Normal thymus in infancy: sonographic characteristics.

A prospective sonographic study of the normal thymus in 56 infants was performed to determine the shape, extent, and echogenicity of normal thymic tissue. The thymus had either a triangular or teardrop shape on longitudinal scans and a trapezoidal or bilobate shape on transverse scans. Typically, the thymus was located anterior to the great vessels and extended down to the upper portion of the heart (in one infant, to the diaphragm). It extended up into the lower cervical area in five infants. The great vessels--including the superior vena cava, aorta, and the pulmonary artery--were well imaged through the thymus. In six infants the thymus encircled the left innominate vein. The echogenicity of the thymus was homogeneous, similar to that of the liver and spleen. The intensity of thymic echogenicity was less than that of the liver, spleen, and thyroid gland. This study demonstrates that the thymus has a characteristic sonographic appearance and can be easily identified.