Upper panel: scheme of the voxel selection for the “gradient” path. From an initial position e₀ (black dot), the nine upper voxels are considered and the one with minimal magnetic energy (the darker one) is selected. From that position, the next nine upper voxels are considered to trace the “gradient” path. Lower panel: for simplicity, an example of the gradient path in a bidimensional magnetic field B is given. The white dashed line indicates the “gradient” path from the initial position e₀ (black square). The color scale indicates a lower magnetic field magnitude when darker (and purple) and a higher ∣B∣ when lighter (and yellow).