The Trail of Invisible Light: A Century of Medical Imaging
Magnetic Resonance Imaging
An MRI unit consists of a large cylindrical superconducting magnet, devices for
transmitting and receiving radio waves, and a complex computer system. When a patient lies inside the unit, the strong magnetic field causes the hydrogen nuclei in the body to line up. A low-frequency radio wave is pulsed through the magnet into the patient. The hydrogen atoms absorb the energy released by the radio waves. This disrupts the uniformity of the nuclei. When the radio-wave stimulation stops, the nuclei return to their original state and emit energy in the form of weak radio signals. The strength and length of these signals--and |
28. MR images of the brain. Courtesy of Hanna Damasio, MD, Department of Neurology, and the Division of Nuclear Medicine, Department of Radiology, UIHC. |
| therefore the kind of image produced--depend on the properties of the organ or tissue involved. A computer translates the signals into highly detailed cross-sectional images. The images are essentially maps of the locations of water or hydrogen in the body. |
MRI doesn't read tissue density -- it surveys the location of hydrogen atoms in the body (mostly in water and fat) to create a picture of tissue structure. The technique does not display bone as well as X ray and CT scan. But because bone is largely invisible to it, MRI does a much better job of revealing the normal anatomy and diseases of the soft tissues that bone otherwise obscures--such as the spinal cord or the base of the brain.
29. Fluoroscopic examination. From Electro-Therapeutics and Roentgen Rays, Mihran Krikor Kassabian, MD, J.B. Lippincott Company. 1907. |
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