Effect of ultrasonic transducer frequency on the registration of ultrasound to CT vertebral images

Researchers of computer-assisted surgical systems are seeking to reduce the invasiveness of spinal procedures through the use of intra-operative ultrasound (US). Given a favorable registration of vertebral US images to pre-operative CT scans, the individual vertebrae in physical space would be mapped to the patient's corresponding image space. In this work a method is proposed for transcutaneous localization of a lumbar vertebra in US images and a subsequent registration of vertebral surfaces from US and CT. In this study, US scans of a life-size plastic spine phantom were obtained using B-mode transducers of frequencies 3.5 and 4.5 MHz. The spine was immersed in a water tank and images from the L2 vertebra were captured in the transverse plane. A point-to-surface registration that is a modification(1) of the Besl/McKay algorithm(2) was applied to extracted US vertebral surface points and a triangulated surface representation of corresponding CT scans. The results of this registration have been qualitatively assessed, and both data sets visually align along the entire L2 vertebra. Presently, more than 250,000 lumbo-sacral spinal surgeries are performed annually(3); consequently, minimizing the intervention in this region could have an extensive positive effect for both the procedure and the patient.