Evaluation of cerebral radiation injury using MRI and Proton MRS

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Brain necrosis is the most serious side effect in the central nervous system after irradiation. We investigated the usefulness of magnetization transfer imaging (MTI) and proton magnetic resonance spectroscopy (Proton-MRS) for the early detection of cerebral radiation necrosis in the normal brain of cats.A single 60 Gy dose was irradiated with a 1.5*2.0 cm vertex portal in the right cerebral hemisphere of the cat, using a 12-MeV electron beam. MR images and Proton-MRS were obtained, using a 1.5 T MR imaging unit. Coronal fast spin-echo imaging was performed with TR=3000 msec and effective TE=88 msec. 2D-GRASS imaging was per-formed with TR=600 msec and TE=4 msec and every 50 msec off resonance magnetization transfer pulses were used for evaluate magnetization transfer ratio (MTR). Proton-MRS were obtained from a volume (10*10*10mm3) of interest by using point resolved spectroscopy pulse sequence. Repetition time and echo time were 2000 ms and 136 ms, respectively.Irradiated white matte … More r revealed an abnormally high MR signal intensity on T2-WI in all cats 3.5 to 4.5 months after irradiation. Proton spectra in the affected region showed an apparent increase of the choline-containing com-pounds/creatine-phosphocreatine ratio and a trend of a decreasing N-acetylaspartate/creatine-phosphocreatine ratio compared with the contralateral non-irradiated brain in all cats. These changes occurred at least one month earlier than abnormalities detected on MR images. MTI also showed abnormally low MTR at the irradiated area earlier than conventional MR images. Microscopic examinations revealed the regions of high signal intensity on T2-WI were demyelination and contained flank necrosis.We speculate that, just before radiation necrosis, cell membrane kinetics was changed as membrane synthesis was disturbed and/or membrane degradation was accelerated. In vivo Proton-MRS and MTI could be a potential tool for detecting radiation injury, even when conventional T2 image shows no signal change. Less