Rats don’t like having their brains scanned any more than humans do, so studying their brains with positron emission tomography (PET) usually requires they be asleep. Now researchers at Brookhaven National Laboratory in New York have developed a Rat Conscious Animal PET (RatCAP) imager, which allows a rat to wear a scanner even while it moves around. The advance will make it possible for the first time to study metabolic activity throughout the brain in freely moving rodents, says high-energy physicist Craig Woody, co-author of the paper  published in Nature Methods.
PET imaging, which measures how a mildly radioactive tracer is taken up in tissue, is widely used in brain imaging to measure changes in metabolic activity that correspond with a particular behavior or occur in response to a stimulus such as a drug. For RatCAP, the Brookhaven researchers used a solid-state array of photosensors that they developed and miniaturized. This imager is placed on a rat’s head and attached to a readout system that encircles the animal’s head like a very high-tech flea collar. A flexible arm relieves the weight of the scanner and allows the rats to move freely about their cage, move their heads up and down, and turn 360° before having to turn back in the other direction.
To test the system, the researchers injected rats with a tracer to measure dopamine levels in the brain—first in a conventional PET scanner under anesthesia, then wearing RatCAP. Although the resolution and sensitivity of RatCAP was comparable to the commercial scanner, co-author Daniela Schulz, a behavioral neuroscientist, says RatCAP provided a different pattern of dopamine activity in the rats' brains, suggesting that anesthesia may alter neurochemistry more than researchers had realized.
Schulz says that because PET is widely used for humans as well as for rats, the new technology has “immense translational value." She plans to continue to use the system to track dopamine signaling during sex and other rat behaviors and says that other researchers have expressed interest in using it to study anxiety and seizure disorders. Co-author Paul Vaska, a medical physicist, says that the researchers have been talking with companies about commercializing the technology.
“This small, compact, lightweight PET camera, there are a lot of amazing technological achievements embodied inside that,” says physicist William Moses of Lawrence Berkeley National Laboratory in California. “I’m really excited to see it coming together.”