Figure 3. Variation of the image quality indices with the optical density (OD) in the lung area OD_lung for posteroanterior chest radiography at constant beam quality (70 kVp and 1 mm aluminium added filter). (a) Areal contrast index C_a in the lung, cardiac and diaphragmal areas and in the ribs. •, lung; , cardiac; *, subdiaphragmal; +, ribs. (b) Low contrast visualization index P_low for the lung, cardiac and diaphragmal areas and high contrast visualization index P_high in lung field. •, P_low lung; , P_high lung; , P_low cardiac; *, P_low subdiaphragmal.