 Several of the patients with atrial fibrillation require transesophageal echocardiography TEE performed prior to cardioversion or atrial fibrillation ablation procedures. There is opportunity to enroll most of these patients in our proposed study. There has been essentially no work published about the use of echo contrast materials in TEE other than agitated saline, particularly the use of Optison. Thus, the investigators will be exploring a new field in echocardiography and echo contrast. 