Obliteration of Segmental Pulmonary Artery

intrapulmonary density can usually be accomplished by obtaining posteroanterior ( PA) and lateral chest roentgenograms. However, when one is confronted with an abnormality within the lung, only the PA chest film may be available for appraisal. In the presence of multiple infiltrates in both lungs, it may be difficult to locate precisely the segmental distribution even after viewing the lateral film. Physicians involved with roentgenographic interpretations should therefore be acquainted with the vanous radiographic signs that assist in the segmental or lobar localization of intrapulmonary densities. In viewing the PA chest roentgenogram, there is superimposition of two or more lung segments on one another in all areas except the apices. Indeed, an opacity appearing next to the right heart border may reside in the medial segment of the middle lobe or the medial basal or posterior basal segments of the right lower lobe. In such an instance, a useful radiographic sign for localization of an opacity adjacent to the border of the heart, or aorta, is the “silhouette sign.” The principle of localization of opacities of water density utilizing the “silhouette sign” is as follows: “An intrathoracic lesion touching a border of the heart, aorta, or diaphragm will obliterate that border on the roentgenogram. An intrathoracic lesion not anatomically contiguous with a border of one of these structures will not obliterate that border.” Thus, an infiltrate in the medial segment of the middle lobe usually obliterates the adjacent contiguous heart border, whereas the heart border is preserved in lower lobe involvement. A detailed study of the “silhouette sign” is available elsewhere.” Obliteration of the borders of pulmonary vessels has been noted to occur with parenchymal infiltrates.3’4 The application of this finding for localizing an infiltrate to a lobe or segment has not previously been emphasized. It is the premise of this report that when a segmental artery, or a branch thereof, is found to have its borders obliterated by an infiltrate (“obliteration sign”), the infiltrate is located within the same segment as the vessel so affected. The principle involved is the same as that described for the “silhouette sign.” The clinical application of the “obliteration sign” for the localization of infiltrates is dependent upon the recognition of specific lobar or segmental arterial branches within the lung. It is often difficult to locate the various segmental arteries on the PA chest roentgenogram. The distribution of arteries within the lung parenchyma is indeed quite variable. However, a general pattern of branching does exist. Arterial branches within the basal segments of the right lower lobe can frequently be identified by their pattern of distribution and tracing their origin from the pars basalis of the right pulmonary artery. The segmental artery of the posterior basal