Illustrates the limitations of setting a single threshold value to segment a mammogram for measurement of density. (a) Aerial view of mountains in the South Island of New Zealand. The altitude of the snow line varies so that a single value is not adequate to separate the snow-covered (dense) from bare (fatty) regions. (b) A schematic illustration of this problem. The edge and density brightness thresholds are denoted by the horizontal dashed lines Because of the reduction in thickness of the breast near the periphery, the brightness of a region of dense tissue in the mammogram (between the two vertical dashed lines) falls below the density threshold and so is excluded from the measurement. Similarly, fatty tissue in an area of the breast that is thicker than average can be inappropriately registered as dense tissue.