physicu principle of MRI

Magnetic resonance imaging (MRI)
is noninvasive method of mapping the internal structure and certain aspects of function within the body.
It uses nonionizing electromagnetic radiation and appears to be without exposure-related hazard
Name of MRI previously is nuclear magnetic resonance (NMR),first described by Bloch and Purcell in 1946 .

NMR has been used extensively as a laboratory method for studying the properties of matter at the molecular level (NMR spectroscopy
In applications to medicine, it is now commonly referred to as magnetic resonance (MR).
MR describes the phenomenon whereby the nuclei of certain atoms, when placed in a magnetic field, absorb and emit energy of a specific or resonant frequency.
The hydrogen (1^H) atom inside body possess “spin”
• In the absence of external magnetic field, the spin directions of all
atoms are random and cancel each other.
• When placed in an external magnetic field, the spins align with the
external field.
• By applying an rotating magnetic field in the direction orthogonal to
the static field, the spins can be pulled away from the z-axis with an
If radiofrequency of hydrogen is applied ,aproportion of the protons change alignment ,flipping through apreset angle , rotate in phase with one another.fallowing this radiofrequency pulse,the protons return (realign )to their original postion