Many people ask about the safety of radiation in imaging, such as bone scans, CT scans, and PET scans.  PET scans give higher amount of radiation than CT or T99 bone scans.  Read more here - Naturally-occurring "background" radiation exposure.  (MRI and ultrasound do not use radiation, so may be alternatives in some cases.)  

Radiation dosage is usually measured using "millisieverts", or mSv.  
Some estimates say the average person in the U.S. receives about 3 mSv per year from naturally occurring radioactive materials in the environment.  Read more here - Naturally-occurring "background" radiation exposure.  

Men also ask about the safety of radiation treatment for prostate cancer.  

Radiation Treatment dosage is measured using Gray's, or Gy, which is the measurement of the 'absorbed dose' of radiation.  Since different types of body tissue absorb radiation doses at a higher or lower rate, Gy is used in prostate cancer radiation dosage measurements.