Brief Answer:
A medical procedure should be necessary.

Detailed Answer:
I read your question and I understand your concern. A brain aneurysm is a very serious issue, but on the other hand it is really lucky that it was detected now while it hasn't ruptured.

An aneurysm is an abnormality in the wall of the blood vessels forming a bulge or a ballooning in the wall. In itself it may not cause any symptoms in most cases, but if it ruptures it causes bleeding in the brain which may be very serious at times fatal.

Many people have aneurysms without knowing it, estimates vary from 1.5 to 5% of the population. Most never rupture, only around 0.5 to 3% of cases so before deciding whether to undertake any procedure about it or to abstain from treatment it is looked at factors which predispose it to rupture such as size, location, form etc.

In your case though a 12 mm aneurysm is of a considerable size and is considered to have a high rupture risk so it should intervened in my opinion. As I said before it is lucky for it to have been discovered now before any serious bleeding happened.

Now there are two options which should be considered. The first is open surgery which accesses your brain and puts a clip which excludes the aneurysm from the circulation. The second option is endovascular coiling, meaning entering the blood circulation with a catheter through a vessel usually in your groin, traveling with the catheter to the aneurysm site and inserting coils into the aneurysm which prevent blood from flowing into the aneurysm. While the first is done by a neurosurgeon, the second can be done by an interventional neuroradiologist, a neurosurgeon or a trained neurologist, it varies among centers.

How to choose between the two methods. Well first step is up to the neurosurgeon and the physician doing endovascular procedure, they will study carefully aneurysm location, architecture, its relation to the adjacent structures etc because there might be factors rendering one or the other option technically more difficult, dangerous or impossible. If all you've had up to now is a CT of blood vessel, at times they might want to do an angiogram (entering with the catheter as described above and injecting contrast in the vessel to have a clear visualization of it) to better study it before making a decision.

If after consideration both methods are considered equally feasible, and the medical center has experience and expertise in both (at times clinics may be more oriented and good at one of procedures - but big medical centers in US usually are good and experienced at both) then nowadays endovascular treatment is the most preferred option perhaps, because while similarly efficacious it is as you can well imagine easier on the patient than open surgery, quicker post-op recovery, shorter hospital stay.

I remain at your disposal for further questions.

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