Some traumatic brain injuries can be seen on an MRI scan. Many, however, cannot. The structural 
damage caused by the trauma is just too subtle. Of course, even subtle changes in brain structure can cause profound changes in brain functioning. Simply because the brain injury cannot be seen on an MRI, it doesn’t mean that it doesn’t exist.
While an MRI (upper photo) shows abnormalities in the brain’s structure, a PET scan (lower photo) shows abnormalities in the brain’s functioning. PET scans do this by measuring the distribution of metabolic activity in the brain. The parts that are not experiencing the expected biochemical activity have a functional abnormality. The PET scan, then, can serve as objective evidence of a brain injury that can’t be seen on an MRI.
PET scans can help prove that a victim has suffered a traumatic brain injury. But before a judge will allow the jury to see the PET scan, he has to be convinced that the PET scan reliably depicts what the testifying expert says it depicts,. The judge will also need to be convinced that PET scanning is widely accepted in the medical community.
Doctors frequently use PET scans to find changes in brain functioning in an Alzheimer’s patient or an epileptic. They less commonly use them to diagnose victims of traumatic brain injuries. This is one reason why some judges are reluctant to allow PET scans to be used in a trial of a traumatic brain injury case. These judges, however, are often more willing to allow the PET scans to be shown to the jury when the scan was ordered by the victim’s treating doctor for the purpose of diagnosis or treatment – before any lawsuit was filed.