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Magnetic resonance imaging is one of the major computerized imaging modalities available at any large metropolitan hospital and in many facilities in smaller cities and towns. This imaging technology provides capabilities for physicians to obtain images of pathology that are different from those obtained with other modalities and are often complementary. Additionally, magnetic resonance systems can provide information unavailable by any other imaging method. The initial clinical installations were in major research medical centers in the United States and Europe. Since these first installations of the early 1980s, systems have been commercialized by major manufacturers that are installed in all types of facilities worldwide. These include installations in dedicated separate buildings as well as in multistory hospitals. Systems that are mounted in trailers so they can be moved from location to location by truck as required with minimum setup times are equal in performance to most stationary installations. These latter systems are complete imaging facilities that can go where they are needed. Such is the revolution in medical imaging that originated from the work of Block and his coworkers at Stanford and Purcell and his colleagues at Harvard, both of whom reported their independent discoveries of nuclear magnetic resonance in Physics Reviews in 1946 (Block, Hansen et al. 1946; Purcell, Torrey et al. 1946). Working independently, these researchers described nuclear magnetic resonance (NMR) in bulk matter using different descriptions of what turned out to be the same phenomenon. The Stanford researchers described the precession of nuclear magnetization in a magnetic field, which produced an electromotive force in a radio frequency coil by induction, called “nuclear induction.” The Harvard group investigated the transitions of magnetic nuclei between different quantized states while in a magnetic field and the absorption of radio frequency energy at resonance, called “nuclear magnetic resonance.” After publication of the work of the two groups, discussions resolved the fact that both were describing aspects of the same phenomenon. In 1952 Block and Purcell shared the Nobel Prize in physics for this discovery. Many researchers have developed techniques that have moved these discoveries from physics into different applications. Chemists quickly found that the NMR frequency of a particular nucleus depends in part on the chemical structure of the material due to shielding effects on the nucleus from the cloud of electrons surrounding it. This causes a small shift in the frequency of the signal that became known as the “chemical shift.” In situations where different parts of the molecule exist in different chemical environments, a set of small frequency changes is observed. The recording of these chemical shifts as an aid in determining molecular structure is known as NMR spectroscopy.
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