Functional magnetic resonance imaging, or fMRI, has been used by neurosurgeons since the 1990s to scan for brain tumors and to diagnose nervous system disorders. Some in the intelligence community want to use it as a tool to identify terrorists or during an interrogation.

The adoption of fMRI is not surprising given the limitations of lie detection techniques such as a polygraph test, says Jonathan Marks of Pennsylvania State University.

The imaging uses magnetic fields to detect tiny changes in blood oxygen levels in the brain. Active neurons take up more oxygen than inactive ones and these tiny changes are believed to be signatures of cognitive processes.

Marks says there is evidence fMRI is being used to interrogate suspected terrorists despite concerns it may not be reliable.

"There can be all sorts of reasons for recognizing a name or a photograph or responding to a particular word," said Marks. "I lived London, listening to reports of IRA bombings. My brain would light up if you mentioned the word semtex -- a plastic explosive."

Marks analysis is published in the American Journal of Law and Medicine.