how to keep the PR of canadian status? 13462005-12-02 21:11:29

I am doing the Phd study in U.S.A. but I am a PR of Canada, the trouble is I have to study in U.S.A around 4 to 5 years, I am afraid that if I donot go back to Canadan during these year, my PR status will expire. I do not know if anyone of you have a good suggestion for me to keep my PR status and at the same time I can finish my study in U.S.A?
Thank you so much!

你在美国什么签证？加拿大太乏味，没有发展前途。