A normal urinary bladder filled with urine typically appears as a thin-walled, anechoic structure. In contrast, when a bladder is decompressed by a Foley catheter, it is almost empty, and you can see the Foley balloon (a round, cyst-like structure since it is fluid-filled) inside. Below are examples.

The images below show a distended urinary bladder (anechoic region), even though a Foley catheter is in place. This suggests an obstructed catheter, which may result from a blood clot, debris or a stricture. In the first picture, ‘D’ points to a bladder diverticulum.

Additionally, here’s an example of a straight or intermittent urinary catheter in the bladder. Unlike a Foley, there’s no balloon visible, and a Foley wouldn’t typically be inserted this far into the bladder. The bladder isn’t empty because this image was taken shortly after the catheter was inserted.