The chest x-ray shows a C-shaped ring in the middle of the cardiac shadow; this is due to mitral annular calcification.
It is typically seen in hypercalcaemic states (e.g. end stage renal disease). It is also associated with infective endocarditis, embolic transient ischaemic attacks, and AV conduction disturbance.
Reference: Grainger & Allison’s Diagnostic Radiology Essentials.