Image formation, Steps 2 & 3 - Phase and frequency encoding. For step 2 of the image formation process, a phase encoding gradient, GP, is applied in a direction along the selected image plane (in this case the phase encoding direction is along the y direction). This causes a range of phase shifts of the proton magnetic moments dependent on their position along the gradient as well as the slope and duration of the gradient. For step 3, following the phase encoding gradient, the frequency encoding gradient, GF, is applied also in the plane of the selected slice but perpendicular to the phase encoding direction. The MR signal echo is measured during this period. The frequency encoding gradient determines the Larmor frequency according to position along its direction (in this case, the x direction). The detected MR signal from the slice of tissue is therefore comprised of many different frequencies. The field of view is predefined and matched to a specific range of frequencies, referred to as the receiver bandwidth. See also Additional File 1.