Question

how does a mri work? describe the physics of it.

Answer 1

here you have a link
http://bestphysicsvideos.blogspot.dk/2011/05/how-mri-works.html

Answer 2

Atoms in your body have magnetic dipole moments. When you put these atoms in a magnetic field, the dipole moments will align with the field. If you apply a second field
of the right frequency ,you can cause the dipoles to flip their orientation briefly. When the dipole realign themselves with the field, they give off light. By seeing where the light(not the visible kind) comes from, and what frequency it has, you can determine where the atoms are and what kind they are.

See http://phet.colorado.edu/en/simulation/mri for a cute simulation you can play with.