Image Segmentation and Analysis of Flexion-Extension Radiographs of Cervical Spines

Abstract

We present a new analysis tool for cervical flexion-extension radiographs based on machine vision and computerized image processing. The method is based on semiautomatic image segmentation leading to detection of common landmarks such as the spinolaminar (SL) line or contour lines of the implanted anterior cervical plates. The technique allows for visualization of the local curvature of these landmarks during flexion-extension experiments. In addition to changes in the curvature of the SL line, it has been found that the cervical plates also deform during flexion-extension examination. While extension radiographs reveal larger curvature changes in the SL line, flexion radiographs on the other hand tend to generate larger curvature changes in the implanted cervical plates. Furthermore, while some lordosis is always present in the cervical plates by design, it actually decreases during extension and increases during flexion. Possible causes of this unexpected finding are also discussed. The described analysis may lead to a more precise interpretation of flexion-extension radiographs, allowing diagnosis of spinal instability and/or pseudoarthrosis in already seemingly fused spines.

Figure 1

Image segmentation via K-means clustering: (a) original image; ((b)–(e)) S _k, k = 1 ⋯ 4, respectively; (f) inverse of S ₃ after erosion.

Figure 2

Segmentation of SL line: (a) selection of contours within distance δ from σ ₁ = ∂R ₁; (b) least-squares fit to SL line within ROI (dashed line) and a scaled plot of local curvature of SL (continuous trace).

Figure 3

Least-squares fitted SL line (dashed line) and resulting curvature (continuous line): (a) extension; (b) flexion.

Figure 4

Fitted SL line (dashed line) and resulting curvature (continuous line) from a second subject: (a) flexion; (b) extension. Adjacent levels are marked with A and B, respectively.

Figure 5

Fitted plate contour (dashed line) and osculating circle (continuous) fitted to the plates mid-point: (a) flextion; (b) extension.

Figure 6

Average curvature measured in 1/L during flexion (solid line) and extension (dashed line).

Figure 7

Fitted plate contour (dashed line) and osculating circle (continuous) fitted to the plates mid-point: (a) neutral; (b) flexion; (c) extension; (d) plate curvatures in 1/L units.