Measurement of tissue oxygen extraction ratios from venous blood T₂: Increased precision and validation of principle

It has recently been shown that parenchymal oxygen extraction ratios (OERs) can be quantified using the absolute T₂ of venous blood draining from this tissue (Oja et al., J Cereb Blood Flow Metab 1999;19:1289-1295). Here, a modified Carr-Purcell-Meiboom-Gill (CPMG) multiecho experiment was used to increase the efficiency and precision of this approach and to test the applicability of the two-compartment exchange model for spinecho BOLD effects in pure venous blood. Relaxation measurements on bovine blood as a function of CPMG interecho spacing, oxygen saturation, and hematocrit provided the baseline relaxation and susceptibility shift parameters necessary to directly relate OER to T₂ of venous blood in vivo. Using an interecho spacing of 25 ms, the results on visual activation studies in eight volunteers showed T₂(CPMG) values increasing from 128 ± 9 ms to 174 ± 18 ms upon activation, corresponding to local OER values of 0.38 ± 0.04 and 0.18 ± 0.05 during baseline activity and visual stimulation, respectively. These OER values are in good agreement with literature data on venous oxygenation and numbers determined previously using a single-echo approach, while the measured T₂s are about 20-40 ms longer.