MRI (Magnetic resonance imaging) is a non-invasive imaging technique that uses a combination of a large magnet, radio frequencies, and a computer to generate images of internal structures and organs of the body without using x-ray radiation and surgery.

MRI has been developed based on the principles of nuclear magnetic resonance. NMR (nuclear magnetic resonance) was discovered by Felix Bloch and Edward Purcell in 1946. During the 1950s and 1960s, NMR (nuclear magnetic resonance) was used for chemical and physical molecular analysis. In 1971, Raymond Damadian showed that the nuclear magnetic relaxation periods of tissues and tumors differed. In 1973, Paul Lauterbur used a back projection technique similar to that used in CT scans and demonstrated magnetic resonance imaging on small test tube samples. In 1977, Raymond Damadian came up with the first whole body MRI scanner and performed a whole body scan in 4 hours and 45 minutes. In 1980, Edelstein and coworkers could acquire a single image in approximately 5 minutes using Ernst's technique. In 1986, this time was reduced to approximately 5 seconds. In 1993, functional MRI was developed which showed the mapping of the function of the various regions of the human brain.

An MRI machine is a large tube-shaped machine running through the magnet from front to end. The magnet creates a strong magnetic field around the patient and sends pulses of radio waves from the scanner. These waves force the nuclei of hydrogen atoms out of their normal position. As the nuclei realigns into their normal position they send out radio waves on their own. The computers or the scanners identify these signals and analyze and convert them into pictures with minute details of the part of the body being examined. MRI scans are now widely used to examine organs, detect tumors and in the diagnosis of many forms of cancer and injuries in bones and tissues.