An analytical model of tissue relaxation and restricted diffusion in human blood is presented. The blood tissue model is composed of three different compartments: red blood cells, plasma, and macromolecular protons. The relaxation rate constants and free diffusion coefficients of intracellular and extracellular water may differ. Analytical formulas for signal loss due to relaxation and diffusion in the Carr-Purcell Meiboom-Gill and pulsed-field-gradient multispin echo experiments for this tissue model are derived. The model is fitted to the experimental data for human blood with various concentrations of Gadolinium contrast agent. The obtained model parameters are realistic. The validity and sensitivity of the model are also discussed.

Diffusion and T2 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 T2 spectra of blood and how relaxation weighting changes the curvature of the diffusion curves, thereby demonstrating the close coupling between diffusion and T2 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-T2 coupling in tissues with more complicated multiexponential T2 behavior.