Magnetic resonance(MR) is based upon the interaction between an applied magnetic field and a nucleus that possesses spin.
Atoms consists of three fundamental particles: protons(positive charge) neutrons(no charge) and electrons(negative charge).
A third property of the nucleus is spin or intrinsic spin angular momentum.
The nucleus can be considered to be constantly rotating about an axis at a constant rate or velocity. A limited number of values for the spin are found in nature; that is, the spin, I, is quantized to certain discrete values. There are three groups of values for I:zero, half-integral values, and integral values.
The 1H nucleus, consisting of a single proton, is a natural choice for probing the body using MR techniques for several reasons. 1. It has a spin of 1/2 and is the most abundant isotope for hydrogen. 2. Its response to an applied magnetic field is one of the largest found in nature. 3.The human body is composed of tissues that contains primarily water and fat, both of which contain hydrogen.
In addition to its spin, a positively charged nucleus also has a local magnetic field or magnetic moment. This associated magnetic moment is fundamental to MR.The magnetic moment for the nucleus is parallel to the axis of rotation, This orientation of the nuclear spin and the charges induced in it due to the experimental manipulations that the nucleus undergoes provide the basis for the MR signal.