End-of-life questions are complicated by the uncertainty of whether a patient in a deep vegetative state will ever regain consciousness or recover, and doctors have been baffled by patients who they thought were all but dead coming back to life after an extended period of unconsciousness.

For those cases where patients do seem to wake from the dead, it’s most likely that they were in a minimally conscious state, where there is some awareness or response to stimuli. Such patients have a better chance of recovery than those in a vegetative state, where there are no signs of awareness or response to stimuli.

Diagnosing consciousness is tricky; oftentimes brain activity can be observed, but that doesn’t necessarily translate into consciousness on the part of the patient. The most well-known and standard test for determining awareness is the Coma Recovery Scale-Revised (CSR-R), a behavioral test. Now researchers have discovered that a particular type of brain imaging, positron emission tomography, may be able to determine which vegetative patients will recover.

In a study out Tuesday, scientists looked at 126 patients who had experienced severe brain damage. Researchers from the University of Liége in Belgium tested whether using PET with the imaging agent fluorodeoxyglucose (FDG) or another imaging technique called functional MRI (fMRI) could distinguish between a vegetative and a minimally conscious state.

Overall, the FDG-PET combination was better than the fMRI method at distinguishing between the two states. FDG-PET was also 74% accurate at predicting recovery within the next year. Additionally, 12 of the patients in the study group who showed some brain activity on the FDG-PET scan were diagnosed by the CSR-R method as behaviorally unresponsive, but 9 of them later recovered some consciousness.

“We confirm that a small but substantial proportion of behaviourally unresponsive patients retain brain activity compatible with awareness,” says study leader Professor Steven Laureys from the University of Liége in Belgium.

But diagnosing consciousness through brain imaging is far from an exact science. It’s often unclear how to interpret brain activity. In 2011, researchers from the University of Western Ontario reported that they had successfully used an electroencephalogram (EEG) to record brain signals that suggested awareness in patients in a vegetative state. However, when a team of scientists from Weill Cornell Medical College tried to replicate their data a few years later, they discovered that the original researchers didn’t account for false-positives. When they went back over the data, accounting for interfering factors like muscle activity and EEG blips, they were unable to replicate the results.

What the study does indicate, though, is that PET may be needed for confirmation of consciousness. “Our findings suggest that PET imaging can reveal cognitive processes that aren’t visible through traditional bedside tests, and could substantially complement standard behavioural assessments to identify unresponsive or ‘vegetative’ patients who have the potential for long-term recovery,” Laureys said in a statement.

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