The frequencies absorbed in magnetic resonance are in the megahertz (million cycles per second) and gigahertz (billion cycles per second) ranges. The absorption frequencies for any particular substance are directly proportional to the strength of the magnetic field. This characteristic is unique to magnetic resonance.
There are two distinct subcategories of magnetic resonance. One of these, for which the absorbing particles are electrons, is known by either of two interchangeable names: electron spin resonance (ESR) or electron paramagnetic resonance (EPR). The other category, in which the absorbing particles are atomic nuclei, is called nuclear magnetic resonance (NMR).2
The resonances are actually the absorptions of particular frequencies and are found in EPR and NMR arises from some of the most fundamental properties of matter. A general theory of EPR and NMR must be derived from quantum mechanics, but a classical analogy provides some insight. In the familiar model, every atom has a massive nucleus containing N positively charged protons and a number of uncharged neutrons. Outside the nucleus are N negatively charged electrons in various "orbits" or "distributions." The number N, called atomic number, identifies the atom in the periodic table. 3
Every electron and proton possesses, in addition to charge and mass, an indestructible amount of angular momentum or "spin," the property that keeps a gyroscope spinning after the driving force is removed. Because moving charge always has magnetic effects, the combination of charge and spin makes every electron and proton a spinning permanent magnet, the strength or "magnetic moment" being several hundred times greater for electrons.
A spinning electron can be compared to the spinning wheel of a toy gyroscope. When the gyroscope's spin axis is off vertical, and the bottom end of the axis rests loosely on a support, the unexpected happens. Instead of falling farther from the vertical under the force of the wheel's weight, the spin axis rotates steadily around the vertical at a fixed angle from it, a motion known as "precession." In EPR the electron is spinning and a microscopic magnet. The externally applied magnetic field supplies the extra force, and the electron's spin axis precesses. Resonant absorption occurs when the source frequency is synchronous with the precession frequency. A similar explanation applies to NMR. 4
Magnetic Resonance Image Production
Figure 1 illustrates the a simplified block diagram of generic MR imaging mechanism demonstrating the elements essential for the recognition and creation of MR signals and the demonstration of MR images. These components are as follows:
1) The RF mechanism (a) for the production of the magnitude of the RF magnetic field with the help of a coil for transmitting mode, amplifier and a transmitter, b) for the detection of the free induction decay (FID), which is the result of the net magnetization to an RF pulse this is done by the help of a signal demodulator, a coil for