D i.e. Left upper lobe (posterior pa) - Consolidation of apical poion of left upper lobe (LUL) will obliterate or obscure the aoic knuckle (knob) poion of cardiac shadow on chest x-ray (Mnemonic-LUL sounds like Knuckle") * - Consolidation of lingula (anterior) obscures left hea border (i.e. left cardiac silhouette) and consolidation of righ middle lobe (RML) obliterates right hea border (right cardiac sihoutte). Mn= "Lingula is left and RML is right". Radiographic Mediastinal Signs Silhouette's (Obscured Margin) Sign - The normal radio-opaque margins of soft tissue mediastinal structures (such as hea, aoic arch, hilum, azygoesophageal recesses) and diaphragm are visible on chest x-ray because they are outlined by adjacent air containing radiolucent lung (i.e. b/o normal soft tissue air inteface/contrast). -Silhouette (obscured margin) sign is based on the principle, which says that if two structures have approximately the same radiographic density, and are in intimate contact with each other, then the interface between them is obliterated". However, if there is even a small gap between them, then they are seen with their borders as separate entities. An intrathoracic lesion touching a border of hea, aoa or diaphragm and having similar radiographic density (opacity) will obliterate that border on X-ray. And a lesion not anatomically contigious with a border of these structures or having markedly dissimilar density will not obliterate that border. - Silhouette's invisible or obscured border sign is caused by any intrathoracic opacity eg consolidation, pneumonia, mass, fluid, atelectasia of same density as the adjacent normal structure. This sign is used to diagnose variety of lung/chest conditions and to localize it to a specific lobe /region of lung. Silhoutte sign is very useful in localizing lung lesions as all structures forming cardiac silhoutte (hea border, ascending descending aoa, aoic knob & hemidiaphragm) are in contact with a specific poion of lung. Silhoutte structure (which may be obscured by pathology) Lung lobe/poion in contact Upper right hea border (Superior RUL (Right upper lobe, anterior vena cava), Ascending aoa & Upper pole of hilum = Superior right mediastinum or Right Aoic border segment) & anterior segment of RML Right hea border/Right cardiac RML (Right middle lobe, medial) sithoutte >>> Right medial lower lobe Aoic knudcle/knob/arch LUL (Left upper lobe apico-posterior segment) Upper left hea border (left superior LUL (Left upper lobe anterior mediastinum) segment) Left hea border (Left cardiac silhoutte) Lingula (anterior) segment of LUL Anterior hemidiaphragm Lower lobe (anterior- basal segments) Position of oblique fissure is best index of lower lobe volume. Left hemidiaphragm, Descending aoa Left lower lobe (LL) Right hemidiaphragm Right lower lobe (RLL) - Obscuration of right hea border (right cardiac silhouette) on PA viewQ and movement of horizontal fissure and lower half of oblique fissure toward one another on lateral projection is diagnostic of right middle lobe collapse. With the exception of absence of horizontal fissure on left, radiological features of lingual collapse are similar to right middle lobe collapse. - In right upper lobe collapse, lateral end of horizontal fissure moves upward & medially (towards superior mediastinum) & its anterior end moves upward towards apex. And the upper half of oblique fissure moves anteriorly. Both fissures become concave superiorly. Cervico-Thoracic Sign It is based on the fact that upper most border anterior mediastinum ends at the level of clavicle while middle (higher) & posterior (highest) mediastinum projects above clavicle. Therefore, a well defined mass (which is sharply outlined by apical lung) above the clavicles is always posterior, whereas an anterior mass being in contact with soft tissues (of same density) rather than aerated lung, is ill defined (have unsharp borders) Hilar-Overlay Sign - It allows differentiation of true cardiomegaly from large anterior mediastinal masses. In cardiomegaly the hilum is displaced laterally whereas in the presence of anterior mediastinal mass the hilum is seen projecting medial to the lateral border of mass. With the mediastinal mass, the hilium is seen through the mass whereas with cardiomegaly the hilum is displaced so that only its lateral border is visible. Hilar Bifurcation (Convergence) Sign - It differentiates hilar masses from vascular structures in cases of hilar enlargment. - If the vessels (eg pulmonary aeries) are seen to arise directly from the hilar shadow (i.e. vessels converge into lateral border of cardiac silhoutte) then the enlargement is vascular, but if they appear to arise medial to the lateral aspect of the hilar shadow, the enlargment is caused by an extravascular mass.