Zeugmatography is one of the earliest forms of medical magnetic resonance imaging. It combines nuclear magnetic spectroscopy with techniques for scanning with radio waves to create images of internal human anatomy, and is particularly useful for locating tumors and other abnormal tissues.
This non-invasive imaging method is based upon the principle that hydrogen nuclei in a strong magnetic field will absorb pulses of radio frequency photons. (Many elements could be used in theory, but hydrogen is used because it's ubiquitous.) The nuclei then re-emit the pulses as radio waves which can then be reconstructed into computerized images of cross-sections of the human body.
The zeugmatograph ultimately detects the spin the hydrogen nuclei get when they're exposed to strong magnetic fields. This spin differs according to the density gradient of the water the hydrogen atoms are found in, so the spin indicates tissue density. The machine does not measure the concentration of hydrogen atoms per se.
This imaging process was developed in 1973 by Paul C. Lauterbur, who is the head of the department of medical information sciences and the director of the Biomedical Magnetic Resonance Laboratory at the College of Medicine at the University of Illinois at Urbana-Champaign. Lauterbur coined this technique's name from the Greek word "zeugma," which means "to join together".
There are two main versions of this technique used today: Fourier Zeugmatography (which exploits phase and frequency shifts) and Rotating Frame Zeugmatography.
Reference: http://www.knaw.nl/heinekenprizes/lau_med/1989.htm