The Washington University team say that specific areas of the brain are active when thinking about upcoming events.
The Proceedings of the National Academy of Sciences study could help doctors trying to understand damage inflicted by strokes, injuries or diseases.
The findings tally with damage spotted in the brains of patients who have lost the ability to 'think ahead'.
The brain remains the most poorly understood organ of the body, but the use of MRI scans to examine the way they work has taken off in recent years.
When patients or volunteers are placed in the functional MRI scanner and asked to think or move in a particular way, specific areas of the brain 'light up' on the scan image, corresponding with increased electrical activity in those regions.
The technique has developed to the extent that scientists can almost know what patients are thinking about simply by looking at the brain areas they are using.
The latest project looked at one of the qualities thought to be unique to humans - the ability to create a mental picture of events that have not yet happened.
The researchers placed 21 volunteers into the MRI machine, then contrasted the scan results when they were asked to imagine vividly future events and recollect past memories.
The resulting images showed clear differences between a birthday already experienced, and a birthday yet to come.
In particular, when looking ahead, three particular areas of the brain were activated - the left lateral premotor cortex, the left precuneus and the right posterior cerebellum.
These brain areas are already known to be involved in the imagining of body movements, suggesting that when the human brain is thinking about the future, it does so in terms of distinct movements and actions that will happen at that point.
Examples from other research include the activation of the part of the brain involved in swinging the arm when volunteers were asked to think about playing baseball.
The test results are in line with other studies of patients who have suffered brain damage in roughly the same areas, and are no longer able to imagine future events.
The researchers wrote: "Perhaps one of the most adaptive capacities of the human mind is the ability to fashion behaviour in anticipation of future consequences.
"Much of our everyday thought depends on our ability to see ourselves partaking in future events."
However, they made clear that further research would be needed to unlock the precise way that brain works when thinking about the future.