Objective: Acute pancreatitis(AP)is an inflammatory that affects multiple organs and systems,a variety of inflammatory factors are closely related to their occurrence and development.In the development of acute pancreatitis,spleen as the the largest immune organ plays an important role,and they influence each other.Nevertheless,the splenic complications appeared in AP are relatively rare.This study attempts to use intravoxel incoherent motion(IVIM)diffusion weighted MR imaging to explore the true diffusion coefficient(D),perfusion fraction(f),and pseudo-diffusion coefficient(D*)of the spleen in AP and normal control,and their variation based on the severity of AP.Materials and Methods: A total of 53 patients with AP admitted to our institution between May and December 2015 as well as 15 healthy volunteers were included in this study.Abdominal MRI(GE 3.0T)were performed within 72h after admission,including routine plain scan,tri-phase dynamic enhancement,and IVIM Diffusion Weighted Imaging.There were Multi-bvalue(0、50、100、300、500、800、1000、1500s/mm2)in the study,D,D* and f were calculated by the workstation.The severity of AP was graded by the MRI severity index(MRSI)as mild(0-3 points),moderate(4-7 points),and severe(8-10 points).The D,D* andf values of the spleen both in AP and normal control were analyzed and were compared with the severity of AP.Results: Of the 53 patients with AP,patients with mild,moderate,severe AP were 35.8%(n=19),50.9%(n=27),11.3%(n=6),respectively.In AP and the normal control,the splenic D,D*,and f values were,respectively,(0.621±0.435)*10-3mm2/s and(1.039±0.470)*10-3mm2/s,(70.80±42.55)*10-3mm2/s and(129.6±62.23)*10-3 mm2/s,(0.340± 0.123)and(0.221± 0.118).Compared with those in the normal control,the splenic D,D* values in AP were apparently decreased,while the f value were increased(p<0.05).In mild,moderate,and severe AP,the splenic D values were,respectively,(0.682±0.371)*10-3mm2/s,(0.713±0.526)*10-3mm2/s,and(0.747±0.408)*10-3mm2/s.The D* values were,respectively,(82.7±59.5)*10-3mm2/s,(65.1±26.6)*10-3mm2/s,and(59.4±37.5)*10-3mm2/s.The f values were,respectively,(0.292±0.118),(0.312±0.123),and(0.354±0.148).With the increasing of AP severity,the splenic D and f values were increased,while the D*value was decreased(P<0.05).Conclusion: In patients with AP,the splenicD,D* values are apparently decreasing,f value increase.With the increasing of AP severity,the splenic D and f values are increased,while the D* value is decreased gradually.In patients with AP,the splenic perfusion microculation increases,additionally,which increases apparently following the increasing of the AP severity.