Which is better for detecting ovarian tumors ultrasound or cat scan, my pelvic ultrasound said solid mass, ct said no mass?

They are complementr: They both have value. Since it does not show up on ct, we can be sure that it is not big enough to worry about it. Yet you need to find out the size of the mass seen on the ultrasound and have a follow up in 2-3 months time through your gynecologist. This is one way to be sure and confirm it once more. It may or may not be real. Only follow up will clarify it.

Complimentary: Ct ; ultasound are complimentary imaging modalities. For smaller masses in appropriate location ultrasound may have an advantage especially if done transvaginally. For larger masses ct may have the advantage of not being blocked by bowel gas and being less degraded by large body habitus. More specific info in your case would be helpful. Size of mass? How was the sono done? Ct with oral and IV cont.