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 thermally conductive backing block thermally and acoustically coupled to the array of transducer elements and the integrated circuit which attenuates or scatters ultrasound energy received from the transducer elements; a probe connector configured to connect the transducer probe to an ultrasound system; a cable connected between the probe case and the probe connector and containing a plurality of electrical conductors coupled between the probe case and the probe connector; 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 pump, coupled to the fluid loop, which pumps fluid through the loop; and a heat exchanger which acquires heat generated by the transducer array and integrated circuit for dissipation outside the closed fluid loop, wherein the heat exchanger further comprises fluid channel, coupled to the closed fluid loop, and passing through the thermally conductive backing block; characterized in that the thermally conductive backing block is formed of a material having a thermal conductivity at least as great as that of graphite and comprises a composite structure of acoustic dampening material, wherein the composite structure further comprises a plurality of holes in the backing block which are filled with acoustic dampening material; and the holes further comprise a plurality of cylindrical holes formed in the backing block and extending from a top surface of the backing block to a bottom surface of the backing block, such that the holes are angled at a non-parallel angle with respect to a direction normal to a plane including the array of transducer elements. 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 thermally conductive backing block thermally and acoustically coupled to the array of transducer elements and the integrated circuit which attenuates or scatters ultrasound energy received from the transducer elements; a probe connector configured to connect the transducer probe to an ultrasound system; a cable connected between the probe case and the probe connector and containing a plurality of electrical conductors coupled between the probe case and the probe connector; 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 pump, coupled to the fluid loop, which pumps fluid through the loop; and a heat exchanger which acquires heat generated by the transducer array and integrated circuit for dissipation outside the closed fluid loop, wherein the heat exchanger further comprises fluid channel, coupled to the closed fluid loop, and passing through the thermally conductive backing block; characterized in that the thermally conductive backing block is formed of a material having a thermal conductivity at least as great as that of graphite and comprises a composite structure of acoustic dampening material, wherein the composite structure further comprises a plurality of holes in the backing block which are filled with acoustic dampening material; and the holes further comprise a plurality of cylindrical holes formed in the backing block and extending from a top surface of the backing block to a bottom surface of the backing block, such that the holes are angled at a non-parallel angle with respect to a direction normal to a plane including the array of transducer elements. a closed fluid loop extending through the cable from the probe case to the probe connector; a pump, coupled to the fluid loop, which pumps fluid through the loop; and a heat exchanger which acquires heat generated by the transducer array and integrated circuit for dissipation outside the closed fluid loop, wherein the heat exchanger further comprises fluid channel, coupled to the closed fluid loop, and passing through the thermally conductive backing block; characterized in that the thermally conductive backing block is formed of a material having a thermal conductivity at least as great as that of graphite and comprises a composite structure of acoustic dampening material, wherein the composite structure further comprises a plurality of holes in the backing block which are filled with acoustic dampening material; and the holes further comprise a plurality of cylindrical holes formed in the backing block and extending from a top surface of the backing block to a bottom surface of the backing block, such that the holes are angled at a non-parallel angle with respect to a direction normal to a plane including the array of transducer elements.