Abstract: Objective To evaluate the diagnostic value of 3.0 T diffusion-weighted imaging with background suppression (DWIBS) magnetic resonance (MR) for lymph node metastasis in rectal cancer.Methods Thirty-five patients with rectal cancer who underwent preoperative routine MRI +DWI examination were enrolled in the study and were treated by rectal cancer resection plus lymph node dissection.Metastatic and non-metastatic lymph nodes were confirmed by postoperative pathology.Apparent diffusion coefficient (ADC) values,long-axis and short-axis diameters of lymph nodes were measured.Receiver operating characteristic (ROC) curve was used to assess the diagnostic efficacy of ADC,long-axis and short-axis diameters for differentiating metastatic lymph nodes from non-metastatic lymph nodes.Results A total of 151 lymph nodes were confirmed with exact location in 35 patients.Sixty-five metastatic lymph nodes and 86 non-metastatic lymph nodes were identified.The ADC values of metastatic lymph nodes and non-metastatic lymph nodes were (0.86±0.14)×10-3 and (0.94±0.16)×10-3 mm2/s respectively.The long-axis diameter were (9.78±3.13) and (7.90±1.77) mm,respectively.The short-axis diameter were (7.65±2.00) and (6.45±1.19) mm,respectively.There were statistically significant differences between metastatic and non-metastatic lymph nodes in ADC values,long-axis diameter,and short-axis diameter (all P<0.01).The areas under the ROC curve of ADC value,long-axis diameter,and short-axis diameter were 0.648,0.706,and 0.692,respectively.Optimal cutoff values for these parameters were 1.05 ×10 -3 mm2/s,7.95 mm,and 5.90 mm,respectively,and the corresponding sensitivities and specificities were 93.8% and 30.2%,75.4% and 61.6%,90.8% and 38.4%.Conclusions Quantitative measurement of ADC value may reflect the degree of diffusion restriction of metastatic lymph nodes by DWIBS at 3.0 T MR.Accurate diagnosis of metastatic lymph nodes in rectal cancer demands comprehensive evaluation combining ADC value with diameter measurement.