How MRI Works!!

The body is largely composed of water molecules which each contain two hydrogen nuclei or protons. When a person goes inside the powerful magnetic field of the scanner, the magnetic moments of these protons align with the direction of the field.
A radio frequency electromagnetic field is then briefly turned on, causing the protons to alter their alignment relative to the field. When this field is turned off the protons return to the original magnetization alignment. These alignment changes create a signal which can be detected by the scanner. The frequency the protons resonate at depends on the strength of the magnetic field. The position of protons in the body can be determined by applying additional magnetic fields during the scan which allows an image of the body to be built up. These are created by turning gradients coils on and off which creates the knocking sounds heard during an MR scan.
MRI is used to image every part of the body, and is particularly useful for neurological conditions, for disorders of the muscles and joints, for evaluating tumors, and for showing abnormalities in the heart and blood vessels.

Physics principle

Nuclear magnetism

Subatomic particles such as protons have the quantum mechanical property of spin. Certain nuclei such as 1H (protons), 2H, 3He, 23Na or 31P, have a non–zero spin and therefore a magnetic moment. In the case of the so-called spin-1/2 nuclei, such as 1H, there are two spin states, sometimes referred to as "up" and "down". Nuclei such as 12C have no unpaired neutrons or protons, and no net spin; however, the isotope 13C (referred to as "carbon 13") does.
When these spins are placed in a strong external magnetic field they precess around an axis along the direction of the field. Protons align in two energy "eigenstates" (the "Zeeman effect"): one low-energy and one high-energy, which are separated by a very small splitting energy.

Imaging

A number of schemes have been devised for combining field gradients and radio frequency excitation to create an image:
· 2D or 3D reconstruction from projections, much as in Computed Tomography.
· Building the image point-by-point or line-by-line.
· Gradients in the RF field rather than the static field.
The 3DFT technique is rather similar except that there is no slice selection and phase-encoding is performed in two separate directions.
Scanner construction and operation
The major components of an MRI scanner are: the main magnet, which polarizes the sample, the shim coils for correcting inhomogeneities in the main magnetic field, the gradient system which is used to localize the MR signal and the RF system, which excites the sample and detects the resulting NMR signal. The whole system is controlled by one or more computers.

Magnet

The magnet is the largest and most expensive component of the scanner, and the remainder of the scanner is built around it. The strength of the magnet is measured in tesla (T). Clinical magnets generally have a field strength in the range 0.1—3.0 T, with research systems available up to 9.4 T for human use and 21 T for animal systems
Just as important as the strength of the main magnet is its precision. The straightness of the magnetic lines within the center (or, as it is technically known, the iso-center) of the magnet needs to be near-perfect. This is known as homogeneity. Fluctuations (inhomogeneities in the field strength) within the scan region should be less than three parts per million (3 ppm). Three types of magnets have been used:

Gradients

Gradient coils are used to spatially encode the positions of protons by varying the magnetic field linearly across the imaging volume. The Larmor frequency will then vary as a function of position in the x, y and z-axes.Gradient coils are usually resistive electromagnets powered by sophisticated amplifiers which permit rapid and precise adjustments to their field strength and direction.

Radio frequency system

The radio frequency (RF) transmission system consists of an RF synthesizer, power amplifier and transmitting coil. This is usually built into the body of the scanner. The power of the transmitter is variable, but high-end whole-body scanners may have a peak output power of up to 35 kW, and be capable of sustaining average power of 1 kW. The receiver consists of the coil, pre-amplifier and signal processing system. While it is possible to scan using the integrated coil for RF transmission and MR signal reception, if a small region is being imaged, then better image quality (i.e. higher signal-to-noise ratio) is obtained by using a close-fitting smaller coil. A variety of coils are available which fit closely around parts of the body such as the head, knee, wrist, breast, or internally, e.g. the rectum.