Used to image soft tissue structures such as interior of complex joints?

If you are asking what is used for imaging of soft tissues in joints; an MRI (MRT, KST, ...) At this point, MRI (preferably 3T MRI) is the best. Ultrasounds can also be used (for example for rotator cuff evaluation), or sometimes an arthro-CT (normal CT is not that good). A diagnostic arthroscopy is a possiblity, but I suppose that no longer falls under "imaging". Plain x-rays will not show soft tissues (well, they will, but very vague).