During this stage, the MRI system first conducts a pre-scan to determine the larmor frequency (centre frequency) at which the hydrogen protons are spinning. Once the centre frequency has been determined (which is around 63.6 MHz for external magnetic field of 1.5 T and gyromagnetic ratio of 42.57 MHz/Tesla), a radio Frequency (RF) pulse is sent from the transmission coil with a frequency that matches the centre frequency of the system. Only those protons, which have their frequency of the spin the same as that of the frequency of the RF pulse, will respond to the signal (hence the name resonance was derived in magnetic imaging). Thus by sending a radio frequency pulse (RF pulse) at the centre frequency, with certain strength and for a certain period of time, we can rotate the net magnetization through 90 degrees into x-y plane from Z-axis.This process is called excitation and during this the protons have been excited to a higher energy state and start spinning in phase.