It's very easy to miss an aneurysm on an MRI tailored to screen for MS. A large aneurysm would likely be seen, but a small (but no less deadly) vascular defect could easily be missed if it was asymptomatic, and thus, not something the radiologist would be looking closely for. An MRA would show it nearly every time.

An aneurysm forms when the vessel wall weakens and cannot withstand the pressure of the arterial bloodflow. It begins to balloon out, which thins it even more, until it finally gives way - like a bubble gum bubble popping.