Measurement of tissue oxygen extraction ratios from venous blood T(2): increased precision and validation of principle.

It has recently been shown that parenchymal oxygen extraction ratios (OERs) can be quantified using the absolute T(2) 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 spin-echo 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 T(2) of venous blood in vivo. Using an interecho spacing of 25 ms, the results on visual activation studies in eight volunteers showed T(2)(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 T(2)s are about 20-40 ms longer.