This study explores the relationship between myocardial fibrosis and patient outcomes in Atrial Fibrillation (AF), specifically after catheter ablation. It aims to use Cardiac CT, an accessible tool, to measure left ventricular extracellular volume (ECV) as an indicator of fibrosis. The study will assess if higher ECV levels correlate with increased risks of AF recurrence, hospitalization, and poor cardiac function recovery. Positive findings could make ECV a key factor in deciding AF treatment strategies.

Atrial Fibrillation (AF) is the most common arrhythmia and is associated with significant healthcare-related expenses. With the aging population, the prevalence of AF is increasing. AF frequently co-exists with heart failure (HF) with reduced (HFrEF) or preserved (HFpEF) ejection fraction. The relationship between AF and HF is complex, and one condition constitutes a risk factor for the other. Myocardial interstitial fibrosis is a hallmark of myocardial remodeling occurring as a consequence of risk factor exposure leading to both AF and heart failure. However, the role of interstitial fibrosis in the prognosis of patients with atrial fibrillation with and without heart failure remains unknown.

AF has a significant impact on patients' well-being and quality of life. In addition to coping with uncomfortable symptoms, individuals with this condition face an increased risk of future adverse events like frailty, stroke, dementia, and all-cause mortality. To address these concerns, many patients opt for procedures such as AF ablation, hoping for a cure. While the results of this procedure show promise, a small percentage of patients who undergo AF ablation may experience a recurrence of the arrhythmia or find that the participant's heart's systolic function does not fully recover. The investigators aim is to investigate whether there is a connection between myocardial fibrosis and poor outcomes following AF ablation.

Cardiac MRI (CMR) has been the gold standard methodology of quantification of interstitial fibrosis. Diffuse interstitial fibrosis can be quantified by extracellular volume (ECV) imaging. CMR-based left ventricular ECV has been associated with adverse outcomes in multiple cardiovascular disease states. However, the availability of CMR is confined only to tertiary centers with expertise. Cardiac CT is an alternative way to detect and quantify interstitial fibrosis by quantification of ECV. CT-derived ECV has been shown in multiple studies to have an excellent correlation with CMR-derived ECV. Cardiac CT is widely available and has a higher spatial resolution. Since as part of the standard pre-ablation routine, patients will undergo a cardiac CT scan to accurately map the anatomy of the participant's heart, investigators ought to measure ECV parameters during this visit.

Inclusion Criteria:

• Patients must be at least 21 years old at the time of enrollment.
• Patients must be able to give informed consent.
• Patients with symptomatic paroxysmal/persistent atrial fibrillation (AF).
• Eligible patients must be determined to be suitable candidates for ablation to treat AF by their cardiologist and/or electrophysiologists regardless of this protocol.
• Patients are scheduled to have a pre-procedure cardiac CT upon the discretion of their electrophysiologists

Exclusion Criteria:

• Known allergy to iodinated contrast media.
• Elevated serum creatinine (> 1.5mg/dl) OR calculated creatinine clearance of < 60 ml/min (using the Cockcroft-Gault formula).
• Patients with a positive urine or serum pregnancy test will be excluded from this study.
• Patients with heart transplantation.
• Patients with polymorphic ventricular tachycardia (VT) or presenting in electrical storm.
• Presence of any other history or condition that the investigator feels would be problematic.
• Patient with iodinated contrast administration for other reason 24 hours prior to investigational CT.
• Patients who have a heart pacemaker, metallic foreign body (metal sliver) in their eye, or who have an aneurysm clip in their brain, mechanical heart valves and prosthetics that contain metal.
• Patients with severe claustrophobia.

Two independent blinded trained readers will detect, interpret and quantify interstitial fibrosis using CT images, to ascertain intra- and inter-observer variability. The readers will determine the region of interest (ROI), myocardial and blood pool attenuation values in the pre-contrast and 5 minutes delayed acquisition.