What happens to individual protons after an RF pulse is applied?

When an RF pulse is applied in MRI, it affects the alignment of individual protons in the magnetic field. Initially, these protons align with the main magnetic field (B0), creating a net magnetization vector in the direction of B0.

After the application of an RF pulse, the protons begin to absorb energy and are temporarily displaced from their alignment. This energy allows them to move into a higher energy state, and as they return to their equilibrium state, they start to precess around the magnetic field. During this process, the protons that are elevated to this higher energy state precess in a coordinated and synchronized manner, which is termed as precessing "in phase."

This synchronized motion of the protons leads to the creation of a net rotating magnetization in the transverse plane, which is critical for the detection of the MRI signal. The precession out of phase or disorganization would not generate such a clear MRI signal.

The key to understanding this process lies in recognizing that the RF pulse is designed to disturb the precession of the protons and allow for coherent signal formation, which is captured by MRI scanners. Thus, the answer captures the essence of how the RF pulse influences the behavior of protons in the