what is a gradient vector
The gradient is: Note we have calculated only the magnitude of the gradient. But like the main magnetic field, the gradient (G) is a vector, possessing both magnitude and direction. By convention, the direction of the main magnetic field is designated to be the z-axis. For completeness, therefore, we should say the gradient (G) is a vector of magnitude 2 mT/meter in the z-direction. Alternatively, we could write G = Gz = 2 mT/meter. Since in our example the magnetic field did not change either direction or magnitude in the x- or y-directions, we can also state Gx = 0 and Gy =0.