I spoke with the radiologist on call at DRMC and he told me this hospital uses Sitz markers to test colonic transit times. A patient takes one tablet (Sitz marker) and over a 3 day period comes back for KUBs to be taken. Based on the location of the markers, they estimate the transit time.
This article lists different methods of doing that.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3270312/