From the two definitions given by anongoodnurse, it sounds like only "self-imposed" is actually invited, as the action was voluntary, of one's own free will or design, or done by choice, whereas "self-inflicted" just means that the injury was done by oneself, and doesn't seem to carry the idea that it was necessarily done by choice.
The example given under self-inflicted was died of a self-inflicted gunshot wound. If you were cleaning your weapon but didn't make sure it was clear, looked down the barrel, accidentally pulled the trigger and thereby got shot in the face, you would have died of a self-inflicted gunshot wound. Getting shot would not have been voluntary, of one's own free will or design, nor done by choice as you didn't invite death, you didn't want to get shot in the face, but through your own negligence you got killed.
So it makes sense to me (based off of the two definitions) to say that self-imposed is something that someone does to themselves that they want to happen, while self-inflicted is something that someone does to themselves, whether they wanted to do it or not.