The soft tissue rim sign is used to distinguish a ureteric calculus from a phlebolith. The former appears as a calcific density with a surrounding soft tissue rim, representing the edematous ureteric wall. On the other hand, phleboliths usually have invisible walls (up to 8% may have a soft tissue rim sign 2) but may have a comet tail sign.
This sign may be absent in cases of a larger (>4 mm) and/or ureterovesical calculus 3.