Three-dimensional spinal bone imaging with medical ultrasound for epidural anesthesia guidance

Epidural anesthesia procedures are performed in approximately 75 percent of all childbirths in the United States; however, failure rates for these procedures are as high as 40 percent across all cases and 70 percent in the obese population. Due to the limitations associated with X-ray-based fluoroscopy, including exposure to ionizing radiation and lack of portability, the standard of care is to perform epidurals using palpation - i.e. without medical imaging. High-quality 3D imaging of spinal bone surfaces, obtained using a portable ultrasound device, could potentially overcome these challenges and offer physicians a safe imaging tool to increase success rates. A 4-channel handheld ultrasound system with custom 0.95 cm diameter piston transducers focused at 5 cm were designed and fabricated. The system was then used to image an excised lumbar spine in a water bath over a mechanically scanned 90 mm × 50 mm 2D plane. Positional information regarding the surface of the lumbar spine bone at each point across the 2D plane was estimated in MATLAB by detecting the position of maximum envelope-detected signal above the noise level. 3D topographical images were rendered and qualitatively compared to photographs of the actual spine. Additionally, 3D printed models of anatomically accurate lumbar vertebrae were fabricated and embedded in a graphite gelatin phantom. Imaging was conducted with a mechanically scanned 5 MHz piston transducer, 3D images were formed, and the correlation between the known surfaces and ultrasound estimated surfaces was computed. Images of the excised spine using the handheld ultrasound system yielded excellent correlation to optical images. Automated measurements (performed in MATLAB) of spinous process and epidural gap dimensions across the axis of the spine were within 0.5 mm of the actual dimensions. 3D images of the printed spine model produced a 0.88 correlation coefficient between estimated and true spine surface locations. Results in this paper demonstrate the feasibility of a handheld ultrasound device for low-cost, safe, and portable imaging of spinal bone anatomy in 3D, which possesses the potential for improving the success rates of epidural anesthesia procedures.