a, b) Enlargements of the areas near the Fig. 8 e and f origins, respectively. The upper abscissae are labeled with Yeq, the fractional oxygenation of RBC hemoglobin – running from right-to-left (Yeq = 0 represents completely deoxygenated (paramagnetic) hemoglobin). This would be the situation for CR-free blood. The lower abscissa [CRb] range in oxygenated blood produces the same range of transmembrane susceptibility difference as would the full range of blood oxygenation (Yeq, top abscissa scale) in the absence of contrast reagent. In other words, blood oxygenation Yeq of 1.0 produces the same |Δχ| as would [CRb] = 0 mM in fully oxygenated blood, and Yeq of 0 produces the same |Δχ| as would [CRb] = 6.1 mM in fully oxygenated blood (with 37% Hct).