What is the difference between ASL and fMRI interpretation?

Hi all,
I am familiar with the physics and acquisition methods of ASL and fMRI.
ASL measures perfusion, and fMRI metabolic rate, both a proxy for brain activity.
From a measurement/imaging point of I am familiar with their different qualities like SNR, sensitivity etc.
However, from a physiological/interpretation point of view, what is the difference between they way they proxy brian activity, and in which cases is this difference relevant?
Thanks in advance!

ASL is a more direct measure of blood flow: it acquires pairs of images where one has tagged the blood (resulting in decreased signal) and the other has not (the control image). This subtraction also makes for more normal white noise than the BOLD signal. On the other hand, the timing of the ASL post label delays is crucial (you must provide enough time from label to imaging for the blood to move into the imaging slices, and have enough time for it to wash out for the control slice). The delay in post label delays and the pairs also tends to limit the temporal sampling rate and the number of slices acquired with ASL. For looking at task-related behavior, both make strong assumptions about the a normal coupling of the neurovascular response. I would suggest the book FSL book for further details on ASL.

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More generally, I wonder what the differences are between perfusion and metabolism as proxy for neuronal activity. For the assumtions made in perfusion I’ll take a look at the book, thanks!