In vivo pons motion within the skull

Boundaries of the pons and clivus were determined using a gradient-based edge detector (thick lines). The edge points of the pons were fitted into an ellipse using a direct least-squares method (thin lines; also showing the long and short axes).

Reference, displaced and the transformed reference images with coordinate systems used in the data analysis. (a) Reference image with coordinate systems O₀ and O₁. (b) Displaced image with O₂ containing the clivus. (c) Reference image in (a) was rigidly transformed to maximize the mutual information between the two rectangular regions in (a) and (b).

Boundaries of the pons (the fitted ellipse) from the reference image were transformed into the displaced image (a), and zoomed in (b). The displacement of the pons was obtained by comparing the pons locations, and then decomposed into components as shown in the figure.

Displacement of the pons as a function of the head flexion angle parallel and normal to the clivus surface during flexion in supine postures (17 experiments from a total of 15 subjects). Positive values were defined as the pons moving caudally toward the foramen magnum and away from the clivus surface. The displacement increased significantly with the increase of the head flexion angle. The displacement measurements in both directions were significantly larger than zero.

Displacement of the pons as a function of the head flexion angle parallel and normal to the clivus surface during flexion in prone postures (10 experiments from a total of 10 subjects). Negative values in the parallel direction indicate that the pons moving away from the foramen magnum during the head motion. The displacement increased significantly with the increase of the head flexion angle. The displacement in the parallel direction was significantly larger than zero, but not in the normal direction.

Displacement of the pons along gravity for each subject in neutral (14 experiments from a total of 14 subjects) and flexion (11 experiments from a total of 11 subjects) positions. Subjects #12–#14 only had the displacements in the neutral posture but not in the flexion posture. Average displacement along gravity in flexion postures (1.27mm, 95% CI: 0.86–1.68mm) is 40% larger than the average displacement in neutral postures (0.91mm, 95% CI: 0.69–1.14mm).