On frontal chest radiographs, this sign presents as a curvilinear density projecting over the right retrocardiac region, signifying left atrial enlargement (Fig. 9).³² The curvilinear line represents the inferolateral margin of the left atrium.³³ The double density sign may be observed in patients without cardiac disease; however, there is a semiquantitative measurement to estimate the left atrial diameter and better estimate whether it is a real finding.³³ Higgins et al found that, on PA radiographs of adult patients, if the left atrial dimension is defined as the distance from the midpoint of the double density to the inferior wall of the left mainstem bronchus, a distance greater than 7 cm was consistent with a diagnosis of left atrial enlargement, confirmed on echocardiography.³³ The measurement was found to be an unreliable sign in the evaluation of pediatric patients with a double density sign on PA radiographs.³³