Chang sign

Chang sign is a radiologic sign for detecting pulmonary embolism in X-ray films. It refers to the dilatation and abrupt change in calibre of a previously normal descending pulmonary artery on a chest X-ray film.^[1] Chang sign usually appears within 24 hours of the onset of chest pain due to pulmonary embolism,^[2] and the maximal dilatation of the descending pulmonary artery often occurs in two to three days after the onset of pain.^[3]

Chang sign is absent in case of co-existing pneumonia or other conditions causing central opacities, where the descending pulmonary artery cannot be visualised in the X-ray image.^[3]