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Transducer motion estimation can be used to synthesize 3D ultrasound images from multiple B-mode (2D) images. This enables intuitive ultrasound volume imaging using low-channel count systems with reduced transducer complexity, power consumption and beamforming computational load. However, it is difficult to accurately and precisely track transducer motion in three dimensions. In this paper, we investigate combining motion estimates from optical tracking and ultrasound decorrelation measurements, to produce an improved composite estimate. Individual modality and combined tracking performance is evaluated using mechanical (i.e. motion stage) and freehand scanning. A lumbar-region spine phantom was used as a target, in a speckle background, with overlaid porcine tissue to simulate skin texture and provide realistic phase aberration. Results indicate that for this system, decorrelation tracking is accurate for elevational displacements up to 1mm, with errors of <; 0.07 mm. Optical tracking maintains errors <; 0.07 mm for displacements up to 3mm. Unfortunately, the presence of ultrasound gel introduces a significant measurement bias, -0.129 mm for a 1mm translation. A combined estimate mitigates this effect, reducing the 1 mm translation estimation error to <; 0.07 mm. Using combined motion estimates, we show it is possible make useful 3D images using a B-mode transducer system.