Ultrasound-CT registration of vertebrae without reconstruction.

PURPOSE: While robust and accurate, our previously developed volume-to-volume ultrasound-CT registration of vertebrae required that the 2D ultrasound slices be reconstructed into a 3D volume, a time-consuming step that increased the total registration time per vertebra. We have modified our registration technique to a slices-to-volume strategy to eliminate the ultrasound reconstruction step in order to make the total registration time more practical intraoperatively.
METHODS: The slices-to-volume registration is achieved by performing backward scan line tracing on individual ultrasound slices as they are acquired, and then registering them as a group to the posterior vertebral surface extracted from the pre-operative CT image. The technique is validated using a lumbosacral Sawbones phantom and the lumbosacral section of three porcine cadavers.
RESULTS: The slices-to-volume registration reduced the total registration time per vertebra from 8 to 4 min. The registration accuracy and robustness of the slices-to-volume registration were found to be equal or superior to those of our previous volume-to-volume registration. In addition, a trade-off was found between registration accuracy and registration speed by changing the number of ultrasound slices used in the registration.
CONCLUSIONS: The slices-to-volume ultrasound-CT registration significantly reduces the total registration time per vertebra, making this automated technique more practical intraoperatively.