Sort of? Not quite?
Ugh, I just wrote this long-ass post and lost it. Whatever, here's the short version:
Stroke is a clinical diagnosis, not a radiologic one.
CT will catch hemorrhages or large infarcts just fine. That's the important stuff: big infarcts cause big tissue damage. Tiny infarcts cause tiny tissue damage. MRI's superiority over CT is in that it catches smaller infarcts, and catches infarcts early. If I see a patient that screams "stroke" and the noncontrast CT is negative for hemorrhage, it's ischemic until proven otherwise. I don't need an MRI at the less than six hour stage and, while having one would be nice for malpractice reasons, whatever, I don't actually need one for your care. I may follow-up with an MRI if it's less certain that it's a stroke: tumors, infection, etc. that look like strokes. But, key - those aren't pressing, time-wise, the way a stroke is, if they look like a stroke ("look like" implies things about the nature of the underlying condition.)
After 6 hours we've passed the TPA stage and we're looking at clot removals. Here we need to know the exact territory of the infarct. If I'm bothering with imaging, yes, MRI/MRA, but that's not about finding the kind of stroke - it's about telling the interventionalist where to go.