Here is an image of the right kidney obtained from an end-stage kidney disease patient. At first glance, it appears as expected: thin parenchyma, prominent peri-renal fat as you would commonly find in these patients. A small simple cyst is seen in the inferior pole.

However, on fanning the transducer anteriorly, you see a huge anechoic cyst, measuring about 7.3 cm in its widest diameter. As the rib shadow was obscuring the view and the patient could not be repositioned, a phased-array transducer was used to get a better look at the cyst (because it has a small footprint), albeit with compromise of the image quality (kidney looks hazy).

Here are the CT scan images demonstrating the cyst-kidney anatomic relation.

Another example: imaged using a handheld ultrasound device.

Below is a short video discussing a similar case, as well as other important pitfalls in imaging renal cysts

Take-home point: When performing renal ultrasound, do not forget to fan the transducer properly before drawing conclusions. Do not be under the impression that fanning is just to detect ‘small’ stones/cysts; even larger lesions can be missed as illustrated above.