Comparison of hypercapnia‐based calibration techniques for measurement of cerebral oxygen metabolism with MRI

MRI may be used to measure fractional changes in cerebral oxygen metabolism via a metabolic model. One step commonly used in this measurement is calibration with image data acquired during hypercapnia, which is a state of increased CO₂ content of the blood. In this study some commonly used hypercapnia‐inducing stimuli were compared to assess their suitability for the calibration step. The following stimuli were investigated: (a) inspiration of a mixture of 4% CO₂, 21% O₂ and balance N₂; (b) 30‐s breath holding; and (c) inspiration of a mixture of 4% CO₂ and 96% O₂ (i.e., carbogen). Measurements of BOLD and cerebral blood flow made on nine subjects during the different hypercapnia‐inducing stimuli showed that each stimulus leads to a different calibration of the model. We argue that of the aforementioned stimuli, inspiration of 4% CO₂, 21% O₂ and balance N₂ should be preferred for the calibration as the other stimuli produce responses that violate assumptions of the metabolic model. Magn Reson Med 61:391–398, 2009. © 2009 Wiley‐Liss, Inc.