A 50-year-old male patient was admitted for fever and low abundance of haemoptysis.
We performed a chest computed tomography without and with intravenously administered contrast material at 25 seconds in supine position and at 3 minutes in prone position. The CT scan showed neither proximal pulmonary embolism nor pneumonia. We incidentally detected a well-defined, homogeneous, low-attenuation and non-enhancing mass located at the anterior mediastinum (about 2 cm in diameter). The 3 minutes scan acquisition showed no wall enhancing and identified a change of the size of this mass (increased in diameter).
Pericardial diverticula are rare benign congenital malformations due to abnormal fusion or lack of fusion of mesenchymal lacunae through diverticula retaining communication with the pericardial space. 
Usually, patients have no symptoms and often pericardial diverticula are incidental findings on CT or cardiac MRI. Most commonly, they are found in the cardiophrenic angle (70 % of cases)  and less frequently in the anterior mediastinum.
The main imaging diagnostic criterion opposing to pericardial cyst is the change in size or form with changes in body position or breathing. 
The late enhancement technique is useful to detect an enhanced wall for the differential diagnosis of cystic thymoma, particularly in this location.
Differential Diagnosis List