The question of parallax-effect on radiographic assessment of short trachea in infants and children.

Disproportionately short trachea, can be recognized in AP chest radiographs of infants and older children taken during quiet respiration and showing an evaluable air tracheobronchogram, by the thoracic vertebral level of the carina (normally at T4 in neonates and infants, and at T5 in children two years of age or older). To establish the extent to which a parallax-effect of differing angles of X-ray beam relative to the patient can influence the radiologic assessment of carinal level, we measured the tracheal-vertebral distance at the carina in 29 lateral radiographs of neonates, infants and children, prepared diagrams of the loci of carinal beam intercept of the vertebral column for different angles of beam to body, from tracings of lateral radiographs of two 2-week-old infants, one with trachea of normal length and one with short trachea; of a nine-year-old child with short trachea, and of a ten-year-old with normal trachea, and made radiographs of a postmortem tracheobronchogram of a two-day-old infant at different beam angles. We conclude that tube-body angles of the order of 10-15 degrees from vertical at 27 inches (68.6 cm) or at 40 inches (101.6 cm) FFD do not significantly affect the apparent thoracic vertebral level of the carina in AP chest radiographs, and that a beam angle of 20 degrees or more from vertical is necessary to change the apparent level of tracheal bifurcation by one vertebral body, depending on the patient's age and on whether the patient's position relative to the X-ray beam is lordotic or anti-lordotic.(ABSTRACT TRUNCATED AT 250 WORDS)