Oxygen Extraction Fraction Definition
where CMRO2 is the cerebral metabolic rate for oxygen and Ca is the oxygen content, the product of the total hemoglobin concentration [Hbtot] in mM and the arterial oxygen saturation fraction (Ya). CBF is cerebral blood flow. FOE varies from organ to organ and depending on the level of activity.15 FOE measurements for the whole body give a range of about 0.15 to 0.33.15 That is, the body consumes 15% to 33% of the oxygen carried. The heart and brain are likely to have consistently elevated EPF levels during active states.15 The mean cerebral FEO was 0.292 (SD = 0.06) measured by NIR spectroscopy and partial jugular vein occlusion in 41 preterm infants (median pregnancy, 29 weeks; , 27 to 31 weeks).18 In this study, there appeared to be no association between cerebral FEW and gestational age or postnatal age (median, 9 days; 6 to 19 days).18 However, when cerebral FEFT was measured in the first 3 days after birth, there was a significant decrease in brain FOE between days 1 and 2. indicating an increase in CBF and cerebral oxygen supply (Fig. 17.5).14,54,60 How to cite this article: Chang, F.-Y. et al. Determination of the oxygen extraction fraction by magnetic resonance imaging in models of dogs with internal carotid artery occlusion. 6, 30332; doi: 10.1038/srep30332 (2016).
The original phase-based method of regional oxygen metabolism used a multi-echo sequence.16 Multiecho phase values correspond to a straight line, which is a phase relative to TE, and a new phase for a TE is extrapolated from the line. The difference between the new phase and the measured phase is due to noise. Multi-echo sequences eliminate the effect of noise on phase measurements. SD estimates in Yv due to noise are <5% of oxygen saturation, with a signal-to-noise ratio of only 10.16, while an ordinary phase image has an average SNR between 40 and 70. This means that thermal noise in this SNR range results in proportionally low error values. In addition, multi-echo sequences require a triple calculation of simple echo sequences, making it difficult to apply the former in clinical practice. Venous blood from the upper sagittal sinus was collected before and immediately after MRI. The venous blood oxygen saturation level, SvO2, was measured using a portable i-STAT clinical analyzer (Heska, Fort Collins, CO) developed for veterinary use. SvO2 values are calculated on the basis of the value of the partial pressure of oxygen at the half-saturation of haemoglobin (P50), blood pH and pCO2. This device is calibrated for animals such as dogs, cats and horses. In the normal pH range (pH = 7.4), the P50 is 32.7 mmHg (15) for rats, 31.5 mmHg (16) for dogs and 34.1 mmHg for cats (17). Therefore, the P50 value for rat blood is within the range used for the calibration of this blood gas analyzer.
In order to maintain an airtight environment during blood collection, the following procedure was used to remove air contamination. A custom-made 1 ml syringe with a .26 gauge needle was used to draw venous blood. The blunt demand peak was fixed with PE10 pipes positioned through the burr hole drilled into the superior sagittal sinus. The needle and tube were filled in advance with heparin rinse (10 units/ml) to remove air pollution (O2) and prevent blood clotting. Arterial blood oxygen saturation, SaO2, was measured before and immediately after MR using a pulse oximeter placed on the rat`s hind leg. Non-invasive measurement of the brain oxygen extraction fraction reflects the balance between oxygen supply and tissue oxygen consumption and is therefore important for many clinical and research studies. Approaches have been proposed that use the dependence of blood oxygen content on the R2 relaxation rate in the blood (23) or the susceptibility of blood in large venous vessels (i.e. jugular veins) (24) to estimate the overall part of oxygen extraction from the brain. The recently proposed qBOLD model (25) measures regional OEF values, providing a new tool for researchers and clinicians that can be easily implemented on commonly available MRI scanners. Previous results (14) obtained with a trio 3.0T CT scan of the whole human body (Siemens Medical Systems, Erlangen, Germany) were well consistent with the established results of the PET studies.
Our current study is designed to validate the qBOLD approach to measuring OEF. Although CT, MRI and functional MRI have largely replaced SPECT and PET for the evaluation of acute stroke patients, PET imaging is still used in determining the oxygen extraction fraction in patients considered for corrective surgery for obstructive carotid artery disease. The current gold standard for measuring cerebral blood flow is PET imaging with water labeled with 15O, H215O. In a recent study, strong correlations between MRI and PET brain circulation indices were found (Vakil et al., 2013). In addition, new radiopharmaceutical developments have been made that can provide PET/SPECT with unique information on the pathophysiology of stroke. For example, brain cell death, specifically the death of the “neurovascular unit,” which includes the neuronal cell and its associated endothelial and glial cells, is the primary neurological substrate of stroke. Therefore, information on the extent and dynamics of this brain cell death (apoptosis) as well as tissue hypoxia can be valuable for better diagnosis and follow-up of neuroprotective treatment in stroke patients. Hypoxia was imaged with nitroimidazole-based radiopharmaceuticals such as 18F-fluoromisonidazole (FMISO) and 18F-fluorazomycin-arabinoside (FAZA).