Radiologist here. 👋🏻
You are right it does not require contrast. The scan uses a special technique to reduce signal from tissue that isn’t moving so only things that are moving produce high signal and show up “bright.” Since blood is the only thing really moving in the brain - voila!
The lung motion is be a problem but it is still done in some situations and the radiologist just tries their best with any motion artifacts. Since the blood vessels are so big in the chest it kind of works out, but it turns out better with contrast.