Magnetic resonance imaging (MRI)

Magnetic resonance imaging (MRI) allows physicians to examine tissues and organs inside the body by observing the response of atoms exposed to a strong magnetic field. In a closed MRI, the patient lies inside a machine in a 48-inchtube; in an open--or "accessible"--MRI the patient lies on a table. More sensitive than X-ray spectroscopy, MRI does not rely upon potentially harmful radiation. Instead, a powerful electromagnet creates radio waves that cause hydrogen atoms in the body to release energy. The magnet can map this energy from almost 360 degrees, projecting images to a computer which gives an extremely high-resolution picture. Because MRI can scan through bone, it can probe the brain in search of a tumor, to assess stroke damage, or identify degeneration. It also "sees" under nerve coverings to help diagnose multiple sclerosis;reports joint injuries in muscle and ligaments; identifies blocked blood vessels; helps diagnose heart, liver and kidney disease; and is among the most powerful tools for detecting tissue abnormalities such as cancer.The series ofscientific developments leading to the invention of MRI actually began in the late 1930s. Isidor Isaac Rabi (1898-1988), an American scientist, designeda process by which the magnetic strengths of atomic nuclei could be recorded.Firing a vaporized beam of silver through a magnetic field, he noted that the nuclei behaved like spinning tops, and that they wobbled at very precise frequencies; when radio signals that matched the frequencies of the wobble wereapplied, the nuclei reversed their spin. This phenomenon, called magnetic resonance, was very easy to observe, and much could be learned about the structure of the atom by knowing the resonance frequency. After World War II, two other American scientists, working independently, devised improvements upon Rabi's process, making it more precise and eliminating the need to vaporize (and thus destroy) the sample. Scientists Felix Bloch (1905-1983) and Edward Purcell (1912-1997) shared the 1952 Nobel Prize for Physics for nuclear magnetic resonance (NMR) spectroscopy, which gained an immediate place in nuclear laboratories as the most precise tool for studying molecules. Chemists had earlier discovered that a nucleus' wobble revealed information about surrounding molecules, and that each atom and molecule carried a "signature wobble." By recognizing these signatures, researchers could use NMR to identify thecomposition of unknown chemical samples.

The most important application was forthcoming, however. Raymond V. Damadian(1916-) was the first to realize that NMR could be used on living tissue. Hetested first on animals and then on humans, finding that the process was excellent for detecting areas of disease inside the body--areas that had previously required exploratory surgery to locate. NMR was especially useful for detecting cancer, since cancer cells carry their own signature resonance frequency. About this same time, the Swiss physical chemist, Richard R. Ernst (1933-), was working upon improving the NMR process yet again. By changing the radiosignals, Ernst succeeded in making the technology more sensitive and easierto interpret--improvements that paved the way for the development of MRI, forwhich Ernst was awarded the 1991 Nobel Prize for Chemistry. MRI enables physicians to create three-dimensional images of large sections of molecules. Itcan define areas of soft tissue too thin to be picked up by X-rays. Using theradio signals, the computer will search for the frequencies of specific types of atoms (such as cancer cells). Once the radio waves are turned off, the atoms emit pulses of absorbed energy; the computer reads these pulses, creating a three-dimensional image of the scanned area. MRI scanners are generally found only in large medical research centers: the equipment is extremely expensive, and a trained radiologist must be present to supervise the procedure. The technology required to perform NMR scanning is much more affordable and easier to use; NMR units can be found in a variety of sizes at most hospitals.Both inventions have become important diagnostic tools. In fact, the Royal Swedish Academy (while announcing Ernst's Nobel Prize) described NMR spectroscopy as "perhaps the most important instrumental measuring technique within chemistry."

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