A brain scanning technique has "far-reaching" potential for predicting the fate of severely brain damaged vegetative patients, research has shown.
Positron emission tomography (PET) imaging was able to identify signs of awareness in patients previously thought to be lost to the world on the basis of standard tests.
Nine of the group later recovered a reasonable level of consciousness.
Overall, the PET scan was able to predict the extent of recovery of isolated brain damaged patients within the next year with 74% accuracy.
In comparison, functional magnetic resonance imaging (fMRI) - another brain scanning technique - was only 56% accurate.
Professor Steven Laureys, from the University of Liege in Belgium, who led the study reported in The Lancet medical journal, said: "We confirm that a small but substantial proportion of behaviourally unresponsive patients retain brain activity compatible with awareness.
"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."
Judging the level of consciousness of severely brain damaged patients can be challenging and distressing.
Traditionally, bedside clinical examinations are used to decide if a patient is in a minimally conscious state (MCS), or a vegetative state (VS), also known as unresponsive wakefulness syndrome.
Vegetative patients show no sign of either awareness or stimuli response and their chances of recovery are poor.
A third state, locked-in-syndrome, describes patients who are unable to respond to stimuli but remain conscious.
The standard assessment procedure uses a rating system called the Coma Recovery Scale - Revised (CSR-R) which scores responses to a wide range of stimuli.
However, up to 40% of severely brain damaged patients are misdiagnosed using these tests.
A PET scan tracks the uptake of glucose by nerve cells. It is often used to diagnose and assess brain cancer, since tumours consume a lot of glucose, but can also monitor activity in different parts of the brain.
The study looked at the effectiveness of PET and fMRI in diagnosing vegetative and minimally conscious states among 126 brain damaged patients, some of whom had been in their situation for more than 20 years. Also included in the group were four patients with locked-in-syndrome.
Results were compared with the findings of CSR-R behavioural tests.
A third of the 36 PET scanned patients diagnosed as "behaviourally unresponsive" by the CSR-R assessment - who included both vegetative and locked-in patients - were found to have brain activity consistent with "minimal" awareness.
Of this group, nine subsequently re-gained a higher level of consciousness.
Prof Laureys said: "Repeated testing with the CRS-R complemented with a cerebral... PET examination provides a simple and reliable diagnostic tool with high sensitivity towards unresponsive but aware patients.
"fMRI during mental tasks might complement the assessment with information about preserved cognitive capability, but should not be the main or sole diagnostic imaging method."
Dr Michael Bloomfield, from the Medical Research Council Clinical Sciences Centre in London, said: " This really exciting study suggests for the first time that a brain scanning technique called PET could be used in the future to predict the likelihood that a patient may 'wake-up' a long time after a severe brain injury.
"However, much more research is needed to work out how accurate PET scans might be at doing this and make the technique reliable enough to be potentially used in specialist centres.
"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."