An external ultrasound probe assembly capable of scanning a three-dimensional volume is provided. The ultrasound probe assembly contains a plurality of ultrasonic transducers disposed along a longitudinal axis of the probe assembly. The plurality of ultrasonic transducers is disposed on a mechanism operable to reciprocally pivot the plurality of ultrasonic transducers enabling the plurality of ultrasonic transducers to scan the entire three-dimensional volume. A helically disposed electrical interconnection member may be disposed about a pivot axis of the plurality of ultrasonic transducers and may electrically interconnect the plurality of ultrasonic transducers to an ultrasound imaging system. The ultrasound probe assembly may be fluid filled and contain bubble position control and fluid expansion compensation features.