| BACKGROUNDAccurate assessment of lymph node（LN）metastasis is importment for cancer staging and determining the best treatment options for a patient.LN metastasis is a dynamic process that may be reflected by intravoxel incoherent motion diffusion-weighted imaging（IVIM-DWI）.However,previous studies were not adequately designed and have produced controversial results.OBJECTIVEThis study sought to monitor the dynamic process of LN metastasis with IVIM-DWI,and to investigate the impact of disease course on the accurate detection of metastatic LNs by IVIM-DWI.METHODSTwenty female New Zealand rabbits with 2.5-3.0 kg body weight were studied.For each rabbit one thigh was injected with VX2 cells（to induce metastatic popliteal LNs）,and the other thigh was injected with an egg yolk emulsion（to induce inflammatory LNs）.Eight rabbits underwent IVIM-DWI（14 b values）two hours prior to,and 14,21,and 28 days after inoculation（D0,D14,D21,D28）.The short-axis diameter,short-to long-axis ratio（SLR）,enhancement ratio,the signal intensity ratio of lymph node to background,apparent diffusion coefficient（ADC）,true diffusion coefficient（D）,pseudodiffusion coefficient（D*）,and perfusion fraction（f）were measured.Independent samples t test or Mann-Whitney U test were used to analyze the difference between the metastatic and the inflammatory nodes at each time point,and between the metastatic group（metastatic nodes at D14,D21,and D28）and the inflammatory group（inflammatory nodes at D14,D21,and D28）.Three rabbits randomly chosen from the remaining twelve rabbits were sacrificed at each time point to perform hematoxylin and eosin staining and histologic evaluation.RESULTSThe patterns of dynamic change of D*,ADC,and D were different between the metastatic and the inflammatory groups.D*in the metastatic group increased gradually and continuously over time,while the inflammatory group peaked at D14 and then decreased.The value of D*was significantly lower in the metastatic group than in the inflammatory group only at D14（P=0.006）.Both ADC and D values in the metastatic group increased rapidly before D21,and then slowed down or decreased slightly by D28.Both ADC and D values in the inflammatory group decreased until D14 and then increased gradually.The metastatic group had greater values of ADC and D compared to the inflammatory group at both D21 and D28（ADC:P=0.015,P=0.003;D:P=0.005,P=0.006）.Both groups showed similar pattern of dynamic change in the value of f,and the metastatic showed greater value of f only at D28（P=0.003）.The short-axis diameter and SLR in the metastatic group were greater than the inflammatory group by D28（P=0.002,P=0.002）.The signal intensities show no significant difference between groups at any time point（P>0.05）.Cross-sectional analysis showed that the metastatic group had greater ADC,D,and f values（P<0.001,P<0.001,P=0.001）,and no significant difference was observed between groups（P=0.789）.Histopathologic analysis revealed that metastatic nodes were filled with increasing numbers of tumor cells and neovascularization over time.Focal necrosis and destruction of nodal structures gradually appeared and progressed in the metastatic nodes.CONCLUSIONS1.IVIM-DWI may be more sensitive in detecting metastatic nodes compared to conventional imaging parameters（size and signal）.2.IVIM-DWI can reflect the dynamic process of LN metastasis.3.IVIM-DWI can discriminate metastatic nodes and inflammatory nodes,but such ability is influenced by the disease course. |
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