Transmission-Based Vertebrae Strength Probe Development: Far Field Probe Property Extraction and Integrated Machine Vision Distance Validation Experiments

Abstract

We are developing a transmission-based probe for point-of-care assessment of vertebrae strength needed for fabricating the instrumentation used in supporting the spinal column during spinal fusion surgery. The device is based on a transmission probe whereby thin coaxial probes are inserted into the small canals through the pedicles and into the vertebrae, and a broad band signal is transmitted from one probe to the other across the bone tissue. Simultaneously, a machine vision scheme has been developed to measure the separation distance between the probe tips while they are inserted into the vertebrae. The latter technique includes a small camera mounted to the handle of one probe and associated fiducials printed on the other. Machine vision techniques make it possible to track the location of the fiducial-based probe tip and compare it to the fixed coordinate location of the camera-based probe tip. The combination of the two methods allows for straightforward calculation of tissue characteristics by exploiting the antenna far field approximation. Validation tests of the two concepts are presented as a precursor to clinical prototype development.

Keywords: instrumentation; machine vision; microwave; osteoporosis; surgical navigation; transmission; vertebrae.

Figure 1

(a) Photograph of the probe handle pair with the associated camera and AprilTags on opposing handles. The coaxial tips are inserted through holes in the pedicles and subsequently into the main body of the vertebra. α represents the angle between the two coaxes and d represents the spacing between the probe tips. (b) Close-up of the vertebra with arrows indicating the location of the pedicles.

Figure 2

Photograph of a pair of printed AprilTags mounted to a flat surface on one of the probe handles. Arrows are used to point to the AprilTags and the probe handle.

Figure 3

Photograph of the tag array with respect to the camera on the probe handle. The 3D printed green post is the exact mechanical reference for extrinsic calibration. “1” indicates the 3D printed post, “2” indicates the array of reference tags, and “3” indicates the handle with the camera attached to the lab stand.

Figure 4

Photographs of the probe tips from different views. In this case, the probe tips are machined at a 45° angle from the axis of the coax. Points 1 and 2 are the associated acute and obtuse angles, while Point 3 shows the slightly elongated center conductor after machining.

Figure 5

Photograph of the Polaris near infrared camera system for tracking objects with optically coated spheres.

Figure 6

(a) Photograph of the two probe handles mounted to the experimental fixture. The coaxial probes are each oriented at 25° to vertical, and the web camera for the left handle is aimed at the tags (not shown) printed on a face of the right handle. The aluminum fixture allows for the handles to be moved vertically (left) or horizontally (right) while maintaining the same angle orientation. (b) Photograph of the probes with the coaxes submerged in a glycerin–water mixture.

Figure 7

Plot of the S11 phase as a function of frequency for probe 1 after the port extension procedure. Zero phase for S11 is nominally an open circuit, which the probe tip closely resembles.

Figure 8

Plots of the (a) magnitude and (b) phase as a function of frequency for the reference measurement and five different probe tip spacings. Note that the phases have been unwrapped.

Figure 9

(a) attenuation and (b) phase coefficients as a function of frequency for different probe tip separation distances. The values based on the actual liquid dielectric properties are also plotted for reference.

Figure 10

(a) attenuation and (b) phase coefficients as a function of frequency for different probe tip separation distances and three different dielectric liquids—20%, 50% and 80% glycerin mixtures. The values based on the actual liquid dielectric properties are also plotted for reference. Note that the plots for only three distances (1.6, 2.0 and 2.4 mm) are shown for each concentration to improve clarity.

Figure 11

Scatter plot of the differences between the mechanically measured and machine-vision-measured separation distances with respect to the mechanically measured separation distances.

Figure 12

Scatter plot of the differences between the mechanically measured and surgical-navigation-measured separation distances with respect to the mechanically measured separation distances.