Don't worry about a letter of intent. Go talk to her/him in person. Tell them the truth, if you are asking your IM attending for a letter, and you are going into radiology, say so. Attendings that are worth asking will still write you a great letter, and in most cases, will tailor it towards what your specialty.
Asking in person will also allow you to size up your letter writer. You should directly ask, do you feel comfortable writing me a STRONG letter of recommendation? If they hesitate, I would rethink asking that person for a letter. Residency directors are expecting glowing letters, at least from those you worked directly with (your departmental letter may or may not be a little less subjective and a little more objective).
I would however, drop a thank you note. They do take time to write you a letter after all.