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Medical imaging has come a long way since 1895, when german physicist Wilhelm Conrad Rntgen observed strange flickers cast by his cathode-ray instruments. Within months, Rntgen had used the mysterious “x-rays,” as he called them, to produce an image of the bones of his wife’s hand, revolutionizing medicine. For the first time, physicians could peek inside the body without cutting it open or probing an orifice. Today they can practically image our thoughts.

One of the latest technologies for seeing under our skin-functional magnetic resonance imaging (fMRI)-uses the combination of a powerful magnet and radio frequency pulses to see which parts of the brain are active. Neurons themselves are too small to image, but their activity causes changes in the flow of oxygenated and deoxygenated blood around them. For example, when you hear a loud noise, a patch of neurons fires on each side of your brain. Their activity requires an increase in blood supply. The oxygen-rich inrushing blood has different magnetic properties than the deoxygenated blood that it displaces. The magnet and the radio signals inside the functional MRI scanner work together to reveal where blood is rich in oxygen and where it is not. The resulting image shows the two patches of neural activity as bright regions on either side of the brain. From such maps, researchers can determine which parts of the brain are used for speech, vision, auditory and motor skills, and more.

Functional magnetic resonance imaging was first realized in 1990 when Seiji Ogawa, working at what was then AT&T’s Bell Laboratories, announced that he could use the contrasts in blood oxygen levels to create images of regional brain activity. The technique is a step up from diagnostic magnetic resonance, which has been around since the 1970s and produces detailed views of bones, ligaments and other tissue. Another method, called positron emission tomography, does provide images similar to a functional MRI’s-but it requires patients to be injected with radioactive substances.

As fMRI improves, so too will medicine. Recently, researchers at the Medical College of Wisconsin in Milwaukee used the technology to figure out which part of the brain manages our perception of time. Their find could lead to new drugs for patients with Parkinson’s disease, who often experience problems with time perception.

Got a new technology you’d like to see explained in Visualize? Send your ideas to visualize@technologyreview.com.

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