Links you might find of interest:

Arizona Radiation Regulatory Agency (ARRA) http://www.arra.state.az.us/
American College of Radiology http://www.acr.org/default.aspx
American Association of Physicists in Medicine http://www.aapm.org/
Radiological Society of North America http://www.rsna.org/
United States Nuclear Regulatory Commission http://www.nrc.gov/
Health Physics Society http://www.hps.org/
The Society of Nuclear Medicine http://www.snm.org/
Section for Magnetic Resonance Technologists http://www.ismrm.org/smrt/
American Society of Radiologic Technologists https://www.asrt.org/
Society for Cardiovascular Magnetic Resonance http://www.scmr.org/
American Nuclear Society http://www.ans.org/