When a frequency same as the object is applied, it goes through an energy transition. This is called resonance.
This frequency applied is called a radiofrequency pulse (RF pulse)
This is usually at 90 degrees to external magnetic field
RF pulse is applied at the specific Larmor frequency of hydrogen, only hydrogen spins resonate
When this RF pulse is applied, two things happen. Spin up, low energy hydrogen spins absorb energy and move into high energy. Spin down, high energy hydrogen spins lose energy and return to low energy state
If right amount of RF pulse is applied, number of spin up equals number of spin down nuclei. The NMV lies 90 degree perpendicular to external field, i.e. the transverse plane.
Receiver coil is placed in the transverse plane. When NMV passes across transverse plane, a voltage is induced. Once the RF pulse is removed, the voltage decreases. This decrease is called free induction decay.