Placecard Question

The placecards I am using has a Line for Table Number and Name. The line with the name has a M and then a line. (M__________)

What do I do with people who are doctors? ow do I address them?

Let me see if I find an article for addressing invitations. I have an etiquette book at home about this but I'm at work now. Hold on..let me see if I find the article that I have on that here online...