Lesions in this region can be difficult to definitively characterise, however, only a limited differential for these abnormalities can be considered. In the absence of other adenopathy within the mediastinum a large mediastinal nodal mass would be unlikely with the exception of a possible focal lymphomatous mass. The absence of contralateral prominence of the atrial appendage excludes the possibility of posterior atrial enlargement. Right paracardiac masses can include the possibility of thymoma, however, although these can be similarly large, these are generally in the right paracardiac location, effacing the right cardiac border. The most likely cause of these appearances would therefore reflect a bronchogenic cyst.
CT images demonstrate a large abnormality in the subcarinal/azygoesophageal recess. This demonstrates uniform attenuation measuring somewhat more than fluid density (approximately 30 HU). These appearances are typical for a bronchogenic cyst, the density of which is often slightly hyperdense.
Similarly, axial T1/T2, coronal T1/T2 fat sat images demonstrate a uniform unilocular abnormality displacing the adjacent structures but also demonstrating moderately elevated T1 signal. In my experience moderately elevated T1 signal indicative of proteinaceous fluid is more common than the low T1 signal expected of simple fluid in these lesions. No enhancement should be expected.
Bronchogenic cysts are part of the foregut duplication spectrum. Most typically they arise in this region, and can become quite large before noted incidentally on chest radiographs.