Effects of biophysical and physiologic parameters on brain activation‐induced R₂* and R₂ changes: Simulations using a deterministic diffusion model

A central issue in magnetic resonance imaging of human brain function using blood oxygenation level‐dependent (BOLD) contrast is the accurate interpretation of the signal changes that are observed. Using a method that incorporates repeated phase rotation and convolution with a smoothing function to simulate spin diffusion in the presence of magnetic field perturbers, the dependencies of the absolute and relative changes in transverse relaxation rates (δ R 2 * and δ R 2 ) on biophysical and physiologic parameters were explored. First we introduce the modeling methodology. Then we simulate δ R 2 * and δ R 2 as physiologic and biophysical parameters are modulated within the ranges that they vary across subjects and voxels in the brain. The simulations demonstrate that the δ R 2 * and δ R 2 values that occur with activation‐induced changes in blood oxygenation depend most strongly on the resting state blood volume and field strength. The δ R 2 */δ R 2 ratios depend most strongly on the vessel radius and spin diffusion coefficient.