Is it hard to manipulate an MRI?

This morning I heard in the news that people were using MRI to detect lies. Experiments were conducted and MRI was compared with regular lie detectors. Under the experimental design, brain images showed relative (and statistically significantly) more activities when a person was trying to formulate a lie. I can't help thinking about how hard it would be to "pump" activities into my head when I try to lie. To me, it is definitely harder to manipulate my pulse in order to manipulate the regular lie detectors.