Accuracy of Ultrasound Measurements for Skull Bone Thickness Using Coded Signals

The knowledge of skull bone thickness would be helpful for a great variety of surgical interventions of the head. Ultrasound (US) can offer this information intraoperatively in real time. A-mode US measurements of skull bone thickness were performed with different pulse characteristics: 1) in water and 2) by directly coupling a 2.25-MHz US transducer integrated in a handpiece with a soft delay line using coded excitation (CE) (SonoPointer). Mechanical measurements by calipers served as controls. The specimen were 16 nonselected human cadaveric skull bones preserved with formaldehyde. The average difference between the bone thickness measured by the SonoPointer and the mechanical control measurements was 0.04 $\pm$ 0.62 mm. The 95% limits of agreement between the two methods were ${-}\hbox{1.18}$ and 1.25 mm. However, even the gold standard of two repeated caliper measurements had limits of agreement of ${-}\hbox{0.4}$ and 0.42 mm. Using a standard US pulse in water, only 62.5% sample points were measurable, whereas the SonoPointer produced the thickness measurement in 97.9% of points. CE proved to be superior to single burst or needle US pulses. A-mode US measurements of skull bone thickness using the SonoPointer are feasible. It may provide valuable information on skull bone thickness, e.g., for osteosynthesis, calvarial split bone harvesting, implantation of hearing devices, osseointegrated titanium fixtures, and skull base surgery.