1. An ultrasound imaging system, comprising: a transmit subsystem configured to produce high frequency ultrasound energy at a center frequency of at least 25 MHz, wherein the transmit subsystem includes— a transmit oscillator configured to generate a transmit pulse having a pulse frequency centered at the center frequency and a pulse length of a predetermined number of cycles; and an external transducer electrically connected to the transmit oscillator and configured to transmit ultrasound energy based on the transmit pulse; a receive subsystem configured to receive ultrasound echoes emission of the transmitted ultrasound energy scattered by a microbubble contrast agent, wherein the receive subsystem includes at least a first filter configured to filter the ultrasound echoes received at the transducer and output nonlinear ultrasound echoes in a frequency band of interest centered at a subharmonic frequency of the first frequency; and a processing system configured to produce an image from the filtered non-linear ultrasound echoes, wherein the processing system includes— a reference oscillator coupled to the transmit oscillator and configured to produce reference signals at the subharmonic frequency; and a demodulator coupled to the reference oscillator and configured to selectively demodulate the filtered nonlinear ultrasound echoes.