Although sonography is less sensitive than CT for detecting kidney stones, it is the preferred initial imaging modality as there is no risk of radiation, is reproducible, inexpensive, and the outcome is not significantly different for patients with suspected urolithiasis undergoing initial ultrasound exam compared to those undergoing CT scan.

On gray-scale images, stones appear as hyperechoic or bright structures with a posterior “acoustic shadow”. Acoustic shadowing is the black area or signal void seen beyond structures that do not transmit ultrasound waves.

In the Doppler mode, stones exhibit “twinkling sign” or artifact, which refers to a rapidly alternating focus of color Doppler signals mimicking turbulent flow and is more pronounced with rougher stones. It is of note that this sign is more sensitive than shadowing for detection of small stones, which I found to be very helpful in my practice.

Below is a nice example of bladder stone shared by Dr. Martínez demonstrating shadowing + twinkling. In addition, the image reveals the presence of a Foley catheter balloon with linear reverberations within. There is a potential presence of air at the bladder wall-Foley junction, indicated in the labeled image (air appears white on ultrasound). Although not that prominent in this case and possibly related to Foley insertion, it’s important to consider the possibility of emphysematous cystitis if you see multiple irregular (typically with some mobility) white areas adjacent to the bladder wall.