Double J -ureteric stent (ureteral stent):

 

This longitudinal section ultrasound video of the right kidney shows persistent mild hydronephrosis, despite the presence of the double J stent (DJ stent or ureteral stent) in the right ureter.

 

 The above ultrasound video shows the transverse section of the hydronephrotic right kidney with the upper end of the ureteral stent in the distended renal pelvis.
Reference: http://en.wikipedia.org/wiki/Ureteric_stent 
See the following link for article and images of ureteral stent:
See: http://cochinblogs.blogspot.com/2010/04/ultrasound-imaging-of-ureteral-stent.html