Fusion of electromagnetic tracking with speckle-tracked 3D freehand ultrasound using an unscented Kalman filter

Freehand 3D ultrasound (US) using a 2D US probe has the advantage over conventional 3D probes of being able to collect arbitrary 3D volumes at a lower cost. Conventionally, optical and electromagnetic (EM) sensors are used to keep track of the US probe position. Optical tracking provides more accuracy but requires line-of-sight which can be a problem for many applications. Conversely, EM tracking does not have any line-of-sight restrictions, but it has lower accuracy and measurement jitter, and is susceptible to metallic distortions. Ultrasound imaging has the advantage that the speckle inherent in all images contains relative position information due to the decorrelation of speckle over distance. However, tracking the position of US images using speckle information alone suffers from drifts caused by tissue inconsistencies, and overall lack of accuracy. In our work, we examine the possibility for overcoming the limitations of both EM US tracking and freehand, speckle-based US image tracking, through the fusion of these techniques. Even though positions found through speckle-based tracking have very little jitter, the overall error is large, due to drifts in position estimation. By combining the EM and speckle-based tracking information using an Unscented Kalman Filter, we are able to reduce the drift errors as well as to eliminate high-frequency jitter noise from the EM tracker positions. Such fusion produces a smooth and accurate 3D reconstruction superior to those using the EM tracker alone. In addition, we look at the effect of metallic distortions on our fusion and demonstrate improvements over the EM tracker reconstruction.