Paediatric Imaging

Thymus, normal

a gland situated in the anterior mediastinum and the source of T lymphocytes and is consequently important in cell-mediated immune responses. In infancy, the thymus is at its largest size and appears prominent on imaging. At puberty, involution begins. Disease and stress cause thymic shrinkage. This process is mediated by endogenous steroids and occurs rapidly, radiographic change is seen within days. Thymus rebound enlargement to a greater size than previously can occur after recovery, this may occur weeks to months after the cessation of stress. Thymic configuration on chest radiographs is variable. It can be uni- or bilateral, extend from the apex of the lung to the diaphragm, or be focally border forming. It is usually visible on chest radiographs until age 2 and may be border forming until age 5. Several characteristic appearances have been described on plain films. The sail sign (Fig.1) seen in 8.8% of children describes a triangular soft tissue density opacity forming either the right (6.2%), left (2.4%) or both (0.2%) mediastinal borders.The wave sign is normal indentation of the thymus laterally by the adjacent anterior ribs. The thymic notch is an indentation seen at the junction of the thymic and cardiac outlines (Fig.2). The spinnaker or angels wing sign describes the elevation of the thymus in the presence of a pneumomediastinum. The soft, malleable nature of the thymus can result in different appearances on inspiration and expiration films. At CT the thymus is of homogeneous density, it does not exert mass effect or infiltrate adjacent structures. On MRI the thymus has uniform signal intensity, slightly higher than muscle on T1-weighted images and high signal on T2-weighted images. The thymus is quadrilateral in shape in infants and young children and becomes more triangular after age 5 years. The lateral borders are convex in infants, becoming straight or concave in older children. Fatty infiltration occurs in the late teens or early twenties. On ultrasound the thymus is of similar echotexture to liver and spleen.

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Fig.1

Chest radiograph demonstrating the sail sign. The thymic gland is seen as a triangular opacity forming the right mediastinal border (arrow).
Thymus, normal, Fig.1
Thymus, normal, Fig.2