title: A useful sign in differentiation of p-a from a-p chest roentgenogram.
abstract: Differentiation of antero-posterior (a-p) from postero-anterior (p-a) projections of chest roentgenograms is practical with ease and reliability by connecting the two outermost points of the junction of the first and second ribs on the plain film and observing its relation to the superior margin of the lung apex and inferior border of the posterior part of the second rib. Greater than 92 percent accuracy was obtained when this criterion was used to evaluate 500 p-a and 500 a-p chest roentgenogram, regarding the standard technical datas.
mesh_terms: D006801:Humans; D013902:Radiography, Thoracic; D013679:Technology, Radiologic / Q000379:methods