Sonographic study of the thymus in infants and children.

Because of variability in the size of the thymus, imaging in children is a challenge. Difficulties in differentiating an abnormal mediastinal mass from a normal thymus may lead to unnecessary investigations. We performed a prospective sonographic study of the normal thymus in 140 infants and children aged newborn to 8 years to determine the size of the normal thymic gland. Patients were classified into five groups by age: 1) less than 6 months, 2) 6-12 months, 3) 13-24 months, 4) 25-48 months, 5) more than 48 months. The thymus was scanned through the suprasternal, transsternal, and parasternal windows with a Sonos 1000 scanner and a 5.5-MHz sector probe. The thymus was visualized in at least one of the three windows and the width and thickness were measured in 133 of the 140 cases. The mean +/- standard deviation width and thickness were 3.67 +/- 0.64 cm and 1.30 +/- 0.25 cm in boys, and 3.45 +/- 0.59 cm and 1.20 +/- 0.26 cm in girls (p = 0.047 and p = 0.033 in width and thickness, respectively). Regression analysis demonstrated an increase, mainly in the width, with increasing age (width = 3.48 + (0.05 x age) cm, p = 0.009), indicating an increase of 0.05 cm per year. In conclusion, the thymus was clearly and easily visualized on sonograms in the majority of children we studied. This study established the normal size range of the thymus in children less than 8 years old.