Non‐invasive evaluation of blood oxygen saturation and hematocrit from T1 and T2 relaxation times: In‐vitro validation in fetal blood

Purpose We propose an analytical method for calculating blood hematocrit (Hct) and oxygen saturation (sO2) from measurements of its T1 and T2 relaxation times. Theory Through algebraic substitution, established two‐compartment relationships describing R1=T1−1 and R2=T2−1 as a function of hematocrit and oxygen saturation were rearranged to solve for Hct and sO2 in terms of R1 and R2. Resulting solutions for Hct and sO2 are the roots of cubic polynomials. Methods Feasibility of the method was established by comparison of Hct and sO2 estimates obtained from relaxometry measurements (at 1.5 Tesla) in cord blood specimens to ground‐truth values obtained by blood gas analysis. Monte Carlo simulations were also conducted to assess the effect of T1, T2 measurement uncertainty on precision of Hct and sO2 estimates. Results Good agreement was observed between estimated and ground‐truth blood properties (bias = 0.01; 95% limits of agreement = ±0.13 for Hct and sO2). Considering the combined effects of biological variability and random measurement noise, we estimate a typical uncertainty of ±0.1 for Hct, sO2 estimates. Conclusion Results demonstrate accurate quantification of Hct and sO2 from T1 and T2. This method is applicable to noninvasive fetal vessel oximetry—an application where existing oximetry devices are unusable or require risky blood‐sampling procedures. Magn Reson Med 78:2352–2359, 2017. © 2017 International Society for Magnetic Resonance in Medicine.