The CT signs of ureteric obstruction are well known.  On CT KUB it is inportant to determine the site and size of ureteric calculus.  If a stone is not seen in the ureter it may have passed and should be looked for in the bladder.  This is important for the clinician ie (1) the patient can be treated expectantly and (2) the stone is still present thus urine can be collected in an attempt to retrieve the calculus for biochemical analysis.  Note the CT signs of ureteric obstruction can persist for some time after the stone has excited the ureter.