A geologist walks along a mountain slope, running an instrument over the rocks. The instrument shows where iron ore lies beneath the surface and where the rock has cracked. A magnetic map of the brain works in much the same way: it reveals where iron has accumulated in the tissues and where the sheath of nerve fibers is damaged.

After a concussion, such imaging is especially important. A blow to the head makes the brain shift inside the skull, and nerve fibers stretch like rubber bands. If the fiber's sheath, myelin, tears, the signal on the magnetic map changes. Researchers studied the brains of 25 athletes in the first two weeks after mild injury and compared them with the brains of 25 healthy people of the same age.

In the deep nuclei of the brain, which are responsible for movement, no iron accumulation was found. But in one of the regions of the hippocampus, the CA4 zone, the negative signal became weaker. The hippocampus is responsible for memory and is shaped like a seahorse. The CA4 zone is located in its tail and is known for its cells dying first in injuries and epilepsy.

Why does this matter? If a person has a headache or trouble thinking after a concussion, a doctor may not see any damage on a standard scan. A magnetic map with separated signals shows that even with mild injury, changes occur in the brain at the cellular level. Such a map helps explain why some people recover quickly while others suffer from the consequences for months.

A geologist, returning from the mountain, marks on the map cracks that are not visible from the surface. A year later he will come back and check whether they have widened. In the same way, researchers plan to rescan the same athletes several months later to see whether the damaged area heals or the changes remain forever.

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