Distinguishing stent from stone: use of bone windows

To present a simple method to assist in distinguishing stents or nephrostomy tubes from urinary tract stones. Non-contrast computed tomography (CT) is now routinely used for the diagnosis and follow-up of urinary tract stones. When stents or nephrostomy tubes are in place, it may be difficult to differentiate the stent from the stone.

Three representative patients with ureteral stents or nephrostomy drainage catheters (stents/tubes) who were suspected of having residual urinary tract stones were studied by visualizing the suspicious density on CT. Abdominal windows were used for initial viewing, then bone windows were used, and the two techniques were compared. The Hounsfield units (HU) of the various stents/tubes were measured in vitro to establish reference points. The pixel densities of stones have a known range and, in our experience, may be as high as 1600 HU.

Using the abdominal window, no difference was observed in the radiographic appearance of the stents/tubes compared with urinary calculi. However, when viewed in the bone window, the stone appeared less dense than the stent/tube. The in vitro pixel densities of the stents/tubes measured 1600 to 2600 HU compared with the calculi, which do not exceed 1600 HU.

Using the abdominal window, stents and stones may have the same CT appearance. However, the bone window allows a visual distinction between a stent/tube and a stone. This distinction is accounted for by differences in pixel density. This observation allows one to distinguish a stent/tube from a stone on CT in patients in whom a nephrostomy tube or ureteral stent is present.