Theory of contrast agents in magnetic resonance imaging: coupling of spin relaxation and transport.

The role of diffusive transport on the enhancement of nuclear spin relaxation through NMR contrast agents is described by means of diffusion-Bloch equations. These equations are solved in the mean relaxation time approximation [W. Nadler and K. Schulten, J. Chem. Phys. 82, 151-160 (1985)]. A model presented considers relaxation enhancement in tissue in which contrast agents confined to intravascular spaces affect nuclear spin in the extravascular volume. We show how the mean relaxation time depends on capillary density, on permeability, and on diffusion. A second model describes enhanced phase relaxation of liver tissue in the presence of magnetic particles in Kupffer cells. The relationship between relaxation rate and density of Kupffer cells is investigated. The diagnostic value of enhanced nuclear relaxation in the presence of contrast agents is discussed on the basis of the systematic mathematical results obtained.