Visualization of custom drill bit tips in a human vertebra for photoacoustic-guided spinal fusion surgeries

Spinal fusion surgeries require the insertion of screws into the pedicles of vertebrae in order to connect multiple vertebrae with a metal rod and stabilize the spine after an injury or deformity. One outstanding challenge to this surgical procedure is to ensure that a drill tip maintains the correct trajectory when drilling pilot holes for screw insertion. In this work, we demonstrate a photoacoustic imaging system for drill tip tracking that will co-register photoacoustic images with pre-operative CT images. Our approach was tested with custom drill bits containing an optical fiber inside a hollow core with single-hole and multi-hole tips that were inserted in an ex vivo human vertebra. A 32 mm-deep hole was drilled in the pedicle, with the first 13 mm corresponding to the pedicle and the remaining 19 mm extending into the vertebral body. Data was acquired using a 760 nm laser with energies of 1.0 mJ, 2.2 mJ, and 3.4 mJ at the fiber tip. For the single-hole drill tip, the signal was detectable at 0-6 mm depths into the pedicle (SNR: 53.7), which represents 46% of the 13 mm pedicle length. From 6 to 14 mm, the photoacoustic signal was either no longer visualized (SNR: 26.7) or shifted from its expected location in the image due to reflection artifacts. SNR was improved to 31.14 with coherence-based beamforming methods when compared to previously reported conventional delay-and-sum beamformming methods. This enhancement provided clear visualization of low energy photoacoustic signals. Results are generally promising for photoacoustic-guided drilling during pedicle screw insertion.