Measurement difference between Chest CT and Cardiac MRI

I was diagnosed with a Ascending Aortic Thoracic Aneurysm a little over a year ago.  I first had a Echo, which showed a measurement of 4.7cm.  The echo was followed with a Cat Scan of the chest with contrast, which showed a measurement of 5.2cm.  Over the year I had an additional Echo and Chest CT scan every 6 months.  The 3rd CT scan done this May showed the aneurysm had grown to 5.9x5.5cm at the aortic root.  I also have a failing aortic valve.  My cardiologist said it was time for surgery.  She talked with the surgeon and they agreed I could have a Cardiac MRI rather than cardiac cath., I am 38 and my problem is from my rheumatological condition.

Much to my surprise the Cardiac MRI has the aortic root measurement at 3.8x3.5 cm.

Which result is correct, or can all be correct?

[Cleveland Clinic]

I believe that the most of the studies performed used CT as the standard, and so I would suggest that you take this as the correct measurement. MRI is very good as well, but I am not sure if the gadolinium opacified the entire ascending aorta. Given your age you probably don't have coronary atherosclerosis, so you shourl be able to proceed with AV replacement and ascending graft replacement.