Background: Due to the effect of near-field artifacts

the clearity of reconstructed image obtained by coded aperture imaging was decreased.Purpose: In order to explore a way to eliminate the effect and to enhance the image quality

a Geant4 simulation was performed in this work.Methods: On the basis of the principles of near-field artifact generation and elimination

coded aperture imaging of a 300 keV gamma source was simulated by Geant4 at near-field conditions with a mosaicked MURA（19×19） mask by single-and double-mode measurements

respectively.Results: The near-field artifacts were apparent in the images obtained by single-mode measurement

while they were almost eliminated by double-mode measurement.Similar results showed up in the imaging experiments with a commercial gamma camera.Conclusions: The near-field artifacts would be inhibited or even eliminated by the double-mode measurement otherwise from the conventional single-mode measurement

resulting with an enhanced signal-to-noise ratio（SNR） of reconstructed images.