Ultrasonic Detection and Expulsion of Kidney Stones

Abstract

Kidney stone disease afflicts 10% of the U.S. population and severely affects the life quality of patients. There are still many problems with the diagnosis and treatment of renal stones. In diagnosis, X-ray computerized tomography (CT) is the most commonly used technology as it allows urologists/radiologists to locate the stone(s) with high sensitivity. Unfortunately, more and more evidence has been collected that shows that the radiation exposure to patients during CT scans may increase the risk of developing cancer. In kidney stone treatments, extracorporeal shock-wave lithotripsy (ESWL), which breaks the stone with shock waves, is widely used as it is non-invasive and allows the fragments to pass naturally; however, small stone fragments located in the lower pole of the kidney often remain in the kidney, which can cause stones to recur in 50% of ESWL patients within 5 years. Therefore, new technologies that allow for better kidney stone detection and treatment are needed. The twinkling artifact (TA) has been shown to highlight kidney stones during color Doppler ultrasound imaging with high sensitivity for stone detection; however, the instability of the TA has prevented it from being adopted clinically. In this dissertation, the mechanism of the TA was investigated based on raw radio-frequency (RF) data collected from in vitro human kidney stones using the MATLABTM Programmable Verasonics® ultrasound engine. Algorithms, such as beamforming, quadrature demodulation, Doppler processing, etc., were developed to minimize ambiguity in the signal processing. Synthesized RF signals were sent directly into the ultrasound machine in order to separate the acoustic effects from the signal processing effects. It was determined the variability that results in the TA arises from acoustical interactions with the stone. Next, the acoustical effects (i.e., crevice microbubbles, stone ringing, etc.) of the TA were investigated by applying high static pressure (up to 8.5 MPa) on old and fresh human stones where it was determined that microbubbles trapped in crevices on the surface of the stone plays an important role in producing the TA. Modeling simulations were applied to eliminate stone ringing as a possible contributor to the TA. These results have led to the development of new imaging algorithms for better stone detection. A quantitative comparison between the new twinkling image algorithms and the classic color Doppler TA shows that the new imaging techniques are more stable and accurate. Besides improving kidney stone detection, an ultrasound-guided system that is capable of expelling small kidney stones or stone fragments from the kidney has been developed. This device uses acoustic radiation forces and associated acoustic streaming to `push' stones out of the lower pole of the kidney and has been tested successfully in a stone phantom and in many in vivo porcine experiments. Preliminary histological results suggest that the device is safe and that there is no visible thermal or mechanical damage to the kidney. The primary result of this dissertation is insight into the mechanism of the TA, which allowed for the development of new ultrasonic stone-specialized imaging algorithms. In addition, a novel ultrasound technology was developed for expelling small stones or stone fragments from the kidney. Besides furthering science, the results from this dissertation should directly influence patients as it provides improved stone detection and treatment technologies with ultrasound, a non-ionizing alternative to traditional diagnosis regimes, that holds great promise to be adopted clinically.