Kidney stones affect approximately 12% of the global population as of 2018. Currently, the gold standard method of kidney stone location is computed tomography (CT) as the stones are easily visible because they have a higher Hounsfield unit due to the stone’s dense structure.
Currently, there are no other comparable imaging methods for noninvasively locating kidney stones. CT is limited in its use during kidney stone treatment as it is used sparingly in the initial location of stones and in post treatment to confirm if stones are still present. If stones are found early enough and have the correct composition, they can be treated with simple lifestyle changes like increased water intake and diet restrictions. Most often when symptoms of kidney stones arise, the stones are large enough that they are treated with surgical removal or lithotripsy.
Traditional B-mode ultrasound has historically been insufficient in locating kidney stones as it can be very difficult to distinguish stones from the surrounding tissue. Detection rates for ultrasound have been reported to be much lower than CT. In 1996, an artifact was discovered when using Doppler ultrasound that appears as a sparkling mosaic over the stone that was termed the twinkling artifact (TA). In recent years kidney stones have been tested as a clinical source of TAs. The goal of this present work was to explore how stone size and composition affect TAs and the ability to locate stones with TAs in an excised kidney.
Isolated Stone Study
Initial experiments were performed using a wide range of stone types and sizes from 1.31-55.76 mm2 in a cylindrical water tank with degassed water. Degassed water was used to reduce any introduction of microbubbles on the surface of the stones other than possibly due to ultrasound. Stones were suspended on a gauze bridge to limit TA appearance to the stones. All stones tested showed adequate TA signals regardless of stone composition or size.
Excised Kidney Study
To further test TA appearance on stones, they were individually place in an excised pig kidney and scanned in a large water tank with the same ultrasound probe as shown in Fig. 1. The power of the ultrasound pulses was tested to evaluate the ability to use the maximum power for initial location of the stones before lowering the power to a level that would precisely locate the stone and provide general information on its size. This showed no issues with the initial location of the stones with the TA.
Figure 1: Experimental setup for kidney stone scanning in an excised kidney.
Randomized Placement Study
A total of 47 stones were randomly placed within an excised kidney in a large water bath in groups of 5-8 stones per scan. This setup was used to evaluate the robustness of the method in a more clinical situation. The length of the kidney was scanned to locate as many stones as possible with some stones being placed next to each other purposefully. The process of locating and precisely pinpointing the stone is shown in Fig. 2. All 47 stones were located, including the stones placed in the same plane, with only two false positives.
Figure 2: Real-time Doppler scans of the TA over a calcium oxalate monohydrate that is 14.73 mm2 in cross-sectional area. The max voltage of 50 V was used for initial location and the minimum of 23.4 V was used for precision location.