Tech Briefs

The device can be used to study human interaction, dementia, movement disorders, and more.

Patients undergoing a positron emission tomography (PET) scan in today’s bulky, donut-shaped machines must lie completely still. Because of this, scientists cannot use the scanners to unearth links between movement and brain activity. What goes on up there when we nod in agreement or shake hands? How are the brains of people struggling to walk after a stroke different from those who can?

Julie Brefczynski-Lewis, a neuroscientist at West Virginia University, places a helmet-like PET scanner on a research subject. The mobile scanner — designed for studies of human interaction, movement disorders, and more — is based on a scanner developed at Brookhaven Lab for brain-imaging studies in freely moving animals. (Credit: Brookhaven National Laboratory)

To tackle questions like these, Julie Brefczynski-Lewis, a neuroscientist at West Virginia University (WVU), has partnered with Stan Majewski, a physicist at WVU and now at the University of Virginia, to develop a miniaturized PET brain scanner. The scanner can be worn like a helmet, allowing research subjects to stand and make movements as the device scans. This Ambulatory Microdose Positron Emission Tomography (AMPET) scanner could launch new psychological and clinical studies on how the brain functions when affected by diseases from epilepsy to addiction, and during ordinary and dysfunctional social interactions.

The idea was sparked by a scanner developed for studying rats, a project started in 2002 at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory. “I learned about what my friends and colleagues at Brookhaven were doing,” says Majewski, “and decided to build the same type of device for humans.”

The Rat Conscious Animal PET, or RatCAP, scanner is a 250-g ring that fits around the head of a rat, suspended by springs to support its weight and let the rat scurry about as the device scans. Nora Volkow, head of Brookhaven’s Life Sciences division at the time, came up with the idea to image the brains of awake and moving animals.

“I wanted to do PET scans on animals without having to use anesthesia,” says Volkow, who is now the director of the National Institute on Drug Abuse. Unlike humans, animals can’t be told to simply lie still in a scanner. But the anesthesia required to make them lie still muddies the results. “It affects the distribution of the PET radiotracer and inhibits neurons,” Volkow says. A wearable scanner, however, would move with the animal’s brain and eliminate the need for anesthesia. Volkow enlisted the help of Brookhaven scientists and engineers to make the idea a reality.

In designing the small-scale scanner, the team used recent advances in detector technology. For instance, they used dense crystals to convert the gamma photons generated by positron-electron interactions into visible light, along with small light-detecting sensors called avalanche photodiodes. They also used special electronics developed at Brookhaven and built into the compact, lightweight PET detector. Suspending the structure on long springs helped support its weight so rats could wear the scanner while moving around easily.

From Rats to Hats

Word got out about RatCAP as the scientists presented their progress at conferences and meetings. Stan Majewski, then at DOE’s Thomas Jefferson National Accelerator Facility (Jefferson Lab), took notice. He had been working on new methods of breast cancer imaging, applying his high-energy physics detector expertise to the medical field.

Majewski’s Helmet_PET prototype used silicon photomultipliers — a newer, similarly compact but more efficient photodetector than the avalanche photodiodes used in RatCAP. The patent drawing of the prototype was sitting on Majewski’s desk at WVU when Brefczynski-Lewis, a neuroscientist, walked in. The drawing of a helmet-shaped detector on an upright person caught her attention.

“I had always been bothered by this middle zone of the brain you couldn’t reach with other imaging technologies,” she says. “With electroencephalography (EEG) you can’t reach deep brain structures, but with PET and MRI you can’t have motion. I thought Stan’s device could fill this niche.”

After building the first prototype at WVU, the two scientists began using Helmet_PET to image the brains of volunteer patients. After Majewski transferred to the University of Virginia, the team developed a newer model of the device, now known as AMPET. The current imaging cap is designed to scan a standing person and is attached to an overhead support, allowing for some motion.

The AMPET team hopes to start developing a full-brain scanner soon — one that covers the entire head rather than examining a horizontal five-centimeter section like the current ring.

Because AMPET sits so close to the brain, it can “catch” more of the photons stemming from the radiotracers used in PET than larger scanners can. That means researchers can administer a lower dose of radioactive material and still get a good biological snapshot. Catching more signals also allows AMPET to create higher resolution images than regular PET.

But most importantly, PET scans allow researchers to see further into the body than other imaging tools. This lets AMPET reach deep neural structures while the research subjects are upright and moving. In the medical sphere, the scanning helmet could help explain what happens during drug treatments and could shed light on movement disorders.

The RatCAP project at Brookhaven was funded by the DOE Office of Science. RHIC is a DOE Office of Science User Facility for nuclear physics research. For more information, visit here.