confused about major

I have done my undergraduate degree from Indian university in electronics and communication engineering and have done job in one of my field of interest medical image processing and I worked in robotics while my degree projects. Now I am going to apply for masters in US universities but I am confused about what major I have to choose. My long term goal is to be a researcher in medical robotics/ robotics/ artificial intelligence also I can work with nano electronics but I have not done any work or not so much study in that before so I am a bit nervous to choose that. As after masters I will go for PhD so please help me to resolve which of the above field will be favorable. As I have doubts like medical image processing is not a vast and not so many funding is there for PhD students.