If you want to see precisely what the 10 billion neurons in a person’s brain are doing, a good way to start is to track calcium as it flows into neurons when they fire.
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So Jasanoff designed the new sensor to incorporate so-called “superparamagnetic nanoparticles”–extra-strength molecular-sized magnets previously designed for ultrasensitive tumor imaging. They produce large MRI contrast changes capable of producing very high-resolution images.