Few 4mm in right , and few 3mm in left pole of kidneys , shows only on ultrasound and not ct scan? why?

Cysts: It sounds like you are talking about cysts by the measurements. If you had an ultrasound where cysts were seen, drs usually order a ct scan to confirm. Ultrasound is very sonographer dependent. This is why a more objective test like the ct scan is ordered. Most cysts are considered "incidentalomas" meaning they were found, but are not clinically significant. Hope this helps!

Uric acid stones: Uric acid stones dont show up on xray, ct scan but on ultrasound only.