Diffusion MRI requires numerous corrective steps in the data processing pipeline. One such step regards the need for voxel-wise correction of erroneous b-values that ensue from the inherent non-linearity of the magnetic field gradients. To estimate the non-linearity we used a simple image-based method that involves a spherical water phantom to measure voxel-wise the apparent diffusion constant. Deviations from the expected value allow estimation of the local b-value. The b-value maps were validated against the spherical harmonic predictions and subsequently used to correct phantom and in-vivo data.