Consciousness: Confessions of a Romantic Reductionist (The MIT Press)

by Christof Koch

The MRI scanner generates a powerful magnetic field, about 100,000 times stronger than Earth’s magnetic field. The nuclei of certain elements, including hydrogen, behave like miniature bar magnets. When you enter a scanner’s strong magnetic field, the hydrogen nuclei in your body line up with this field. More than half of your body weight is water, which is made up of two hydrogen atoms and one oxygen atom. The MRI scanner sends a brief pulse of radio waves into your skull, knocking the nuclei out of alignment. As the nuclei relax back into their original state, they give off faint radio signals that are detected and processed into a digital image. Such a picture reveals the structure of soft tissue. For example, it demarks the boundary between the brain’s gray and white matter. Magnetic resonance imaging is much more sensitive than X-rays. It has revolutionized medicine by allowing tissue damage, from a tumor to trauma, to be located and diagnosed with negligible risk to the patient.