I was at work last night and spoke with one of the radiographers about saftey of CT scans and conception.
He said, that there is no evidence that xrays or CT's cause problems regarding conception, though not advised for pregnant women.
Though it would be worth mentioning to the radiographer performing scan about your concerns and plans. They might be able to reduce the intensity of the scan to the minimum power and scan as little of your pelvic region as possible, and you could request a lead gown to cover your reproductive organs, (if the scan does not have to go that low,), which if scanning your kidneys and knowing there anatomical position I would assume you would be able to do.
Regarding the CT contrast, it is expelled from your body very quickly, most of it when you do your first wee, so drink plenty of water afterwards and flush your kidneys.

I'm a nurse in an Emergency Department, I am around xrays alot, we do what we can to protect ourselves, by wearing lead gowns, standing behind lead shields or stepping out the room. I always make sure I'm further away that the person taking the xray (they do it all day avery day) I figure I'm not exposed as much as they are. Though once I fall pregnant I will hang up my lead gown and step out of the traumas. Many of my collegues have babies, about 80% of them giving birth to girls. 1 nurse even had triplets (all girls).
It is wise to be careful, and know your facts. I have never heard you need to avoid xrays 4 months prior to conception though.

I hope this helps you.