For something like a stroke, I would assume that it would be clearly visible without any contrast. I have MS, and they can see my lesions just fine on non-contrast scans. It's only if I have had new symptoms that they do a contrast scan, because then they can see which lesions are new/active and which are old.
According to my neurologist, new research have shown that the contrast stays in the body for much longer than previously assumed, so most have started to use contrast only when necessary. 10-15 years ago, I would always get a double scan, one with contrast and one without, but in recent years, I usually only on non-contrast scan.