The present invention includes medical fluid injector systems that detect the contents and/or volume of such contents within a syringe of the system. For example, an RF signal from a first antenna of a medical fluid injector may be transmitted through a syringe associated with the medical fluid injector. At least some of the transmitted RF signal may be received by a second antenna of the medical fluid injector An amount of the RF signal received by the second antenna may be measured to provide information regarding the contents and/or volume of such contents within the syringe.