Pneumomediastinum with ring around the artery sign on chest radiograph

Plain chest radiographs of a 22-year-old-woman who presented to the emergency department with 10 days of sore throat and cough.
(A) A radiograph of the chest in the posteroanterior projection reveals linear lucencies overlying the upper chest and neck (arrows) consistent with pneumomediastinum.
(B) A radiograph of the chest in the lateral projection reveals the "ring around the artery" sign (arrow), a finding seen in pneumomediastinum.
(C) A magnified view of the "ring around the artery" sign (arrow).