Abstract: Objective  To investigate the characteristics of signal changes of the diffusion weighted imaging(DWI) of magnetic resonance imaging(MRI) for cerebral infarction in different periods. 
Methods  A total of 130 patients with cerebral infarction confirmed by MRI clinically were collected, and underwent both magnetic resonance plain scan and DWI examination simultaneously. Based on the time interval between onset and magnetic resonance examination, these patients were divided into hyperacute group(＜6 h, n=30), acute group(6-72 h, n=40), subacute group(3-10 d, n=30), chronic group(＞11 d, n=30). Apparent diffusion coefficients(ADC) of infarct foci and contralateral normal tissues were determined, and the characteristics of DWI signal changes in cerebral infarction in different periods were analyzed. 
Results  DWI of hyperacute lesion showed high signal, and the ADC value of lesion tissue was significantly lower than that of normal tissue, suggesting significant difference(P＜0.05). DWI of acute lesion showed high signal, the ADC value of lesion tissue was lower than that of tnormal tissue, and the difference was statistically significant(P＜0.05). DWI of subacute lesion showed slightly high signal, which was lower than that of acute lesions, and the ADC values of 29 cases of lesions were slightly lower than those of normal tissue, whereas 1 case of lesion was higher than that of normal tissue, with significant difference(P＜0.05). DWI of chronic lesions showed slightly higher signal in 13 patients, and equal or low signal in 17 patients. There was no significant difference between the ADC value of lesion tissue and that of normal tissue(P＞0.05). 
Conclusion  DWI can make a definite diagnosis of the signal changes of cerebral infarction in hyperacute, acute and subacute phases, with higher sensitivity to hyperacute and acute lesions. The changes of DWI signal and ADC value of cerebral infarction are in line with the characteristic time-evolution law, and DWI combined with conventional MRI can be used to infer the period of cerebral infarction.

Key words: cerebral infarction, magnetic resonance imaging, apparent diffusion coefficient