Previously, we learned that parapelvic cysts can mimic hydronephrosis. However, any large cyst can be confused with hydro if you do not scan the kidney in both planes. For example, this transverse scan of the left kidney was performed in a hospitalized patient with acute kidney injury. At first glance, there appears to be an anechoic area in the renal pelvis suggestive of hydronephrosis.

On longitudinal scan, you can see that the pelvis area is not dilated and there is a large cyst in the upper pole, which was mimicking hydro on the transverse scan. In addition, there were some other small cysts in the same kidney suggestive of acquired cystic kidney disease.

The same cyst may also be confused with hemoperitoneum in a trauma patient especially if its origin from the kidney is obscured by the rib shadow and you do not scan in multiple planes.