ASL - Analysis with FSL

Dear all, I am currently working on a project about ASL. I processed the first 10 participants with FSL and the results I obtained shows quite low values for gm (gm perfusion mean(SD) = 63.34(12.86), wm perfusion mean (SD) = 33.66(8.99)).
As far as I know, the mean values should be around 80 in gm, 20 in wm. Do you think there are some issues during acquisition/analysis? since the values do not respect the hipothetical values I should have obtained…

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