Keywords: 
• radius of the rootmeansquared of relativedistance /
• twopion correlation function /
• CoreHalo model /
• imaging technique /
• Gaussian fit

Abstract: We characterize the spatial dimension of pionemitting source in highenergy heavy ion collisions with the radius of rootmeansquared of twopion relativedistance R_rmst. Based on the imaging technique， one can calculate R_rmst modelindependently. Applying this method to the Core-Halo model and comparing the results with that of Gaussian fitted， the results indicate that due to the effects of longlived resonance decay， the values of R_rmst show a larger spatial size of pionemitting region. Meanwhile， for the chaotic source the chaoticity factors calculated by imaging technique are larger than that of Gaussian fitted.