From reading on these boards for a number of years now, people have reported a number of times that a CT scan missed diverticulosis but a subsequent colonoscopy confirmed it. Yes, some are diagnosed with a CT scan, but do not consider a negative finding conclusive. A negative finding with the colonoscopy would be conclusive.
I have never read of an ultrasound confirming diverticulosis, and personally doubt that it could. I would imagine an MRI would, but have never heard of it used for that.