From the CGS Part B Medicare Tip of the Week...
Be descriptive in your office/progress notes; if you order an x-ray or other test, write down in your note that you ordered this service. Verbal orders are not sufficient. Even if the x-ray or lab is being done in your office you must still have clear written documentation that you ordered this service. - 09.19.11
Source: http://www.cgsmedicare.com/partb/pubs/news/index.html.