POTENTIAL OF T2 RELAXATION-TIME MEASUREMENTS FOR EARLY DETECTION OF RADIATION-INJURY TO THE BRAIN - EXPERIMENTAL-STUDY IN PIGS

PURPOSE: To investigate the MR T2 relaxation time and histologic chang es after a single-fraction 25-Gy dose of radiation to the brain of pig s. METHODS: The right hemisphere of 10 Meishan pigs was irradiated wit h a single dose of 25 Gy at the 90% isodose, using a 12-MeV electron b eam. T2 relaxation time was measured within three regions of interest in the brain: those that had received 90%, 70%, and 40% of the total d ose, respectively. T2 kinetics over time was compared with histologic studies. RESULTS: Brain T2 values were noted to increase within the ir radiated areas. T2 kinetics were analyzed in three phases: an immediat e transient phase and two long-lasting phases. These two long-lasting phases were correlated with the detection of ventricular compression a nd necrosis, respectively. The T2 increase within the 90% region of in terest was 19%, 22%, and 26% for phases I, II, and III, respectively. T2 measurements within other regions of interest were not significant. CONCLUSION: Although our results suggest a dose threshold for T2 vari ations, brain T2 values increased after irradiation at a level at whic h disease could not be seen on conventional MR images. This illustrate s the value of using conventional MR imaging in a quantitative manner to assess molecular tissue abnormalities at earlier stages of developi ng diseases.