Stroke remains the most dangerous and frightening complication of atrial fibrillation (AF). Numerous factors predisposing to peripheral embolism in patients with AF have been well defined, documented and included in the CHA2DS2VASC score. Although proper anticoagulation minimizes the risk attributable to "known" risk factors, stroke may still occur. Thus, "unknown" risk factors may play an important role in stroke risk stratification in patients with AF. The investigators assume that one of the important "unknown" risk factor is left atrial appendage (LAA) morphology. The ASSAM study is planned to include 100 patients after ischemic stroke or transient ischemic attack (TIA) and known status of anticoagulation at the time of stroke. The control group will consist of 100 patients scheduled for AF ablation without a history of stroke or TIA.

Inclusion Criteria:

• history of ischemic stroke or transient ischemic attack
• history of AF/AL
• known status of anticoagulation at the time of stroke (treatment dose, INR level)

Exclusion Criteria:

• hemorrhagic stroke
• serious renal impairment with GFR <30 ml/min
• hyperthyroidism
• allergy to the contrast agent
• mental inability to sign the informed consent
• receptive aphasia