Precise knowledge of the $$$T_{2}$$$ of blood ($$$T_{2b}$$$) is required for spin-echo based blood oxygenation determination methods such as TRUST (T2 Relaxation Under Spin Tagging). Several groups have analyzed the $$$T_{2b}$$$ but previous studies have been limited by non-physiologic measurement conditions including the use of nonhuman blood [3] and the lack of calibration for lower HCT commonly found in congenital anemia syndromes. In this study we characterized the $$$T_{2b}$$$ dependence on hematocrit (HCT) and oxygen saturation in human blood at 3T over the physiologic range of hematocrit and blood oxygenation. For validation, we also measured sagittal sinus saturation in 35 normal controls and 12 patients with chronic anemia.This study was performed under an IRB approved protocol with informed consent/assent. Imaging was performed on a Philips 3T Achieva scanner with 8 channel head coil. T2b calibration was performed using the blood of three subjects (1F, 33+15.1 years). Blood samples were centrifuged and separated into packed red cells and plasma. Suspensions were recombined to create blood samples with HCT ranging from 13%-52%. A custom, 37o C temperature controlled blood imaging reservoir was used for in vitro imaging measurements. Blood was deoxygenated external to the scanner in a 37o C temperature controlled glove box using 5% CO2 /95% N2 gas mixture that maintained the pH at approximately 7.4. Upon desaturation, blood was transferred to the imaging reservoir and agitated every 2-3 minutes to minimize sedimentation. Saturation was measured using a hemoximeter (OSM3 Radiometer). Following localization, TRUST was measured using a pulse sequence designed by Lu and colleagues [4]. Arterial spin labeling was disabled. For any given saturation, 1/T2b was well described by the relationship: $$\frac{1}{T_{2b}}={R_{2}}=\frac{1}{T_{2o}}+K(1-Y)^{2}$$ where T2o is the T2 of oxygenated fully saturated blood, Y is the oxygen saturation and K is a constant dependent on several parameters including inter-echo spacing (τ) and hematocrit. The joint dependency of 1/T2b, hematocrit and oxygen saturation was well described by a hyperplane: $$\frac{1}{T_{2b}}=A*HCT+B*HCT*(1-Y)^{2}+C*(1-Y)^{2}+D$$ where A,B,C,D are empirical fitting coefficients. Two dimensional least squares was used to find A,B,C and D. For clinical correlation of the newly derived calibration curve, TRUST imaging was performed in 35 healthy controls (CTL) and 13 patients with chronic anemia (ACTL). The T2prep duration was 0, 40, 80 and160 with τ =10ms and standard parameters [4]. T2 value was derived after mono-exponential fitting of the difference of control and tagged images. For clinical correlation of the newly derived calibration curve, TRUST imaging was performed in 35 healthy controls (CTL) and 13 patients with chronic anemia (ACTL). The T2prep duration was 0, 40, 80 and160 with τ =10ms and standard parameters [4]. T2 value was derived after mono-exponential fitting of the difference of control and tagged images.One hundred and twenty six blood measurements of HCT, oxygen saturations and 1/T2b were used for model fitting. 1/T2b rose linearly with the square of oxygen extraction (Figure 1A). The slope of this relationship steepened with increasing hematocrit. Figure 1B demonstrates an approximately linear relationship between K and HCT, K=.0566 *HCT+ 2.87, R2= .907. Model parameters where found to be A=8.13, B=121, C=5.46 and D=2.46, R2= 0.977. Our model predicted a mean saturation of 0.592 and 0.634 in the CTL vs ACTL whereas the Lu bovine model [3] predicted mean saturation of 0.634 and 0.629 respectively. Bland Altman analysis between saturation predicted by our model and Lu model is shown in Figure 1D. The agreement is highly nonlinear with hematocrit. The Lu calibration saturation estimates an average of 5% higher absolute saturation. However, there is linear decrease in the bias for HCTs less than 35%. Figure 2 compares the sagittal sinus saturation estimates calculated using both techniques in CTRL and ACTL. We report the first description of changes in 1/T2b of human blood across the entire physiological range of hematocrits and oxygen saturations. By controlling for physiological variables such as temperature, spontaneous blood sedimentation and pH, we are confident our data represents the most accurate surrogate physiological account of human T2b blood to date. Our model well characterized the relationships between 1/T2b , hematocrit and oxygen saturation . These data suggest that human T2b is longer than that of bovine blood (3), leading to larger predicted AVO2 consistent with invasive measurements. Importantly, our data allows accurate TRUST oximetry in patients having hematocrits less than 35%, commonly observed in patients with chronic anemias. Care must be taken in the measurement and quantification of SvO2 for even deceivingly small errors is saturation will lead to much larger errors in oxygen extraction fraction and cerebral metabolic rate.