Basic Study for Ultrasound-Based Navigation for Pedicle Screw Insertion Using Transmission and Backscattered Methods

<div><p>The purpose of this study was to understand the acoustic properties of human vertebral cancellous bone and to study the feasibility of ultrasound-based navigation for posterior pedicle screw fixation in spinal fusion surgery. Fourteen human vertebral specimens were disarticulated from seven un-embalmed cadavers (four males, three females, 73.14 ± 9.87 years, two specimens from each cadaver). Seven specimens were used to measure the transmission, including tests of attenuation and phase velocity, while the other seven specimens were used for backscattered measurements to inspect the depth of penetration and A-Mode signals. Five pairs of unfocused broadband ultrasonic transducers were used for the detection, with center frequencies of 0.5 MHz, 1 MHz, 1.5 MHz, 2.25 MHz, and 3.5 MHz. As a result, good and stable results were documented. With increased frequency, the attenuation increased (<i>P</i><0.05), stability of the speed of sound improved (<i>P</i><0.05), and penetration distance decreased (<i>P</i>>0.05). At about 0.6 cm away from the cortical bone, warning signals were easily observed from the backscattered measurements. In conclusion, the ultrasonic system proved to be an effective, moveable, and real-time imaging navigation system. However, how ultrasonic navigation will benefit pedicle screw insertion in spinal surgery needs to be determined. Therefore, ultrasound-guided pedicle screw implantation is theoretically effective and promising.</p></div

Experimental Systems.

<p>An acoustic water bath was used to measure the speed of sound, broadband ultrasound attenuation, and backscatter coefficients. The ultrasonic system for transmission and backscattering measurements is shown in (A) and (B) respectively.</p

Relationship Between Attenuation and Frequency.

<p>(A) Reference signal(dashed line) and signal transmitted through bone(solid line) for the 1.5-MHz center-frequency transducer pair on specimen 1. (B) Relationship between attenuation and frequency for the 1.5-MHz center-frequency transducer pair on specimen 1. (C) Relationship between attenuation and frequency for 5 transducer pairs on 7 specimens.</p

Relationship Between Velocity and Frequency.

<p>The speed of sound decreased as the ultrasonic frequency increased, which indicated a negative dispersion.</p

A-Mode Signal in Backscattering Measurements.

<p>Normalized amplitude at different specimen thicknesses for the 5 transducers was exhibited. The amplitude increased significantly when the thickness of the specimens reduced to about 0.6 cm.</p

Ultrasound Attenuation of the Seven Specimens.

<p>For the same specimen, the ultrasound attenuation increased while the amplitude decreased with an increase in the frequency.</p