Fig. 1.

Simulated (2D) examples of diffusion weighted measurements. The direction along which diffusion weighting was applied is shown by a dashed line and the measured signal along that direction is indicated by the distance of the round marker from the origin. The left panel shows the case where diffusivity is three times greater along the y-axis than along the x-axis. The right panel demonstrates the case where diffusivity is equal in all directions. The extension of this to 3D is straightforward, though a little tricky to demonstrate in a figure. If we extend the figure in the left panel to 3D and assume that the diffusivity along the direction perpendicular to the paper is the same as for the x-direction, the points sampled on the resulting surface would form a “red blood cell” seen from the side. Correspondingly in the right panel if we assume equal diffusivity in all three directions the resulting surface would be a sphere.