Kommerell diverticulum
A Kommerell diverticulum is a rare aortic arch anomaly. It is bulbous configuration of origin of left subclavian artery. However, it was originally described as a diverticular outpouching at origin of right subclavian artery with left aortic arch.
Radiographic features
Plain film
A small rounded density may be seen laterally towards the left of the trachea.
Barium Swallow
An impression can be seen from left side of oesophagus simulating a double aortic arch.
CT / MRI
Typically shoes bulbous enlargement of proximal subclavian artery is seen at its origin from aortic arch, posterior to oesophagus.

Aberrant subclavian and Kommerell diverticulum CT
Article Author : Dr. Praveen Jha, Radiopaedia
References
1. Wolfgang Dahnert. Radiology Review Manual. 6th Edition.
page revision: 2, last edited: 03 Oct 2010 14:03