Relaxation enhancement of the transverse magnetization of water protons in paramagnetic suspensions of red blood cells

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Abstract
The enhancement of the water proton transverse relaxation, ΔR2, brought about by a difference between intra and extracellular paramagnetic susceptibilities in a suspension of red blood cells (RBC) has been evaluated both experimentally and theoretically in terms of (i) the refocusing interval, Δ180, of a CPMG pulse sequence, (ii) the difference in paramagnetic susceptibility, and (iii) the shape of the cell surface. At a hematocrit of 45, the increase in the relaxation enhancement, ΔR2, with increasing Δ180, was a factor of two greater for the naturally biconcave RBC, than for the quasi-spherical RBC in hypotonic suspensions. This difference could be modeled in terms of a transmembrane correlation time, τ = 5.5 ms, across an RBC surface characterized by a demagnetizing factor which differs by 0.13 from that of a sphere. The increase in ΔR2 with increasing magentization difference between the RBC and its surroundings was found to be marginally less than quadratic, both experimentally and from the model.