CT and MRI features of urolithiasis along with its associated complications
Urinary tract calculi (i.e., “liths” or stones) may vary in size from less than 1 mm to many centimeters (“staghorn” calculi) and are typically seen as round, oval, or polygonal foci of very high attenuation on CT within the renal parenchyma, collecting system, ureter, or bladder. Occasionally, matrix calculi and concretions of protease inhibitors (used to treat patients with human immunodeficiency virus [HIV] infection) may be difficult to visualize on CT. MRI is less sensitive than CT for the detection of urinary tract calculi, especially when small in size. However, larger calculi may sometimes be visualized as very low signal intensity foci, often surrounded by very high T2-weighted signal intensity urine within the urothelial system.
When there is obstruction of the ureter by a calculus, which most often occurs at the ureteropelvic and ureterovesical junctions, CT and MRI findings may include dilation of the upstream ureter and collecting system; a thickened edematous segment of ureter completely surrounding the calculus (“soft tissue rim” sign); delayed enhancement of the ipsilateral kidney (“delayed nephrogram”); perinephric or periureteral stranding; renal enlargement; and perinephric fluid or urinoma formation (owing to rupture of a calyceal fornix with urine leak) that shows enhancement on excretory phase images.
Scanning of patients in the prone position is useful to differentiate between a recently passed calculus within the urinary bladder and a calculus that is impacted within the ureterovesical junction. The former will drop to the dependent bladder surface, whereas the latter will remain trapped within the ureterovesical junction.