Calibrated-fMRI is highly promising to quantify human brain function via mapping changes of cerebral metabolic rate of oxygen. While the R2’-based approach is easily applicable, systematic differences to the well-established hypercapnia-calibration have been reported. We present data from an ongoing study in seven healthy young subjects correlating calibration factors M from R2' vs. hypercapnia. We hypothesized better correlation after methodological improvements in R2'-mapping and pseudo-continuous arterial spin labeling (pCASL). Our results confirmed this hypothesis, with good correlations between both fMRI-calibrations. However, we found potentially confounding hypercapnia effects on pCASL. Thus, our results suggest benefits of gas-free R2’-calibration for future applications.