Missed Chest Wall

The Tomophan® consists of four stair-stepped chest wall missing tissue gauges. The two gauges on the left and right of the center of the Test Object are in the same x-y plane, while the two gauges farthest from the center on opposite sides of the Test Object are in the same x-y plane. Thus, measurements of the missing chest wall can be made in two locations along the wall at two different slice heights. The chest wall gauges have 12 steps in 0.5mm increments, rising 6mm into the phantom. 

The missing chest wall measurement calculates the distance from chest wall to where the signal can be detected. The slice that brings the gauge into the sharpest focus is used to evaluate how many steps are seen. Using the graded step incrementation rule to calculate the amount of chest wall seen, multiply 0.5 by the number of steps seen in the image. Subtracting that number from the expected amount of chest wall, 6mm, will determine the amount of chest wall lost. 

Figure 8.1: An example of how to make the missing chest wall measurement from the CT image of the chest wall gauge. With only 4 steps seen in the image, that means only 2mm of the 6mm gauge is visible (4 steps x 0.5mm). Therefore, 4mm of chest wall is missing.