In the US, a "specialist assistant" does not have an M.D. They could work in radiology. "Registered Specialist Assistants are skilled in a group of clinical procedures that make up a specialized clinical service. Registered Specialist Assistants work under the supervision of a New York State licensed, currently registered physician qualified to practice in the medical specialty that utilizes the Registered Specialist Assistants' specialized skills. Currently, there are three categories of specialist assistant licensed by the New York State Education Department: orthopedic specialist assistant, urologic specialist assistant and radiologic specialist assistant."
http://www.op.nysed.gov/prof/med/rsa.htmAs far as I know, only a "physician" would have an M.D.
A "medical assistant specialist" also does not have an M.D.
http://jobdescriptionandresumeexamples.com/medical-assistant...The closest might be a physician specializing in radiology/imaging or physician assistant (PA, not MD) specializing in radiology/imaging.
http://www.augusta.edu/mcg/radiology/faculty_if.php