Therapeutic irradiation is commonly used in the treatment of malignant brain tumors and has the potential to cause severe injury to normal tissues adjacent to a lesion. Computed tomography (CT) can be used not only to visualize tissue damage but to monitor the development of a lesion using advanced quantitative CT techniques. We have used a postprocessing dual energy CT method to quantify changes in tissue composition following large single doses of X-rays to the normal canine brain. The results suggest that these techniques will provide more information about the character of a lesion than is obtainable from conventional imaging procedures.