The Rest of the Basic Physics Behind MRI   When the protons are precessing about the axis of the magnetic field, they can either precess or spin at high or low energy states.   The atoms start out in low energy spins (N-S-N-S).   and can be switched to high energy spins (N-N-S-S).     When the RF coils apply the radio frequency waves to the sample, resonance occurs because the RF waves are made to match the Lamour frequencies of the protons. *Resonance is the transfer of energy from one system to another.* When resonance occurs, the protons absorb energy from the RF waves. This energy is enough for the proton to jump from the low energy state to the high energy state.   The blue line shows how much energy it takes for the proton to switch from a low to a high energy state.   **The value of E is based on E=hv, where h=6.626 x 10^-34 J s and v is the Lamour frequency, which is based on the gyromagnetic ratio of the sample and the strength of the external magnetic field.**   This energy is supplied by the frequency of the RF waves, which is equal to the Lamour frequency of the sample.   The Abs. E. is the energy of the frequency of the RF waves.   Why are we interested in energy and frequency?   When the RF waves cause the protons to switch energy levels, they are also switching their frequencies. The RF coils in the MRI machine are able to detect these frequencies and the computer plots them according to their image characteristics and positions in the sample.