93975 involves examining both the arterial inflow and venous outflow of an organ. It is not necessary that all organs are examined. 93975 can be coded if only one organ such as the liver is studied, if it is studied in full. For paired organs, both should be studied for complete. 93976 would be coded if only arterial inflow or only venous outflow is studied, or only 1 of paired organs (1 kidney) CPT says that it should include spectral and/or color doppler. ACR says that correct use of the codes requires BOTH spectral and color doppler.
(see the ACR US User's Guide.)