One. Get a second opinion from a thoracic surgeon at a major cardiovascular care center like Mayo, UCSD - La Jolla, University of Minnesota Twin Cities, or University of Texas Houston. Make sure that you see a thoracic surgeon and ask for a referral from your cardiologist. As soon as you get to 5 cm they're going to want to do surgery so it sounds like you're close and you want to make very good friends with a good thoracic surgeon.

Two. I personally get scans annually, but for the first five years after I had a complete aortic dissection suddenly, I got scanned every six months with contrast. It's a lot but it's imperative that you know what's going on. 18 months seems like a long time between scans when you're as close to 5 cm as it sounds like you are. I'm not a doctor, but I'm also a survivor of an aortic dissection and I can tell you that you do not want to go through this on emergency basis. You need to know exactly what's going on inside your chest and the best way to find that out is to visit with the person who is going to repair it surgically.
Three. There are no shortcuts to the surgery. It is a median sternotomy (chest crack) and open-heart surgery. They do not yet have any way to do this any other way at this time. Don't be fooled by the word "stent" with respect to your aortic aneurysm. When they say stent, they're not talking about the kind of stent that your father-in-law got fed up through his femoral artery and got balloned to clear a clogged artery. What they're talking about is a dacron tube that is sewn in from the top of the aorta where it exits the heart down to some other location to repair the aneurysmal tissue. The aortic graft that I received is 13 inches long and 32mm wide. Your aorta is somewhere between the diameter of a C battery (26mm) and a D battery (34mm). Surgery is usually recommended when you're at 50mm (5cm).

Peace.