In a single centre study, the investigators will use SPECT (Single Photon Emission Tomography) imaging to measure dopamine levels in the brain. MRI (Magnetic Resonance Imaging) scans will assess brain structure and function. The investigators will test whether treatment with methylphenidate improves cognitive functions in TBI patients who have ongoing cognitive problems, whether the mechanism involves a normalisation of brain functioning and whether brain dopamine levels can predict the magnitude of any improvement in symptoms.