You are about the age when biscuspid aorta valve (BAV) syndrome (congential) manifests itself with an enlarged aorta. The condition is two leaflets that close over the valve opening rather than three. This could be the underlying cause for your back pain...usually lower back.
Your doctor wants to rule out BAV, measure aorta size, and maintain a reference, etc. You shouldn't have any concern about a very serious problem as the MRI should have provided enough information that it isn't large enough to rupture. For your information, an aorta larger than 4.0cm would be considered an aneursym and 5.0cm or greater may begin to be a concern. Lets us no the outcome of the CT scan, and if you have any questions, we are available to help.