Billing 78804 and Contrast

I have a patient that came in on 01/21 and had contrast injected for a two day scan. 4 hours after the injection (still on 01/21) was the first scan and the second scan was on 01/22. Would I bill 78804 and contrast on 01/21, contrast on 01/21 and scan on 01/22 or both on 01/22? Thank you