Figure 1.

External force on brachial artery ultrasound image, longitudinal view. (a) Brachial artery, unprocessed longitudinal view. (b) Sobel edge map of brachial artery, longitudinal view. The snake is attracted to the edges, which are smoothed gradients in the image. The longest edges are horizontal and follow the artery walls, especially the bottom distal wall. (c) External force vector field on brachial artery, longitudinal view. Arrows show direction and relative magnitude of external image gradient force as grayscale brightness change per unit distance. Brighter, longer arrows indicate greater force. The forces are mostly vertical, orthogonal to and pointing towards the edges in panel B. Vertical gradients and horizontal edges are more prominent on ultrasound because of its higher axial resolution, which may affect segmentation and measurement interpretation.