults it is typically bilobed and V-shaped with two small processes extending into the neck [20]. On MR, the normal thymus has homogeneous signal on T1 which is slightly brighter than muscle [14]. On T2, the signal becomes much brighter than muscle [14]. On opposed phase chemical shift imaging the normal thymus and hyperplastic thymus have been shown to demonstrate decreased signal intensity due to the presence of fat within the tissue [13]. Thymic heterogeneity on fat saturation sequences should be considered pathologic [14]. Gallium 67 will accumulate in the normal thymus in up to 60% of children under 2 years of age and also in activated thymic tissue following chemotherapy and systemic illness [14]. I-131 uptake has also been reported in hyperplastic thymic tissue that does not contain ectopic thyroid tissue [14]. The thymus can also be visualized on Octreotide imaging due to the presence of somatostatin receptors in the normal thymus [14]. The normal thymus can accumulate FDG and be visualized on PET imaging [14