physics of MRI

Magnetic resonance imaging
The basic principle of MRI depend on the fact that the nuclei of certain elements align with the magnetic force when placed in a strong magnetic field
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
At the field strengths currently used in medical imaging ,hydrogen nuclei (protons ) in water molecules and lipids are responsible for producing anatomical imaging .If radiofrequency of hydrogen is applied ,aproportion of the protons change alignment ,flipping through apreset angle ,and rotate in phase with one another. following this radiofrequency pulse ,the protons return (realign )to their original postion .
As the protons realign (relax) they induce aradio signal which ,although very weak ,can be detected and localize by antensana coils placed around the patient .
An image represent the distribution of the hydrogen protons can be built up .
The strength of signal depends not only on proton density but also on tow relaxation times .T1 and T2