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

Purpose: 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. Methods: 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 = R_2,plasma + Hct (R_2,RBC - R_2,plasma) + K ̇ Hct ̇ (1 - Hct) ̇ (1 - HbO₂)². Measured R₂ and HbO₂ levels were fit by the model yielding values of R_2,plasma, R_2,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. Results: Constants derived from the fit were: k = 74.2 [s⁻¹], R_2,plasma = 1.5 [s⁻¹], R_2,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. Conclusion: 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.