<prompt> Consolidation of which poion of the lung is likely to obliterate the Aoic knuckle on X-ray chest:Left LingulaRight Upper lobeApex of lower lobeLeft upper lobe posterior pa  <response>: 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 Silhouettes 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. bo normal soft tissue air intefacecontrast. -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. - Silhouettes 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 lungchest 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 lobepoion 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 borderRight cardiac RML Right middle lobe, medial sithoutte Right medial lower lobe Aoic knudcleknobarch 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. 