T₂ -prepared balanced steady-state free precession (bSSFP) for quantifying whole-blood oxygen saturation at 1.5T

To establish a calibration equation to convert human blood T₂ to the full range of oxygen saturation levels (HbO₂ ) and physiologic hematocrit (Hct) values using a T₂ -prepared balanced steady-state free precession sequence (T₂ -SSFP) at 1.5T.

Blood drawn from 10 healthy donors (29.1 ± 3.9 years old) was prepared into samples of varying HbO₂ and Hct (n = 79), and imaged using T₂ -SSFP sequence at 37°C and interrefocusing interval τ₁₈₀  = 12 ms. The relationship between blood T₂ , HbO₂ , and Hct was established based on the model R2=R2,plasma+Hct (R2,RBC-R2,plasma)+k·Hct·(1-Hct)·(1-HbO2)2. Measured R₂ and HbO₂ levels were fit by the model yielding values of R2,plasma, R2,RBC, and k. T₂ -SSFP and the established calibration equation were applied to extract HbO₂ at the superior sagittal sinus (SSS) in vivo and were compared with susceptometry-based oximetry.

Constants derived from the fit were: k = 74.2 [s^-1 ], R2,plasma  = 1.5 [s^-1 ], R2,RBC  = 11.6 [s^-1 ], the R² of the fit was 0.95. Average HbO₂ at the SSS in seven healthy volunteers was 65% ± 7% and 66% ± 7% via T₂ - and susceptometry-based oximetry, respectively. Bland-Altman analysis indicated agreement between the two oximetric methods with no significant bias.

The calibration constants presented here should ensure improved accuracy for whole-blood oximetry based on T₂ -SSFP at 1.5T. Magn Reson Med 79:1893-1900, 2018. © 2017 International Society for Magnetic Resonance in Medicine.