Each year, doctors perform approximately 60 million magnetic resonance imaging (MRI) scans to treat patients for a variety of ailments. Paul C. Lauterbur and Sir Peter Mansfield share this year's Nobel Prize in Physiology or Medicine for their contributions to the development of this procedure. They will split the 10 million Swedish kronor prize (about $1.3 million), which will be awarded on December 10.

The origins of MRI date back more than 70 years and the 1952 Nobel prize in physics was awarded for work that determined how atomic nuclei exposed to a strong magnetic field behave. But prior to the 1970s, scientists utilized magnetic resonance mainly to investigate the chemical structure of substances. The imaging of people relies on the fact that nearly two-thirds of the human body is water. When exposed to a magnetic field, the protons in these water molecules align themselves with the field like tiny bar magnets. Brief pulses of radiowaves allow the molecules to shift out of this alignment. After a pulse, the nuclei return to their previous state and emit so-called resonance waves, the frequencies of which are then converted into images.

The Nobel committee recognized Lauterbur (above, left), now at the University of Illinois at Urbana-Champaign, for introducing gradients in the magnetic field that made it possible to assemble two-dimensional pictures of structures that could not previously be visualized using other methods. University of Nottingham professor Mansfield (above, right) improved the speed and efficiency with which the detected signals could be analyzed and transformed into an image. In addition, he developed so-called echo-planar scanning, in which changing the gradients in the magnetic field more quickly enables extremely rapid imaging. Within a decade, the technique was being used in clinical practice, the committee notes.

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