Shielded by the skull and packed with fatty tissue, the living brain is perhaps the most difficult organ for scientists to probe. Functional magnetic resonance imaging (fMRI), which noninvasively measures changes in blood flow and oxygen consumption as a proxy for neuronal activation, lags far behind the actual speed of thought. Now, a new technique may provide the fastest yet method of measuring blood flow in the brain, scientists report online today in Nature Methods. The technique, which bounces laser beams off red blood cells, has a resolution of under a millisecond—slightly less time than it takes a neuron to fire—and it has a far higher spatial resolution than fMRI. Even the most powerful fMRI machines, used only on animals, can image only millimeter-wide swaths of tissues including thousands of cells. The new technique, which takes its measurements from sonic waves produced by the beams, can image structures as small as individual blood vessels and cells (see above). Although the technique is not likely to be feasible in humans due to safety concerns, it could provide an important tool to better understand how blood flow and oxygen consumption is related to brain activity. That’s a key question for those relying on cruder and safer tools, such as fMRI, to study the human brain, researchers say. It is also a powerful tool for studying how errant eddies and whorls of blood in blood vessels can sometimes lead to stroke, they say.