Multi-modal registration of speckle-tracked freehand 3D ultrasound to CT in the lumbar spine

A method for registration of speckle-tracked freehand 3D ultrasound (US) to preoperative CT volumes of the spine is proposed. We register the US volume to the CT volume by creating individual US " sub-volumes" , each consisting of a small section of the entire US volume. The registration proceeds incrementally from the beginning of the US volume to the end, registering every sub-volume, where each sub-volume contains overlapping images with the previous sub-volume. Each registration is performed by generating simulated US images from the CT volume. As a by-product of our registration, the significant drift error common in speckle-tracked US volumes is corrected for. Results are validated through a phantom study of plastic spine phantoms created from clinical patient CT data as well as an animal study using a lamb cadaver. Results demonstrate that we were able to successfully register a speckle-tracked US volume to CT volume in four out of five phantoms with a success rate of greater than 98%. The final error of the registered US volumes decreases by over 50 percent from the speckle tracking error to consistently below 3. mm. Studies on the lamb cadaver showed a mean registration error consistently below 2. mm.