Fluid (e.g. blood, urine, serous fluid in cysts) is an excellent transmitter of sound waves and appears black (= anechoic) on a sonogram. On greyscale images, prominent vasculature or vascular malformations can mimic hydronephrosis or cysts depending on the location. We should always obtain a few Doppler images when performing kidney ultrasound irrespective of the suspected etiology.

Here is an example of mild hydronephrosis. Note that there is no internal flow on color Doppler.

Here is another one which actually looks like prominent vasculature at first glance but doesn’t light up on color. Can report it as mild hydronephrosis or just prominent collecting system.