Question regarding getting a visa

[shilpakamin]

My grandparents adopted me when i was in high school.so in all of my certificates and on my passport i have their name.But i am always close to my parents than to my grand parents. now my grand dad is no more and my grandma is not in a position to visit US. I am in US with my husband. I'm on a H4-visa. I want my parents to visit US now. Is it possible for them to get a visa since in none of my documents their name is mentioned. But we do have all of the legal adoption papers in which my parents name is mentioned. I am really confused. How difficult is it for them to get a visa ? please do reply soon.
shili