What is the principle behind MRI?

Hydrogen nuclei have magnetic polarity.
When placed in a strong magnetic field, a portion of the nuclei will align with the field.
These nuclei precess with a characteristic frequency called the Larmor frequency.
Radio waves, called a pulse, may be applied selectively in a particular sequence to excite the nuclei by moving them out of alignment with the magnetic field to some degree (e.g., 90 degrees, 180 degrees).
As the nuclei return to alignment, they emit the absorbed energy in the form of radio waves, which are amplified and transmitted to a computer for image generation.