In an unprecedented trial among comatose patients, Belgian researchers said the scanner, an expensive tool not routinely used for this purpose, could identify those who were unresponsive but showed consciousness.
And among the more severely-injured patients, it could also tell which ones had the potential to recover a degree of consciousness, even though they seemed in a vegetative state, the study said.
The findings may provide an important guide to doctors and families facing the tragic decision as to whether a patient should be kept alive or not, observers said.
"PET imaging can reveal cognitive processes that aren't visible through traditional bedside tests," said Steven Laureys, a professor at the University of Liege, who led the study.
The scan could eventually complement existing tests to identify patients "who have the potential for long-term recovery".
Doctors treating a comatose patient often struggle to determine their level of consciousness.
They broadly have to distinguish between a so-called minimally conscious state (MCS) -- in which a patient shows some evidence of awareness and response to stimuli -- or unresponsive wakefulness syndrome, a vegetative state in which there is neither, and for which recovery prospects are remote.
They use a standardised examination called the coma recovery scale-revised (CSR-R), but 40 percent of diagnoses that use it are wrong.
- Two scanners tested -
The research, reported in The Lancet, put two 3D diagnostic tools to the test.
One was the positron emission tomography (PET) scanner, which used a radioactive tracer in the blood stream to map brain regions activated by the stimuli.
In this case, the tracer molecule was attached to glucose -- the energy used by brain cells when they are at work.
The other was the functional magnetic resonance imaging (fMRI) scanner, which has the same goal but uses a different technique -- it identifies the uptake of oxygen by activated brain cells.
Over four-and-a-half years, Laureys and a team tested the two tools on 81 patients in MCS, and 41 in the vegetative state.
For comparison, the results were measured against four patients with "locked-in" syndrome -- a condition in which the individual is fully conscious but cannot respond behaviourally -- and 39 healthy volunteers.
The patients were scanned after they underwent the CSR-R test.
The PET scan was 93-percent accurate in spotting a patient in MCS, compared with 45 percent with fMRI.
PET images were also more useful in gauging how far a patient could recover, compared to tests conducted a year after the scan.
The PET scan highlighted the weaknesses of relying purely on the bedside test, said the study.
The CSR-R test diagnosed 36 patients as unresponsive.
But the PET scan showed that a third of them appeared to have brain activity consistent with some consciousness -- a prediction that was subsequently borne out among nine in this group.
Diagnosing and treating severely brain-damaged patients is also an ethical question, given the emergence of sophisticated life support systems that can sustain vital functions for years.
Headline-making cases include that of former Israeli prime minister Ariel Sharon, who died in January after spending eight years in a coma.
"This study is welcome because diagnosis and prognostic accuracy in patients with severe brain injury needs to be improved," said Jamie Sleigh of Waikato Hospital in New Zealand and Catherine Warnaby of the John Radcliffe Hospital in Oxford, England.
"For example, intensive care doctors caring for former racing driver Michael Schumacher after his recent serious brain injury refused to offer any firm prognosis to the world's media," they said in a commentary.
In remarks carried by Britain's Science Media Centre, Michael Bloomfield of the Medical Research Council (MRC) described the trial as "really exciting."
?If the results of this study are confirmed in future research, this could have far-reaching clinical, ethical and legal implications, including whether to offer an apparently unconscious patient pain relief and, ultimately, whether treatments that may be keeping someone alive should be continued or not," he said.