Dervan of the California Institute of Technology—"wrote the book [Stark and Bradley, 1988]" on clinical MRI. Stark specializes in imaging the liver and abdomen, where contrast agents have been particularly useful for enhancing MR images. He teaches at Harvard University and also works at the Radiology Department of Massachusetts General Hospital. Both authors are practicing physicians whose primary charge is to produce superior pictures of patients who are referred by other doctors for MR imaging.

Graeme Bydder from the Hammersmith Hospital in London was another of the session's presenters and is, said Dervan, "one of the true pioneers in this area." Bydder explained how the general problem of motion in the area under consideration has led to some refined MRI techniques. John Crues III provides another example of the range of MRI specialists. Teaching on the faculty at the UCLA Medical School and also serving as director of MRI at Santa Barbara Cottage Hospital, Crues told about the increasing value of MRI in imaging the musculoskeletal system, hitherto thought to be the province of the x ray. If there was an exception to the generalization that clinical MRI involves its practitioners in a continual process of analyzing and refining the pictures made of individual patients, Robert Balaban, chief of the Laboratory of Cardiac Energetics at NIH, is it. His laboratory is building new prototype machines in an effort to elucidate some of the subatomic phenomena even most MRI practitioners take for granted.

MRI provides a new way to look inside the body. When compared to the earlier x-ray systems—even sophisticated current versions such as computerized tomography (CT) and positron emission tomography (PET)—MRI is noninvasive, and often dramatically superior in discerning details and subtleties. The enhanced safety and results derive from a different approach altogether. The x ray invades the body in a series of electromagnetic pulses that are targeted to collide with the electrons in the tissue under examination. MRI, however, rests on different interactions: first, since most MR imaging targets the nuclei of hydrogen atoms (1H) found in water (H2O), the interaction is with protons; second, the "disruption" caused by the radiation is far more benign, since the radio frequency (RF) pulses generated travel in wavelengths many orders of magnitude longer—by photons that have energy many orders of magnitude less powerful—than those in the x-ray range of the electromagnetic spectrum. Instead of targeting electrons and thus generating ions in the body, MRI manipulates the inherent magnetic properties of spinning protons, detecting information from how the protons react to the RF pulses and magnetic fields directed toward the target tissue.

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