Figure 21b.  Rib overlying shadows. (a) Photograph of a right eighth rib shows a blue catheter along the superior margin and a guide wire along the inferomedial margin. (b) Radiograph of the same rib shows the catheter (thick line) and the guide wire (thin line) crossing between the external and internal margins at the lateral portion of the rib. The course of the guide wire at the lateral portion of the rib is very similar to the medial border of the anterior overlying opacity. The course of the catheter corresponds to the posterior overlying opacity. (c) Schematic demonstrates the relationship between the rib margins and the overlying opacities. The gray area corresponds to the overlying shadow, which results from overlap of the bone structure; the blue line indicates the superior margin of the rib; and the red line indicates the inferomedial margin of the rib.