Hii aspirant..

In post graduation that is Mtech you can opt for all other departments and computer science as well if you have done your UG that is under graduation BTech in computer science engineering. 

One student is eligible to take or opt the department in post graduation (PG) according to the department they have chosen in UG that is under graduation.

As you have done your BTech that is under graduation in Computer Science Engineering you are eligible for MTech that is post graduation in All the departments.

Hope it helps. 

Best of luck.