[Echographic evaluation of the thymus gland in childhood]

The thymus is a linphoepithelial organ located mostly in the anterior mediastinum and, for a smaller part, in the neck. The gland is particularly developed in the fetus and in children, while it shrinks in young adults following an involution process. We investigated the US appearance of the thymus in normal pediatric subjects to gather information for use in pediatric patients with suspected thymus conditions, using a safe, irradiation-free, technique.

We performed a US examination of the anterior mediastinum and the neck in 30 children (14 males and 16 females) ranging in age 6 months to 11 years. We used a Sonora LOGIC 700 MD General Electrics unit with 7.5-13 MHz linear probes and acquired transverse and longitudinal scans on the chest wall and the neck, integrated with intercostal scans.

The thymus had the same echogenicity as the liver parenchyma in 22 children (73.3%), lower echogenicity in 2 (6.6%, age range 6-12 months) and higher echogenicity with heterogeneous structure in 6 children (20%, age range 8-11 years). The gland was in central and symmetric localization in 20 children (66.6%) while it exhibited a slight deviation leftwards in 7 (23.3%) and rightwards in 3 children (10%). We considered as normal the following values: in the right lobe, 1.4 cm for the AP diameter and 2.5 cm for the longitudinal one; in the left lobe, 1.4 cm for the AP diameter and 2.9 cm for the longitudinal one.

Radiological studies of the thymus are really difficult to perform because the gland is extremely variable in size, extension and shape. The thymus has the same echogenicity as the liver parenchyma and lower echogenicity than the thyroid parenchyma. We believe that US is an effective technique for studying the thymus in all its involution stages and that provides similar information to CT and MRI. Also, US does not use radiations and needs no sedation, which improves safety for young patients.