What about Radiology (diagnostic)
If your school has the equipment, you really just need to find someone interested in imaging to get started. It's not easy if you're the "expert" teaching yourself, but you get a lot more freedom in what you study. IMO, a lot of PIs aren't familiar with the possibilities and limitations of imaging technologies.
From an MD/PhD standpoint, I think imaging is a great road to go down. Once you get your knowledge base up to par, you can really crank out papers. Plus, odds are that no one in your department understands the more advanced techniques so it's really easy to defend.
🙂
A big caution: don't do imaging unless you love physics, math, and computer science. Techs can do scanning - the tough part is to understand and manipulate 3D images. My days consist of trying to figure out wtf all these flipping protons mean from a biological standpoint, writing code to do the manipulations & statistics that I need, etc. It's not a course of study where you do the MRI, get the results, and know 100% - you have to confirm your suspicions with traditional lab techniques, etc.
(Sorry for rambling) One more thing - combining your interests usually leads to interesting research. Example from this thread - cardiac MRI is a very useful tool that is giving new insight to the fluid movement in the heart.