The most common major side effect of radiation therapy for the treatment of primary brain tumors is the necrosis of normal brain tissues (radiation necrosis). Radiation necrosis typically occurs weeks to months following treatment. The diagnosis is suspected when patients have new areas of gadolinium enhancement on magnetic resonance imaging (MRI) scans. Tumor recurrence can also occur within weeks to months following treatment and is represented by new areas of gadolinium enhancement as well. As the management options for radiation necrosis and tumor recurrence are significantly different, distinguishing the two is of critical importance. Conventional MRI, diffusion-weighted MRI (DW-MRI), MR spectroscopy, SPECT, and PET imaging have all been used to try to make this distinction, but the sensitivity and specificity of these techniques have not been clinically useful. Most patients must therefore undergo a risky diagnostic surgical procedure. Diffusion tensor MRI (DT-MRI) is an imaging technique that provides information regarding both the diffusive properties of water as well as the directionality of water movement. A modification of DT-MRI, referred to as two-compartment DT-MRI, appears to be more sensitive than other variations of MRI for the diagnosis of selected types of brain abnormalities.