Normal-sized nodes are demonstrable at C/MRI, but are not visible on CXR. The ease with which enlarged nodes can be recognised at CXR varies according to their location. Nodes in the right paratracheal group are readily identified:they show uniform or lobular widening of the right paratracheal stripe. Enlarged azygos nodes displace the azygos vein laterally and enlarge the shadow that normally represents the azygos vein to over 10 mm in its short-axis diameter. If the lymph nodes beneath the aortic arch become large enough to project beyond the aortopulmonary window they cause a local bulge in the angle between the aortic arch and the main pulmonary artery.
Hilar lymph node enlargement causes enlargement and/or lobulation of the outline of the hilar shadows. The diagnosis of lymph node enlargement on plain radiography depends on the recognition of the edge of a round or oval hilar mass, an analysis that requires a detailed understanding of the normal anatomy of the hilar blood vessels. Subcarinal lymph node enlargement widens the carinal angle and displaces the azygo-oesophageal line, so that the subcarinal portion of the azygo-oesophageal line, which is normally concave towards the lung, flattens or becomes convex towards the lung, an appearance that may be confused with left atrial enlargement. Subcarinal lymphadeonpathy can be appreciated on a lateral radiograph when it manifests with hilar lymphadenopathy. The combination of enlarged lymph nodes creates a rounded mass simulating a doughnut surrounding the mainstem bronchi.
Posterior mediastinal lymph node enlargement causes localised displacement of the paraspinal and paraoesophageal lines.
CT is an excellent method for detecting mediastinal lymph node enlargement. It is usually easy to distinguish between the normal vascular structures and enlarged lymph nodes using contrast medium-enhanced CT. The short-axis measurement provides the most representative
guide to true size, because long-axis measurements vary to a significant degree according to the orientation of the lymph node within the CT section. In the assessment of lymph node enlargement, MRI provides essentially the same information as CT, although its use is limited to selected cases because of longer acquisition times and relatively limited spatial resolution (which may make measurement of individual nodes difficult). MR is not very helpful for detecting calcification. Although high T2 signal may be seen in lymphadenopathy, this finding is rarely specific.
Sarcoidosis.
Sarcoidosis is a common cause of intrathoracic lymph node enlargement. Mediastinal lymph node enlargement occurs at some stage in most patients, with the hilar nodes being enlarged in almost all cases. Additionally, tracheobronchial, aortopulmonary and subcarinal nodes are enlarged in over half the patients. Anterior mediastinal nodes occasionally increase in size, but posterior mediastinal and internal mammary node enlargement is rare. One important diagnostic feature of lymphadenopathy in sarcoidosis is its symmetry. Lymph node calcification may have a stippled or egg-shell appearance.
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