Knowledge of the cerebral metabolic rate of oxygen (CMRO2) is fundamental to understandingtissue metabolism, and therefore one of the key physiologic parameters of interest to clinicalmedicine. Substrate and oxygen delivery to the cells are both mediated by blood flow (vascularmetabolic coupling). Thus, determination of CMRO2 demands knowledge of the change in oxygenconcentration during substrate metabolization, typically expressed as oxygen extraction fraction(OEF), and cerebral blood flow rate (CBF). The latter yield CMRO2 via the following conservationof mass equation: CMRO2 = c×OEF×CBF, with the constant c related to the blood’s oxygencarrying capacity.MRI is uniquely suited for noninvasive evaluation of CMRO2 providing results in absolutephysiologic units of μmol O2 min-1/100g tissue. While quantification of blood flow has been aroundfor nearly three decades, the measurement of hemoglobin oxygen saturation, and thus OEF inthe form of O2 extracted by the tissue from the arterial blood, is of more recent vintage. Twodominant approaches have emerged, both exploiting heme iron magnetism in hemoglobin’sdeoxy state, either by direct measurement of the blood’s bulk magnetic susceptibility (1,2), orindirectly via measurement of the blood water transverse relaxation rate (3,4). Both approachesallow robust measurement of whole-brain CMRO2 (see (5) for a recent review). The most commonT2-based method for CMRO2 measurement is TRUST (T2 relaxation under spin tagging),consisting of a train of non-selective phase-reversal pulses and isolation of veins, followed by EPIreadout (6). Susceptometry based oximetry (SBO) relies on modeling a vein (usually the superiorsagittal sinus) as a long cylinder for which an analytical solution exists to relate the measuredinduced field to intravascular magnetic susceptibility (2). Further, phase-contrast whole-brain CBFis incorporated into the sequence. Some implementations of SBO achieve temporal resolutionson the order of seconds, thereby rendering the method suited for the study of dynamic processessuch as the response to dynamic stimuli (e.g. breath-hold (7)) or gas breathing challenges (1).This talk briefly introduces the fundamentals of MRI oximetry in terms of pulse sequenceand analysis methodology, illustrated with applications to clinical problems in various patientpopulations. Finally, regional oximetric techniques relying on quantitative susceptibility mapping(8) or quantitative BOLD (9) approaches will be briefly touched upon. The lecture’s objective is toacquaint attendees with the principles of MRI brain oximetry.

Acknowledgements

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