Parapelvic cysts can mimic hydronephrosis because of their anechoic nature (like urine, clear fluid in the cysts is also black on ultrasound) and close proximity to the collecting system. Hydronephrosis appears as branching, ‘interconnected’ anechoic area, while parapelvic cysts are seen as ‘non-communicating’ renal sinus cystic masses. In addition, a parapelvic cyst is more spherical as opposed to irregular/cauliflower contour of hydronephrosis and is not connected to the ureter distally. When the sonogram is not clear enough, a CT scan with contrast should be considered to differentiate between these two conditions. On CT, the contrast will not enter the cyst while the collecting system lights up. See one of my previously published case reports for a better idea in addition to below infographic and examples.