Around 1-2% of people are born with a 'bicuspid' aortic valve, with only two cusps instead of the common 'tri-leaflet' valve. People with this valve develop dysfunction of the valve (narrowing or leakage) at a much earlier age. It is also more common for them to develop enlargement of the main blood vessel coming out of the heart, the aorta, and some studies suggest that they are also at higher risk of life-threatening tears in the aortic wall. Current guidelines recommend surgical replacement of the aorta at an earlier stage in these patients. The exact mechanism for the dilatation is not clear, and some studies have suggested greater 'stiffness' in the wall of the aorta. In this study, the investigators propose to 1.) study aortic size, stiffness and flow patterns using MRI in patients with bicuspid and tricuspid valves with a narrowing (aortic stenosis, 2.) measure markers in patients' blood that may be important in the process of expansion, 3.) compare the change in MRI and blood markers at before and 12 months after surgery, in patients who are due to have aortic valve replacement surgery.

Inclusion Criteria:

• Participant is willing and able to give informed consent for participation in the study.
• Age 18 years or above.
• Planned surgical AVR (+/- CABG or aortic root surgery) for severe AS.
• Ability to understand the written information in English.

Exclusion Criteria:

The participant may not enter the study if ANY of the following apply:

• Contraindication to MRI (e.g., non-compatible pacemaker or other device).
• Severe claustrophobia (unable to have MRI scan).
• eGFR<30ml/min (contra-indication to have contrast agent during MRI).
• Contraindication to Adenosine (severe asthma).
• Significant arrhythmia.
• Other valve disease > moderate in severity.
• Female participants who are pregnant or lactating.
• Previous aortic valve or aorta surgery, previous CABG or other surgery involving aortic cross clamping.
• Participant in an interventional cardiovascular clinical trial.

Adult patients listed for a surgical aortic valve replacement (with or without coronary artery bypass graft) for severe aortic stenosis of bicuspid (n=15) and tri-leaflet (n=15) aortic valves will be potentially eligible to participate in this study.