An ultrasonic transducer probe assembly comprising: a probe case; an array of transducer elements located behind an acoustic window in the probe case and configured to transmit and receive ultrasound energy through the acoustic window; an integrated circuit coupled to the transducer elements in the probe case and configured to process signals transmitted or received by the array; a probe connector configured to connect the transducer probe to an ultrasound system; a cable connected between the probe case and the probe connector; electrical conductors; and a fluid-based closed loop cooling system comprising: a closed fluid loop extending through the cable from the probe case to the probe connector; a heat exchanger located in the probe case which acquires heat from the transducer array and integrated circuit; and a pump, coupled to the fluid loop, which pumps fluid through the loop and through the heat exchanger, wherein the closed fluid loop in the cable comprises an inbound fluid conduit and an outbound fluid conduit characterized in that the inbound fluid conduit and the outbound fluid conduit are helically wound and separated from each other within the cable and having a common central passageway through which extend electrical conductors coupled between the probe case and the probe connector, and wherein the helical windings of the inbound fluid conduit and the outbound fluid conduit are in interleaved alternation within the cable so as to be separated from each other within the cable. a probe case; an array of transducer elements located behind an acoustic window in the probe case and configured to transmit and receive ultrasound energy through the acoustic window; an integrated circuit coupled to the transducer elements in the probe case and configured to process signals transmitted or received by the array; a probe connector configured to connect the transducer probe to an ultrasound system; a cable connected between the probe case and the probe connector; electrical conductors; and a fluid-based closed loop cooling system comprising: a closed fluid loop extending through the cable from the probe case to the probe connector; a heat exchanger located in the probe case which acquires heat from the transducer array and integrated circuit; and a pump, coupled to the fluid loop, which pumps fluid through the loop and through the heat exchanger, wherein the closed fluid loop in the cable comprises an inbound fluid conduit and an outbound fluid conduit characterized in that the inbound fluid conduit and the outbound fluid conduit are helically wound and separated from each other within the cable and having a common central passageway through which extend electrical conductors coupled between the probe case and the probe connector, and wherein the helical windings of the inbound fluid conduit and the outbound fluid conduit are in interleaved alternation within the cable so as to be separated from each other within the cable. a closed fluid loop extending through the cable from the probe case to the probe connector; a heat exchanger located in the probe case which acquires heat from the transducer array and integrated circuit; and a pump, coupled to the fluid loop, which pumps fluid through the loop and through the heat exchanger, wherein the closed fluid loop in the cable comprises an inbound fluid conduit and an outbound fluid conduit characterized in that the inbound fluid conduit and the outbound fluid conduit are helically wound and separated from each other within the cable and having a common central passageway through which extend electrical conductors coupled between the probe case and the probe connector, and wherein the helical windings of the inbound fluid conduit and the outbound fluid conduit are in interleaved alternation within the cable so as to be separated from each other within the cable.