This paper presents a fast and efficient method to determine intervertebral disk orientation in a magnetic resonance (MR) image of the spine. The algorithm originates from active contour theory and enforces a shape constraint to avoid leaks through weak or non-existent boundaries. The method represents a vertebra as a rectangle, modeled as a semi-affine transformation applied to the unit square. A regional flow integrated along the rectangle?s perimeter updates the rectangle?s transformation to achieve the segmentation. Further constraints are added so that adjacent rectangles have similar orientation and scale. The orientation of a disk is then inferred from its adjacent vertebrae. Experiments show that the method is fast and effective in detecting the correct intervertebral disk orientation, which is used for transverse image planning.