FIG 2. A–G, Images illustrate the application of the pixel shift program to measure rotation. The first step is to choose a vertebral level from each of the image sets with the thorax rotated clockwise (A) and counterclockwise (B). Note the vertical reference line to the left of each image. The next step is to choose the pivot point and region of interest for the pixel shift analysis (crosshairs and circular cursor in A and B). All voxels outside the cursor are excluded from analysis of motion (C and D). Alignment of the one image with the other before pixel shifting is illustrated by a subtraction image (E), which reveals a mismatch. When the one image is rotated with respect to the other to maximize the correlation, the angle of rotation (illustrated by the reference lines in F) is recorded. Alignment of the images after rotation is illustrated by a subtraction image (G), which shows no mismatch for the vertebral bodies.