Integrated analysis of diffusion and relaxation of water in blood

Diffusion and T 2 relaxation of water both inside and outside red blood cells (RBCs) in human blood were investigated using a hybrid NMR pulse sequence to obtain a more quantitative understanding of the diffusion and relaxation behavior of water in paramagnetic‐doped blood samples. The data were analyzed by both examining the relaxation properties of the system after each diffusion weighting and looking at the diffusion properties at each echo time. The results illustrate how diffusion‐sensitizing gradients affect the T 2 spectra of blood and how relaxation weighting changes the curvature of the diffusion curves, thereby demonstrating the close coupling between diffusion and T 2 relaxation. A three‐pool model, consisting of RBCs, plasma, and macromolecular protons, was used to model the data from the diffusion‐relaxation hybrid experiments. The model was found to describe all the characteristic features of the experimental data well and was used to evaluate the approximations involved in the conventional analysis methods and elucidate the nature of the relaxing and diffusing components. Compared with the separate diffusion and relaxation experiments, the diffusion‐relaxation hybrid experiments are less time‐consuming, result in better parameter determinations, and may be useful in analyzing diffusion‐ T 2 coupling in tissues with more complicated multi‐exponential T 2 behavior.