What percentage of revenue is the suggested standard for an outpatient
medical imaging center to spend in marketing costs, not including
marketing salaries? Is there an industry standard?

No, there is no industry standard.  Most private radiology facilities
market directly to referring physicians by cost efficient ways, such
as brochures, mailings or word of mouth.  This generally runs less
than 1% of their collections.  Some facilities market directly to
consumers, and again, this is highly variable.  I have seen TV
advertisements for "virtual colonoscopy" and "total body PET scan",
but these are generally not paid by health insurance companies. 
Finally, there is the industry-accepted "entertaining" where the
facility wines and dines with the referring MDs, golf outings, Xmas
gifts, etc.  Hard to estimate the cost of of that, as these are
generally written off as "general business expenses".