Using ultrasound to guide the insertion of pedicle screws during scoliosis surgery

Scoliosis surgery involves the insertion of screws and/or hooks into selected vertebrae to secure a pre-bent rod placed along the concave side of the spine. Usually conventional x-rays will be taken before the surgery to plan the alignment and positioning of the pedicle screws. However, reports state that perforation rate range from 6% to 54%. A misalignment of a pedicle screw can potentially cause permanent neurological spinal cord injury or even a life-threatening vascular injury. Because of the importance of positioning and aligning of pedicle screws, we are working on an ultrasound method to guide the insertion of pedicle screws in real time. A pulse-echo immersion experiment was set up to study how well the edges of cortical bone could be detected using a bovine spinous process in-vitro. Two ultrasound frequencies (3.5 MHz and 5.0 MHz) were considered in this study. This preliminary study shows that ultrasound is able to penetrate cortical bone and reflect back from the outer boundary. All interfaces are clearly identified for both frequencies. Strong reflection signals are obtained when the beam is normal to the interface. Derived thickness values from the reflections are comparable with those from micro-CT image. The 5.0 MHz ultrasound frequency provided better resolution than the 3.0 MHz frequency.