Figure 1. Typical PCXI image showing the result from manual particle tracking at one imaging period. The mouse is supine with the lungs to the left and the mouth to the right; MCT is primarily in a left-to-right direction. The first frame (marked in green) from a sequence of 10 is shown with the location of tracked particles in the following frames marked with a red dot. Note that particle motion is not homogeneous; some particles move long distances, some short distances, and some (unmarked , primarily along the dorsal tracheal surface) remain stationary over those 10 frames (Donnelley et al., 2014) .

By utilizing X-ray refraction in addition to conventional absorption, PCXI can create better soft tissue contrast even when the absorption differences are minimal. Increased sample-to-detector distance improves tissue boundaries, provided the X-ray beam is spatially coherent, because of variations in the X-ray refractive indices of the various tissues. This investigation showed that deposited lead dust, typically 5 mm to 12 mm in diameter with a few larger particles present (see Figure 1 in Donnelley, Siu, et al.), was a good marker for tracheal MCT behavior analysis through PCXI. This method also makes it possible to evaluate therapies without using animals in long-term, repeated-measure study designs.