Imaging Anatomy of the Mediastinum Dr. Flip Otto Dept. of Radiology Universitas Academic Hospital

Outline Mediastinal devisions Content of mediastinum Mediastinal contours on PA chest radiograph Cross sectional anatomy of mediastinum Mediastinal lines and stripes on conventional radiography and CT correlation Mediastinal spaces Mediastinal lymphnodes

Mediastinal devisions Devisions used to describe location of pathological processes: Superior mediastinum: Above line from lower border T4 to sternal angle Anterior mediastinum: Between anterior part of heart and sternum Middle mediastinum: Occupied by heart and its vessels Posterior mediastinum: Between posterior part of heart and thoracic spine

Mediastinal contents

Mediastinal contents Superior mediastinum: Anterior mediastinum: Aortic arch and branches Brachiocephalic veins and SVC Trachea Oesophagus Thoracic duct Lymph nodes Nerves Anterior mediastinum: Thymus Mammary vessels

Mediastinal contents cont. Posterior mediastinum: Descending aorta Oesophagus Azygos venous system Thoracic duct Para-aortic, oesophageal and paraspinal nodes Middle mediastinum Heart and pericardium Nerves Lymph nodes Great vessels

Mediastinal contours on PA chest radiograph

Mediastinal contours on PA chest radiograph (from above downwards) Left side: Composite shadows of left subclavian vessels (artery lower and actually forms contour) Aortic knuckle formed by posterior part of arch Small “nipple” where left superior intercostal vein crosses aorta Aortopulmonary window Left main pulmonary artery Left ventricle Right side : Right brachiocephalic vein SVC Right atrium (right heart border) Terminal part of IVC just medial to cardiophrenic angle

Cross sectional Anatomy on CT T3 level 4 3 5 6 1 7 2 1-trachea 2-oesophagus 3-right braciocephalic vein 4-left brachiocephalic vein 5-right brachiocephalic artery 6-left common carotid artery 7-left subclavian artery

Cross sectional Anatomy on CT T4 level 5 1 2 3 1-trachea 2-aortic arch 3-oesophagus 4-superior vena cava 5-arch of azygos vein

Cross sectional Anatomy on CT T5 level 3 1 7 2 11 6 8 5 10 12 4 9 1-main pulmonary trunk 2-right pulmonary artery 3-ascending aorta 4-descending aorta 5-left main bronchus 6-right main bronchus 7-superior vena cava 8-oesophagus 9-azygos vein 10-azygoesophageal recess 11-left superior pulmonary vein 12-left descending lower-lobe artery

Cross sectional Anatomy on CT T6 level 2 3 1 4 6 5 7 1-aortic root 2-right ventricular outflow tract 3-right atrial appendage 4-left atrium 5-descending aorta 6-oesophagus 7-azygos vein

Cross sectional Anatomy on CT T8 level 2 1 5 4 3 7 6 8 1-right atrium 2-right ventricle 3-left atrium 4-left ventricle 5-left ventricular outflow tract and aortic valve 6-descending aorta 7-oesophagus 8-azygos vein

Cross sectional Anatomy on CT T10 level 3 4 2 1 5 1-descending aorta 2-fundus of stomach 3-inferior vena cava 4-oesophago-gastric junction 5-spleen

Mediastinal lines and stripes- Correlation between plain chest radiography and CT Anterior junction line Posterior junction line Stripes: Right paratracheal stripe Left paratracheal stripe Posterior tracheal stripe Posterior wall of bronchus intermedius Interfaces: Right paraspinal line Left paraspinal line Aortic-pulmonary stripe Azygo-oesophageal recess

Anterior junction line

Anterior junction line Apposition of visceral and parietal pleura of anteromedial aspects of the lungs with intervening mediastinal fat Oblique line crossing superior two thirds of sternum from upper right to lower left Increased mediastinal fat or thymus in younger patients lead to appearance as a stripe Superior and inferior V-shaped recesses Seen on 24.5%-57% of frontal chest radiographs Obliteration or abnormal convexity of line suggests anterior mediastinal disease such as thyroid masses, lymphadenopathy, neoplasms, thymic masses or lipomatosis Volume loss or hyperinflation of surrounding lung may displace the line

Abnormal anterior junction line Volume loss in right lung with rightward displacement of anterior junction line following a right middle lobectomy

Posterior junction line

Posterior junction line Apposition of visceral and parietal pleura of posteromedial portion of lungs posterior to oesophagus and anterior to T3-T5 Straight or leftward convex line projecting through trachea May appear as stripe with intervening posterior mediastinal fat Superior and inferior V-shaped recesses Seen on 32% of PA chest radiographs Abnormal bulging or convexity suggests posterior mediastinal abnormality such as oesophageal masses, lymphadenopathy, aortic disease or neurogenic tumours. Volume loss or hyperinflation of surrounding lung may displace the line

Right paratracheal stripe

Right paratracheal stripe Formed by visceral and parietal pleura of right upper lobe in contact with right lateral border of trachea and intervening mediastinal fat. Maximum normal thickness of 4mm. From level of clavicles superiorly to right tracheobronchial angle at the level of the azygos arch inferiorly. Seen on 97% of PA chest radiographs Widening or abnormal contour may be due to paratracheal lymphadenopathy, thyroid or parathyroid neoplasms, and tracheal carcinoma or stenosis. Pleural disease among most common causes for widening.

Abnormal right paratracheal stripe Widening of right paratracheal stripe caused by a large ectopic parathyroid adenoma. Note diffuse osteopenia from hyperparathyroidism.

Left paratracheal stripe

Left paratracheal stripe Formed by contact between left upper lobe and mediastinal fat adjacent to left tracheal wall or wall itself. Extends from aortic arch superiorly to join with reflection from the left subclavian artery. Seen on 21%-31% of PA chest radiographs. May be obscured by contact between left lung and left common carotid artery anteriorly or left subclavian artery posteriorly. Abnormal contour or widening commonly due to pleural effusion, also due to left paratracheal lymphadenopathy, neoplasm or mediastinal haematoma.

Abnormal left paratracheal stripe Widening of left paratracheal stripe with mass effect on the trachea due to large thyroid carcinoma and associated supraclavicular lymphadenopathy

Posterior tracheal stripe

Posterior tracheal stripe Vertical stripe seen on lateral chest radiograph formed by air in trachea and right lung outlining posterior tracheal wall and intervening soft tissues. Up to 2.5mm in thickness When posterior trachea in contact with anterior oesphageal wall may measure up to 5.5mm. Anterior border of retrotracheal space (Raider or retrotracheal triangle) with spine posteriorly and aortic arch inferiorly forming remaining borders. Most common abnormality in retrotracheal space is developmental anomalies of the aortic arch Abnormal thickening also caused by acquired vascular lesion, oesophageal lesions, lymphatic malformations, mediastinitis and post-traumatic haematomas.

Abnormal posterior tracheal stripe Widening of the posterior tracheal stripe due to dilated esophagus in a patient with achalasia.

Posterior wall of bronchus intermedius

Posterior wall of bronchus intermedius Formed when lung within azygoesophageal recess outlines posterior wall of bronchus intermedius which continues to descend 3-4cm after take-off of the right upper lobe bronchus. Seen on 90%-95% of lateral chest radiographs. Thin, vertical or slightly oblique stripe projecting through radiolucent area created by left upper lobe bronchus Normally 0.5-3.0mm thick Pulmonary oedema from congestive cardiac failure common cause for band-like thickening Other causes for thickening include neoplasms and lymphadenopathy

Abnormal posterior wall of bronchus intermedius Diffuse bandlike thickening of the posterior wall of the bronchus intermedius in a patient with pulmonary oedema.

Right paraspinal line

Right paraspinal line Formed by right lung and pleura coming in tangential contact with posterior mediastinal soft tissues Thin white line enhancing edge of right paraspinal line due to positive Mach band phenomenon Straight line from T8-T12 Seen on 23% of PA chest radiographs May be displaced laterally by osteophytes or prominent mediastinal fat Abnormal contour or displacement may suggest posterior mediastinal abnormality such as mediastinal haematoma, a mass or extramedullary haematopoiesis.

Abnormal right paraspinal line Abnormal bulge in right paraspinal line inferiorly due to mediastinal hematoma from multiple right sided transverse process fractures and an associated hemothorax.

Left paraspinal line

Left paraspinal line Formed by tangential contact of left lung and pleura with posterior mediastinal fat, paraspinal muscles and adjacent soft tissues. Extends vertically from aortic arch to diaphragm medial to lateral wall of descending thoracic aorta, but may lie lateral along lower intrathoracic course of the aorta Reported on 41% of PA chest radiographs. Seen more frequently than right because aorta promotes tangential contact of lung Osteophytes, prominent mediastinal fat, tortuosity of descending aorta, mediastinal haematoma, a mass, extramedullary haematopoiesis or oesophageal varices may displace line

Abnormal left paraspinal line Focal lateral bulge in left paraspinal line due to extensive esophageal varices in patient with liver cirrhosis.

Aortic-pulmonary stripe

Aortic-pulmonary stripe Represents mediastinal reflection formed by pleura of anterior left lung coming in contact with and tangentially reflecting over mediastinal fat anterolateral to the left pulmonary artery and aortic arch. Straight to mildly convex crossing laterally over aortic arch and main pulmonary artery. May be elevated in pneumomediastinum. Anterior mediastinal disease such as prevascular lymphadenopathy, thyroid or thymic masses may cause increased convexity laterally.

Abnormal aortic-pulmonary stripe Abnormal contour of the aortic-pulmonary stripe due to lymphoma with anterior mediastinal lymphadenopathy within the prevascular space.

Azygoesophageal recess

Azygoesophageal recess Important mediastinal interface due to differences in density between mediastinum and posteromedial right lower lobe Space lying lateral or posterior to oesophagus and anterior to the spine from level of arch of azygos to aortic hiatus inferiorly Right infra-azygos pleuroesophageal stripe may outline recess when air filled oesophagus and intervening pleura come in contact with right lower lobe Superioly and middle thirds may show mild leftward convexity. Lower third typically straight edge. Superior right convexity may be seen in the young, but is abnormal in the elderly. Abnormal contour may be due to lymphadenopathy, hiatal hernias, bronchopulmonary-foregut malformations, oesophageal neoplasms, pleural abnormalities and cardiomegaly with left atrial enlargement

Abnormal azygoesophageal recess Abnormal contour and right lateral convexity of distal third of azygoesophageal recess due to a large hiatal hernia.

Mediastinal spaces Four named spaces surrounding the central airways: Pretracheal space Aortopulmonary window Subcarinal space Right paratracheal space

Aortopulmonary window

Aortopulmonary window Mediastinal space seen as interface on frontal chest radiographs Lies posterior to aortic-pulmonary stripe Borders: Superior: Inferior wall of aortic arch Inferior: Superior wall of left pulmonary artery Anterior: Posterior wall of ascending aorta Posterior: Anterior wall of descending aorta Medial: Trachea anteriorly, lateral wall of left main bronchus and oesophagus posteriorly Lateral: Left lung and pleura in contact with aortic arch and extending inferiorly to contact left pulmonary artery – forms interface representing AP window on frontal chest radiographs, normal concave reflection along mediastinal side. Convex contour abnormal.

Abnormal aortopulmonary window - causes May be related to disease of contents: left recurrent laryngeal nerve; left vagus nerve; ligamentum arteriosum; mediastinal fat; lymph nodes; left bronchial arteries Abnormal convexity may be due to prominent mediastinal fat, lymphadenopathy, bronchial artery aneurysms or nerve sheath tumours Paralysis of left vocal cord or diaphragm should prompt search for disease in AP window Disease of structures forming borders of AO window e.g. aortic aneurysms can also cause abnormal appearance

Abnormal aortopulmonary window Abnormal bulge in AP window due to significant soft tissue mass within AP window and subcarinal space compatible with metastatic lymphadenopathy in a patient with bronchogenic carcinoma. Also widened right paratracheal stripe due to lymphadenopathy and left lower lobe consolidation.

Mediastinal lymphnodes American Thoracic Society definitions of regional lymph node stations X Supraclavicular nodes 2R Right upper paratracheal nodes 2L Left upper paratracheal nodes 4R Right lower paratracheal nodes 4L Left lower paratracheal nodes 5 Aortopulmonary nodes 6 Anterior mediastinal nodes 7 Subcarinal nodes 8 Paraesophageal nodes 9 Right or left pulmonary ligament nodes 10R Right tracheobronchial nodes 10L Left tracheobroncheal nodes 11 Intrapulmonary nodes

Conclusion Traditional frontal and lateral chest radiography remains a valuable tool in the evaluation of chest disease despite increased reliance on CT, therefore familiarity with anatomic basis of mediastinal lines and stripes as seen on radiography imperative. Knowledge of normal anatomic structures within different mediastinal divisions helps guide formulation of appropriate differential diagnosis

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