This is further info because I think it’s interesting rather that any sort of correction but fMRI doesn’t quite measure blood flow - at least not directly.

Oxygenated and deoxygenated blood have slightly different magnetic properties. So the fMRI is trying to detect from that how oxygenated the blood is, with the assumption that active areas are using more oxygen which causes a small dip then blood flow increases so then there’s an increase that follows over about 5-6 seconds. I don’t know if more advanced things are used now but when I messed about with it you’d measure the change then apply a 6s linear convolution to the signal to estimate activity.

There’s an interesting set of layers of assumptions in all this, and to me the idea that the mri part works at all seems like wild magic.