MR CONTRAST DUE TO INTRAVASCULAR MAGNETIC-SUSCEPTIBILITY PERTURBATIONS

A particularly powerful paradigm for functional MR imaging of microvas cular hemodynamics incorporates paramagnetic materials that create sig nificant image contrast, These include exogenous (lanthanide chelates) and endogenous (deoxygenated hemoglobin) agents for mapping cerebral blood volume and neuronal activity, respectively, Accurate interpretat ion of these maps requires an understanding of the biophysics of susce ptibility-based image contrast, The authors developed a novel Monte Ca rte model with which the authors quantified the relationship between m icroscopic tissue parameters, NMR imaging parameters, and susceptibili ty contrast in vivo. The authors found vascular permeability to water and the flow of erythrocytes to be relatively unimportant contributors to susceptibility-induced Delta R2. However, pulse sequence, echo tim e, and concentration of contrast agent have profound effects on the Ve ssel size dependence of Delta R2. For a model vasculature containing b oth capillaries and venules, the authors predicted a linear volume fra ction dependence for physiological volume changes based on recruitment and dilation, and a concentration dependence that is nonlinear and pu lse sequence dependent, Using the model, the authors demonstrated that spin echo functional images have greater microvascular sensitivity th an gradient echo images, and that the specifics of the volume fraction and concentration dependence of transverse relaxivity change should a llow for robust mapping of relative blood volume, The authors also dem onstrated excellent agreement between the predictions of their model a nd experimental data obtained from the serial injection of superparama gnetic contrast agent in a rat model.