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Objective:This present retrospective study was conducted to evaluate the value of diffusion-weighted magnetic resonance imaging(DWI-MRI)and apparent diffusion coefficients(ADC)in detecting metastatic lymph nodes from nasopharyngeal carcinoma(NPC),especially the lymph nodes with short axis<10 mm.Methods:The data from 34 patients with pathologically confirmed NPC during January 2011 to February 2013 were analyzed.These patients were scanned three times by conventional magnetic resonance imaging(MRl)and DWI before,one month after,and six months after intensity-modulated radiotherapy.DWI was performed using a single-shot echo-planar(SSEPI)MR imaging sequence with b values of 0 and 800 s/mm2.ADC and rADC values of both primary lesions and lymph nodes were calculated and compare.According to the pathological diagnosis,all the 34 patients had non-keratinizing carcinoma and 33 had lymph node metastasis.Totally 158 morphologically abnormal lymph nodes were analyzed.Results:When setting b value was 800,both satisfying images and accurate measurements were obtained.Before radiotherapy,the average ADC of primary lesions was(0.616±0.074)×10-3mm2/s and the 8 lymph nodes with the short axis<10 mm was(0.692±0.069)×10-3mm2/s.The average rADC was(0.513± 0.062)×10-3mm2/s for primary lesions,and(0.576±0.058)×10-3mm2/s for 8 lymph nodes with the short axis<10 mm.One month After CRT,both ADC and rADC values of the lymph nodes increased.The average ADC was raised to(1.335±0.159)×10-3mm2/s for the primary lesions,and to(1.237±0.198)×10-3mm2/s for the 8 lymph nodes.rADC was(1.112±0.133)×10-3mm2/s for the primary lesions,and(1.030±0.165)×10-3mm2/s for the 8 lymph nodes.Six month after radiotherapy,the average ADC was(1.272±0.347)×10-3mm2/s for the primary lesions,and(1.318±0.176)×10-3mm2/s for the 8 lymph nodes.The rADC was(1.082±0.312)×10-3mm2/s for the primary lesions,and(1.097±0.145)×10-3mm2/s for the 8 lymph nodes.The ADC and rADC values of the residual lymph nodes after CRT was significantly higher than that before the treatment(P<0.0001).Conclusions:DWI plays an important role in diagnosing the metastatic lymph nodes especially for the metastatic lymph nodes from NPC with short axis<10 mm.The use of DWI is helpful for making therapeutic decisions and for evaluating effects of the therapy.