A medical scan that can pick up brain tumours could also be used to tell whether a person is lying, US researchers have found.
Traditional lie detection machines are only about 70% accurate
When a person is telling the truth they use different parts of their brain than when people lie, the Temple University team said.
These changes were detected by functional magnetic resonance imaging.
The method may prove more accurate than traditional machines, they told the Radiological Society of North America.
The conventional polygraph lie detector looks for body changes linked with lying such as sweating and changes in blood pressure, heart rate and breathing.
But Dr Scott Faro and his team say the accuracy is limited because people who are telling the truth can show similar changes merely as a result of being anxious about being tested.
Furthermore, those adept at lying can learn how to cheat the polygraph test.
The researchers investigated whether fMRI scans might be able to spot what was happening in the brain when a person was telling a lie.
They asked six of 11 volunteers to fire a toy gun and then lie about what they had done. The other five were asked to tell the truth about what had happened.
Each of the volunteers was then scanned with fMRI while being asked questions by the scientists.
A polygraph test was also carried out for comparison.
In all cases the polygraph and the fMRI accurately distinguished between the volunteers who were telling the truth and those who were lying.
On the brain scans, different areas of the brain were active when the person was lying than when they were telling the truth.
Also, more areas of the brain were activated when the person was trying to deceive the questioner.
Although it is too early to tell whether confident liars could cheat the fMRI test, Dr Faro is hopeful it could be a more accurate way of spotting deception.
Unique areas of the brain are involved in deception
"We plan to investigate the potential of fMRI both as a stand alone test and as a supplement to the polygraph with the goal of creating the most accurate test for deception," he said.
Professor Richard Wiseman, from the Psychology Department at the University of Hertfordshire and who has carried out research into lie detection, said: "I'm sure it would be better than the polygraph.
"The problem with the polygraph is it's a measure of how anxious somebody is.
"Lots of people become anxious when they are attached to the polygraph anyway and good liars are not anxious when they lie.
"With fMRI you are looking at the brain's activity and lying is cognitively quite hard.
"You are having to think what is plausible, what does the person know, what can they go and check on, and so on.
"So, in terms of brain activity, the indicators are likely to be more reliable."
He said the only shortfall was how practical it was to use fMRI routinely because it requires the patient to remain relatively still inside a large, expensive tube-like machine which performs the scanning.
"It's not the sort of thing every police station has in the back, but in the future, potentially in high profile cases, it might be something people want to look at," he said.