After the exclusion of patient prosthesis mismatch, the most common cause of acquired mechanical prosthetic heart valve obstruction is thrombosis or pannus formation, with an incidence of 0.4-6% per year depending on valve type and position. Differentiation of the two entities is of significant importance as valve thrombosis can be potentially amenable to thrombolysis.

Cardiac CT allows for the evaluation of leaflet opening and closing angles, dynamic leaflet motion and the composition of perivalvular masses to aid with differentiating between valve thrombosis and pannus formation. The pannus is found below the aortic prosthesis. They are circular or semicircular extending from the prosthesis ring, and enhance with contrast. Thrombus can be found above or below the valve prosthesis and tends to be irregularly shaped and attached to the leaflet or hinge point. An attenuation cut-off of >145 HU has been recommended for differentiating pannus from thrombus.

The patient underwent redo aortic valve replacement. Subvalvular pannus formation was confirmed to be the cause of leaflet immobility on the explanted valve prosthesis. A bioprosthetic valve was implanted.