Abstract: Objective To investigate the value of magnetic resonance diffusion weighted imaging (MR-DWI) combined with intravoxel incoherent motion (IVIM) in assessing the severity of cerebral infarction in young and middle-aged people and predicting hemorrhagic transformation (HT) after thrombolytic therapy, with the aim of providing a reference for early clinical targeting and intervention planning. 
Methods A retrospective study was conducted on 96 middle-aged and young patients with acute ischemic stroke (AIS). DWI and IVIM examinations were performed on admission, apparent diffusion coefficient (ADC), perfusion fraction (f), pseudo-diffusion coefficient (D*), diffusion coefficient of pure diffusion (D) were obtained from healthy and affected side, and all parameters were standardized. rADC, rf, rD* and rD of patients with different disease severity were compared to analyze the correlation between each parameter and the disease severity of AIS in young and middle-aged people. After intravenous thrombolytic therapy, rADC, rf, rD*, rD and HT-related biological indicators ［cystatin C (Cys-C), fibrinolytic protein 3 (Ficolin-3), complement C1q/ tumor necrosis faction-associated protein-3 (CTRP-3)］ were compared between patients with and without HT at admission. Receiver operating characteristic (ROC) curve was used to evaluate the value of each parameter in predicting HT after thrombolytic therapy for AIS in young and middle-aged adults. 
Results ADC, f, D* and D on the affected side were all lower than those on the healthy side at admission (P＜0.05). The rADC, rf, rD* and rD of severe patients were all lower than those of moderate and mild patients at admission, and lower in moderate patients than in mild patients at admission (P＜0.05). rADC, rf, rD*, rD were negatively correlated with AIS severity at admission (P＜0.05). The levels of rADC, rf, rD*, rD and serum Ficolin-3 and CTRP-3 levels in patients with HT were lower than those without HT, while the levels of Cys-C were higher than those without HT (P＜0.05). rADC, rf, rD* and rD were negatively correlated with serum Cys-C levels and positively correlated with serum Ficolin-3 and CTRP-3 levels in patients with HT at admission (P＜0.05). At admission, the area under the ROC curve (AUC) of the combination of rADC, rf, rD* and rD in predicting HT in young and middle-aged people with AIS after thrombolytic therapy was 0.943, which was greater than that predicted by the combination of serum Cys-C, Ficolin-3 and CTRP-3 (Z=0.195, P=0.043). 
Conclusion DWI combined with IVIM can be used to evaluate the severity of cerebral infarction in young and middle-aged adults, providing reference for clinical prediction of the occurrence of HT after thrombolytic therapy, so as to carry out targeted follow-up treatment and reduce the risk of HT. 

Key words: cerebral infarction, magnetic resonance imaging, intravoxel incoherent motion