When you lie in an MRI machine, you lie in a strong magnetic field and the protons in your body align in the z-direction to give a net magnetic moment. We can now flip all these magnetic moment by sending in a magnetic pulse from another direction. The pulse causes the arrow that represents the magnetic moment to lie flat in the xy plane. After the pulse is gone, the magnetic moment will slowly recover back to the z-direction because this is the lowest energy configuration (remember that the magnetic field in the z-direction remains present during this whole process).

When you lie in an MRI machine, you lie in a strong magnetic field and the protons in your body align in the z-direction to give a net magnetic moment. We can now flip all these magnetic moment by sending in a magnetic pulse from another direction. The pulse causes the arrow that represents the magnetic moment to lie flat in the xy plane. After the pulse is gone, the magnetic moment will slowly recover back to the z-direction because this is the lowest energy configuration (remember that the magnetic field in the z-direction remains present during this whole process).