So, not so much a "common bile duct" as an 'extension-of-the-hepatic-duct' duct. The third paragaph of the link I posted lists a number of reasons the CBD could be dilated, but most of those are way more dilated than you are (i.e. twice the diameter = four times the cross sectional area).
Good luck with the results of your cat scan.
i don't have a gallbladder i had an mri on my back bc of wreck and that was in the report waiting on results from cat scan from today
http://en.wikipedia.org/wiki/Common_bile_duct
Could mean you have a blockage/partial blockage by a gallstone. What does your doctor say?