Retention and growth of urinary stones—Insights from imaging

Recent work in nephrolithiasis has benefited from 2 special kinds of imaging: endoscopic study of patient kidneys with high-quality instruments, and examina- tion of stones with microscopic computed tomogra- phy (micro CT). The combination of these has provided new evidence that there is more than 1 mechanism by which stones are retained in the kidney until they achieve sizes to be clinically relevant. This review de- scribes what is known about the formation of stones on Randall's plaque, the formation of stones on ductal plugs and the ways in which stones may grow in free solution within the calyceal or pelvic spaces. Stud- ies of urolithiasis need to recognize that any group of "stone formers" likely includes patients who differ fundamentally regarding which mechanism of stone formation is the primary route for their stones. Sepa- ration of patients on the basis of which mechanism (or combination of mechanisms) underlies their disease will be important for advancing research in the area of urolithiasis.