A unique peek inside the human brain may help explain how it clears away waste like the kind that can build up and lead to Alzheimer’s disease.

Brain cells use a lot of nutrients, which means they make a lot of waste. Scientists have long thought the brain has special plumbing to flush out cellular trash, especially during sleep — they could see it happening in mice. But there was only circumstantial evidence of a similar system in people.

Now researchers have finally spotted that network of tiny waste-clearing channels in the brains of living people, thanks to a special kind of imaging.

“I was skeptical,” said Dr. Juan Piantino of Oregon Health & Science University, whose team recently reported the findings. “We needed this piece to say this happens in humans, too.”

The study was published in the journal Proceedings of the National Academy of Sciences.

Over a decade ago, scientists at the University of Rochester first reported finding a network they dubbed the “glymphatic system.” Cerebrospinal fluid uses channels surrounding blood vessels to get deep into tissue and move waste until it exits the brain. When mice were injected with a chief Alzheimer’s culprit named beta-amyloid, it cleared away faster when the animals were sleeping.

But it’s been hard to find that system in people. Regular MRI scans can spot some of those fluid-filled channels but don’t show their function, Piantino said. So his team in Oregon injected a tracer into five patients who were undergoing brain surgery and needed a more advanced form of MRI. The tracer “lit up” under those scans, and 24 to 48 hours later, it wasn’t moving randomly through the brain but via those channels just like research had found in mice.

It’s a small but potentially important study that Rochester’s Dr. Maiken Nedergaard predicted will increase interest in how brain waste clearance connects to people’s health.

Additional larger studies in healthy people are needed, and Piantino, whose lab focuses on sleep health, wants to find an easier, more noninvasive test.