CT scan vs Ultrasound

Last year I was diagonosed with a Stage 1 serous borderline tumor.  During the summer I was having abdominal pain and because no cause could be found I requested a CT scan.  The CT results indicated 2 small (1")suspcious nodules in the pelvic region.  My specialist said he wasn't concerned about them because a CT scan of the pelvic region is not as accurate as an ultrasound of the same area.  I had an ultrasound a few months later and things were fine.  Is this true that the ultrasound is more accurate or should I be concerned and following this more closely.

Debbie