ProjectAlign: a real-time ultrasound guidance system for spinal midline detection during epidural needle placement

Ultrasound can provide useful guidance for needle insertion in epidural anesthesia, but image interpretation can be challenging. The aim of this work is to determine the feasibility of a new ultrasound-based system (ProjectAlign) capable of identifying the spinal midline directly at the puncture site. ProjectAlign's main benefit is that it requires no operator interpretation of ultrasound images. Instead, the operator is guided by automatic real-time estimates of spinal midline position projected onto the skin. A simple cross-correlation routine generates increasingly accurate estimates of midline location as the transducer is centred over the spine. A clinical feasibility study was performed to assess the performance of ProjectAlign in identifying the midline in the L2 to L4 lumbar region of 12 subjects. We hypothesized that (i) ProjectAlign can identify the spinal midline within a 5 mm lateral distance of a sonographer's manual marking, and (ii) ProjectAlign is more laterally accurate than palpation in identifying the spinal midline. Both hypotheses were validated by the data. Midline measurement with ProjectAlign generated an RMS error of 2.0 mm, with a maximum error of 5.0 mm. The results of this study support further investigation into the use of ProjectAlign, in particular for obese patients where palpation is most difficult.