We narrow the gap between simulations of nuclear magnetic resonance dynamics on digitaldomains (such as CT-images) and measurements in D-dimensional porous media. We point out with twobasic domains, the ball and the cube in D dimensions, that due to a digitaluncertainty in representing the real pore surfaces of dimension D − 1, there is asystematic error in simulated dynamics. We then reduce this error by introducing localRobin boundary conditions.