The effect of dissolved oxygen on the relaxation rates of blood plasma: Implications for hyperoxia calibrated BOLD. 
To determine the contribution of paramagnetic dissolved oxygen in blood plasma to blood-oxygenation-level-dependent (BOLD) signal changes in hyperoxic calibrated BOLD studies.  Bovine blood plasma samples were prepared with partial pressures of oxygen (pO2 ) ranging from 110 to 600 mmHg.  R1 , R2 , and R2* of the plasma with dissolved oxygen were measured using quantitative MRI sequences at 3 Tesla.  Simulations were performed to predict the relative effects of dissolved oxygen and deoxyhemoglobin changes in hyperoxia calibrated BOLD.  The relaxivities of dissolved oxygen in plasma were found to be r1,O2 =1.97 ± 0.09 ×10-4 s-1 mmHg-1 , r2,O2 =2.3 ± 0.7 ×10-4 s-1 mmHg-1 , and r2,O2* = 2.3 ± 0.7 ×10-4 s-1 mmHg-1 .  Simulations predict that neither the transverse nor longitudinal relaxation rates of dissolved oxygen contribute significantly to the BOLD signal during hyperoxia.  During hyperoxia, the increases in R2 and R2* of blood from dissolved oxygen in plasma are considerably less than the decreases in R2 and R2* from venous deoxyhemoglobin.  R1 effects due to dissolved oxygen are also predicted to be negligible.  As a result, dissolved oxygen in arteries should not contribute significantly to the hyperoxic calibrated BOLD signal.  Magn Reson Med 76:1905-1911, 2016.  © 2015 International Society for Magnetic Resonance in Medicine.