MRI(Magnetic Resonance Imaging) also known as Nuclear Magnetic Resonance Imaging. The name was changed from NMR to MRI since people were afraid of the term nuclear. Ordinary people fail to realize that there are nucleus in every atom. MRI or NMR imaging gets it name from the fact that it is based on the nucleus of atoms.

Some atoms with odd number of protons or neutrons possess a property called spin. Hydrogen atom has the highest spin and also highly abundant in biological matter. This property of hydrogen is used for MRI.

Now when a sufficiently strong magnetic field is applied across the body, the atoms with spin tend to align themselves with the magnetic field. Their alignment maybe parallel or anti parallel. At absolute zero all atoms are in parallel state. However at room temperature there exists both parallel and anti parallel states. We can call this excitation in the presence of magnetic field as state 1. Now if a Radio Frequency wave is applied, the atoms further align themselves depending upon the direction of the RF wave. Lets call this the state 2. When this RF wave is removed the atoms slowly change from state 2 to state 1. In the process they release energy called free induction energy. Now this energy released is sensed and based on various reconstruction algorithms, the image is obtained.

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