(CNN) - It can be one of the most difficult diagnoses for a doctor to make: whether a brain-damaged patient is in a permanent vegetative state and will never wake up, or if he is in a minimally conscious state and may one day recover.
In fact, for patients with significant swelling in the brain, a doctor's outcome prediction is currently "a little better than flipping a coin," researchers Jamie Sleigh and Catherine Warnaby write in The Lancet this week.
However, a new study published with their editorial suggests that some types of brain imaging could make an accurate diagnosis much more likely.
Patients who are in a minimally conscious state (MCS) may respond to stimuli; they may reach for a ball, say their name or look at someone as they enter the room, says Dr. Joseph Fins, an expert in neuroethics and brain injury at Weill Cornell Medical College. Minimally conscious patients have the potential to regain consciousness. Patients who are permanently vegetative do not.
Patients in a persistent vegetative state (VS) are unresponsive, even though their brain shows signs of some activity. The patient is essentially in a coma from which he or she is extremely unlikely to recover.
Both conditions are different from brain death, where the brain has stopped functioning altogether. Patients who are brain dead cannot recover.
Doctors have difficulty distinguishing between minimally conscious and vegetative states because the signs of responsiveness are subtle. "The challenge with MCS is that they only do these behavior intermittently, so it's very easy to miss," said Fins.
The Lancet study authors looked at the potential of functional brain imaging to help doctors predict whether patients will regain consciousness. The study was conducted at a hospital in Belgium. Doctors tracked 81 patients who had been diagnosed as being in a minimally conscious state and 41 who'd been diagnosed as being in a vegetative state.
Up to 40% of patients are misdiagnosed in these situations, researchers wrote in the accompanying editorial. Four patients who were conscious but unresponsive were used as a control group. Doctors looked at functional MRI and PET scans for all patients.
Researchers concluded that PET imaging was better than MRI at distinguishing how likely a patient was to recover. A PET scan was about 74% accurate in predicting whether a patient would recover during the study's one-year follow-up period, compared to the MRI's 56% accuracy rate. That's consistent with neurobiology research, says Fins.
The results did not surprise Fins, who explains that PET scans measure metabolic activity in the brain - the glucose, or energy, that the brain is actually using to function. By comparison, an MRI simply looks at the brain's structure.
A third of the patients whom doctors said were unresponsive showed brain activity similar to minimally conscious patients, according to the study. Nine of these patients eventually recovered consciousness.
"Just imagine," Fins said. "These patients are conscious ... To be segregated from the community because you've been misdiagnosed is a tragic thing. This is complicated stuff."
PET scans may eventually help doctors diagnose patients with serious brain injuries, lead study author Steven Laureys says, but it's not ready for widespread use yet. The study was done in a special unit with trained researchers. Using PET as a tool for diagnosis of brain-injured patients too soon could do more harm than good - leading to more misdiagnoses instead of less.
"Functional brain imaging is expensive and technically challenging, but it will almost certainly become cheaper and easier," Sleigh and Warnaby write.
"In the future, we will probably look back in amazement at how we were ever able to practice without it."