An ultrasound transducer assembly includes an acoustic mirror, an ultrasound transducer positioned to direct a scanned ultrasound beam at the acoustic mirror, wherein the scanned ultrasound beam is reflected by the acoustic mirror to form a reflected ultrasound beam, and an actuating device for moving the acoustic mirror relative to the scanned ultrasound beam so that the reflected ultrasound beam scans a three-dimensional volume. An ultrasound matching fluid may be disposed between the ultrasound transducer and the acoustic mirror. The actuating device may be configured for rotating the acoustic mirror, translating the acoustic mirror or rotating and translating the acoustic mirror. The acoustic mirror may have a single acoustically-reflecting surface of may be a polygon having a plurality of acoustically-reflective surfaces.