Quantification of normal cerebral oxygen extraction and oxygen metabolism by phase-based MRI susceptometry: evaluation of repeatability using two different imaging protocols.

IntroductionGlobal oxygen extraction fraction (OEF) and cerebral metabolic rate of oxygen (CMRO2) were quantified in a test-retest study. Cerebral blood flow (CBF) data, required for CMRO2 estimation, were obtained using dynamic susceptibility contrast MRI (DSC-MRI). OEF and CMRO2 were quantified using two separate data sets, that is, conventional high-resolution (HR) gradient echo (GRE) phase maps as well as echo planar imaging (EPI) phase maps taken from the baseline (precontrast) part of the DSC-MRI time series. The EPI phase data were included to elucidate whether an extra HR-GRE scan is needed to obtain information about OEF and CMRO2, or if this information can be extracted from the DSC-MRI experiment only. MethodsTwenty healthy volunteers were scanned using 3 T MRI on two occasions. Oxygen saturation levels were obtained from phase data measured in the great cerebral vein of Galen, based on HR-GRE as well as EPI phase maps. In combination with DSC-MRI CBF, this allowed for calculation of OEF and CMRO2. ResultsHigh-resolution-gradient echo- and EPI-based phase images resulted in similar OEF spread and repeatability, with coefficients of variation/intraclass correlation coefficients of 026/095 and 023/081, respectively. Absolute OEF values (HR-GRE: 040011, EPI: 035 +/- 008) were consistent with literature data. CMRO2 showed similar repeatability, somewhat increased spread and reasonable absolute values (HR-GRE: 323 +/- 126ml O-2/100gmin(-1), EPI: 279 +/- 089ml O-2/100gmin(-1)). DiscussionIn general, the results obtained by HR-GRE and EPI showed comparable characteristics. The EPI methodology could potentially be improved using a slightly modified DSC-MRI protocol (e.g. with regard to spatial resolution and slice gap).