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Background:In head and neck neoplasm survivors treated with brain irradiation,metabolic alterations would occur in the radiation-induced injury area.The mechanism of these metabolic alterations has not been fully understood,while the altations could be sensitively detected by proton (1H) nuclear magnetic resonance spectroscopy (MRS).In this study,we investigated the metabolic characteristics of radiation-induced brain injury through a long-term follow-up after radiation treatment using MRS in vivo.Methods:A total of 12 adult Sprague-Dawley rats received a single dose of 30 Gy radiation treatment to semi-brain (field size:1.0 cm × 2.0 cm;anterior limit:binocular posterior inner canthus connection;posterior limit:extal acoustic meatus connection;intal limit:sagittal suture).Conventional magnetic resonance imaging and single-voxel 1H-MRS were performed at different time points (in month 0 before irradiation as well as in the 1st,3rd,5th,7th,and 9th months after irradiation) to investigate the altations in irradiation field.N-acetylaspartate/choline (NAA/Cho),NAA/creatinine (Cr),and Cho/Cr ratios were measured in the bilateral hippocampus and quantitatively analyzed with a repeated-measures mixed-effects model and multiple comparison test.Results:Significant changes in the ratios ofNAA/Cho (F=57.37,Pg ＜ 0.001),NAA/Cr (F =54.49,Pg ＜ 0.001),and Cho/Cr (F =9.78,Pg=0.005) between the hippocampus region of the irradiated semi-brain and the contralateral semi-brain were observed.There were significant differences in NAA/Cho (F =9.17,Pt ＜ 0.001) and NAA/Cr (F =13.04,Pt ＜ 0.001) ratios over time.The tendency of NAA/Cr to change with time showed no significant difference between the irradiated and contralateral sides.Nevertheless,there were significant differences in the Cho/Cr ratio between these two sides.Conclusions:MRS can sensitively detect metabolic altations.Significant changes of metabolites ratio in the first few months after radiation treatment reflect the metabolic disturbance in the acute and early-delayed stages of radiation-induced brain injuries.