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Magnetic resonance imaging (MRI) uses the complex but well-understood phenomenon of nuclear magnetic resonance (NMR) to map the distribution of water within the body. The NMR phenomena in bulk materials dates back to the 1940s and uses radio waves and large magnets to initiate an intricate "dance" of the nuclei of the hydrogen atom within water. This is possible because the hydrogen nucleus is magnetic, resembling a tiny compass needle. The characteristic motion started by the burst of radio waves is then detected as a faint radio signal emanating from the wobbling nuclei themselves using a sensitive radio wave detector system.
MRI can do far more than just map the density of water in the body, though. Almost every physical and chemical property we associate with an object - temperature, chemical content, molecular structure, fluidity, flow, electrical conductivity and many others - leaves a subtle mark on the NMR signal we detect. This results in remarkable flexibility in imaging the body with MRI, as water is used in many complex ways that differ across organs in healthy tissue and are altered in disease.
Indeed, MRI has proven to be the method of choice for detecting disease in soft tissue. It has found a range of applications in most parts of the body. In a survey of how medical interventions and technology have affected their ability to treat patients, physicians ranked MRI and CT as the most important innovation. The MRI examination is expensive, but it has nearly eliminated costly and potentially dangerous "exploratory surgery" procedures.
In research, MRI has provided the ability to study disease processes and the response of disease to therapeutic treatment in the only truly relevant laboratory setting: the human body. It has thus proved valuable in honing disease treatments as well for aiding diagnosis. The technique has also proved a powerful research tool for studying human brain function in both disease and health. Using the local changes in blood flow and oxygenation that occur where the brain is relatively active, fMRI can localize brain activity and allow the mapping of function to location. More importantly, functional MRI (fMRI) experiments allow us to study the underlying networks within the brain.