The mechanism of the twinkling artifact (TA) that occurs during Doppler ultrasound imaging of kidney stones was investigated. The TA expresses itself in Doppler images as time-varying color. To define the TA quantitatively, beam-forming and Doppler processing were performed on raw per channel radio-frequency data collected when imaging human kidney stones in vitro. Suppression of twinkling by an ensemble of computer-generated replicas of a single radio frequency signal demonstrated that the TA arises from variability among the acoustic signals and not from electronic signal capture or processing. This variability was found to be random, and its suppression by elevated static pressure and return when the pressure was released suggest that the presence of bubbles on the stone surface is the mechanism that gives rise to the TA.

Purpose
Focused ultrasound has the potential to expel small stones or residual stone fragments from the kidney, or move obstructing stones to a non-obstructing location. The purpose of this study was to evaluate the efficacy and safety of ultrasonic propulsion in a live porcine model.

Material and Methods
Calcium oxalate monohydrate kidney stones and laboratory model stones (2–8 mm) were ureteroscopically implanted within the renal pelvicalyceal system of 12 kidneys in eight domestic swine. Transcutaneous ultrasonic propulsion was performed using a Philips HDI C5-2 imaging transducer and Verasonics diagnostic ultrasound platform. Successful stone relocation was defined as stone movement from the calyx to the renal pelvis, ureteropelvic junction (UPJ) or proximal ureter. Efficacy and procedure time were determined. Three blinded experts evaluated for histologic injury to the kidney in control, sham, and treatment arms.

Results
All stones were observed to move during treatment, and 65% (17/26) were relocated successfully to the renal pelvis (3), UPJ (2), or ureter (12). Average successful procedure time was 14±8 min and required 23±16 ultrasound bursts of ~1 sec duration. There was no evidence of gross or histologic injury to the renal parenchyma in kidneys exposed to 20 bursts (1 sec duration, 33 sec intervals) at the same output (2400 W/cm²) used to push stones.

Conclusions
Non-invasive transcutaneous ultrasonic propulsion is a safe, effective, and time-efficient means to relocate calyceal stones to the renal pelvis, UPJ, or ureter. This technology holds promise as a useful adjunct to the surgical management of renal calculi.

A theoretical approach is developed to calculate the radiation force of an arbitrary acoustic beam on an elastic sphere in a liquid or gas medium. First, the incident beam is described as a sum of plane waves by employing conventional angular spectrum decomposition. Then, the classical solution for the scattering of a plane wave from an elastic sphere is applied for each plane-wave component of the incident field. The net scattered field is expressed as a superposition of the scattered fields from all angular spectrum components of the incident beam. With this formulation, the incident and scattered waves are superposed in the far field to derive expressions for components of the radiation stress tensor. These expressions are then integrated over a spherical surface to analytically describe the radiation force on an elastic sphere. Limiting cases for particular types of incident beams are presented and are shown to agree with known results. Finally, the analytical expressions are used to calculate radiation forces associated with two specific focusing transducers.

Effective stone comminution during shock wave lithotripsy (SWL) is dependent on precise three-dimensional targeting of the shock wave. Respiratory motion, imprecise targeting or shock wave alignment, and stone movement may compromise treatment efficacy. The purpose of this study was to evaluate the accuracy of shock wave targeting during SWL treatment and the effect of motion due to respiration.

Methods: Ten patients underwent SWL for the treatment of 13 renal stones. Stones were targeted fluoroscopically using a Healthtronics Lithotron (5 cases) or Dornier Compact Delta II (5 cases) shock wave lithotripter. Shocks were delivered at a rate of 1-2Hz with ramping shock wave energy settings of 14-26kV or level 1-5. After the low energy pre-treatment and protective pause, a commercial diagnostic ultrasound imaging system was used to record images of the stone during active SWL treatment. Shock wave accuracy, defined as the proportion of shock waves that resulted in stone motion with shock wave delivery, and respiratory stone motion were determined by two independent observers who reviewed the ultrasound videos.

Results: Mean age was 51±15 years with 60% males and mean stone size was 10.5±3.7 mm (range 5-18 mm). A mean of 2675±303 shocks were delivered. Shock wave-induced stone motion was observed with every stone. Accurate targeting of the stone occurred in 60±15% of shock waves.

Conclusions: Ultrasound imaging during SWL revealed that 40% of shock waves miss the stone and contribute solely to tissue injury, primarily due to movement with respiration. These data support the need for a device to deliver shock waves only when the stone is in target. Ultrasound imaging provides real-time assessment of stone targeting and accuracy of shock wave delivery.

"Twinkling" is a widely reported ultrasound artifact whereby kidney stones and other similar calcified, strongly reflective objects appear as turbulent, flowing blood in color and power Doppler. The twinkling artifact has been shown to improve kidney stone detection over B-mode imaging alone, but its use has several limitations. Principally, twinkling can be confused with blood flow, potentially leading to an incorrect diagnosis. Here a new method is reported for explicitly suppressing the display of color from blood flow to enhance and/or isolate the twinkle signal. The method applies an autoregressive model to standard Doppler pulses in order to differentiate tissue, blood flow, and twinkling. The algorithm was implemented on a software-based, open architecture ultrasound system and tested by a sonographer on phantoms and on stones implanted in a live porcine kidney. Stones of 3-10 mm were detected reproducibly while suppressing blood flow in the image. In conclusion, a new algorithm designed to specifically detect stones has been tested and has potential clinical utility especially as efforts are made to reduce radiation exposure on diagnosis and monitoring.

Vibro-acoustography (VA) is an ultrasound interrogation and imaging technique with a variety of applications. Here it was used to identify optimal parameters for detecting and imaging kidney stones in phantoms. The parameters varied included the difference frequency and the position in time of the analysis window used for image construction. Experiments in a water tank were conducted using a focused PVDF membrane hydrophone (receiver) placed in a central opening of an annular, dual element transducer (source), itself mounted on a translation stage. Our source consisted of 90-ms pulses with a center frequency of 2.0 MHz and difference frequencies between 50 and 350 kHz, applied both on and off stone. Variations in the amplitude of the measured ultrasound backscatter and acoustic emissions as a function of difference frequency, between signals from stone and phantom, guided the choice of imaging parameters. The results were detailed images of renal stones measuring 10 dB above the background tissue. These findings suggest that spectral information from the scattering and reverberation of VA induced ultrasound can be used to guide the interrogation and imaging of kidney stones.

Twinkling artifact on color Doppler ultrasound is the color labeling of hard objects, such as kidney stones, in the image. The origin of the artifact is unknown, but clinical studies have shown that twinkling artifact can improve the sensitivity of detection of stones by ultrasound. Although Doppler detection normally correlates changes in phase with moving blood, here the effect of amplitude on the artifact is investigated. Radio-frequency and in-phase and quadrature (IQ) data were recorded by pulse-echo ensembles using a software-programmable ultrasound system. Various hard targets in water and in tissue were insonified with a linear probe, and rectilinear pixel-based imaging was used to minimize beam-forming complexity. In addition, synthesized radio-frequency signals were sent directly into the ultrasound system to separate acoustic and signal processing effects. Artifact was observed both in onscreen and post-processed images, and as high statistical variance within the ensemble IQ data. Results showed that twinkling artifact could be obtained from most solid objects by changing the Doppler gain, yet signal amplitude did not have to be sufficiently high to saturate the receive circuits. In addition, low signal but high time gain compensation created the largest variance.

Residual fragments remain in over 50% of treatments for lower pole kidney stones. A second-generation device based on a diagnostic ultrasound system and scanhead has been developed with a unique algorithm for stone detection and the capability to focus ultrasound to expel residual fragments. Focused ultrasound was applied to a bead on string in a water tank as well as to human stones (<5 mm) implanted in the lower pole of a live porcine model via retrograde ureteroscopy. Histological samples were collected and scored in a blinded fashion for therapeutic exposures and for super-therapeutic levels. The in-vitro bead was visually observed to move under focused ultrasound. Even with progressive manual displacement of the bead, the system continuously tracked and caused bead movement in real time. In the live porcine model, stones were expelled from the lower pole to the ureteropelvic junction in seconds to minutes using pulses at a duty factor of 0.02 and 8 W total acoustic power. Injury was observed no more frequently than in controls. Occurrence of injury rose slightly above control at a duty factor of 0.02 and 80 W and at a duty factor of 1 and 8 W.

An area of active research involves using the radiation force of ultrasound to expel small kidney stones or fragments from the kidney. The goal of this work is real-time motion tracking for visual feedback to the user and automated adaptive pushing as the stone moves. Algorithms have been designed to track stone movement during patient respiration but the challenge here is to track the stone motion relative to tissue. A new algorithm was written in MATLAB and implemented on an open-architecture, software-based ultrasound system. The algorithm was first trained then implemented in real-time on B-mode IQ data recorded from phantom experiments and animal studies. The tracking algorithm uses an ensemble of image processing techniques (2-D cross-correlation, phase correlation, and feature-edge detection) to overlay color on the stone in the real-time images and to assign a color to indicate the confidence in the identification of the stone. Camera images as well as ultrasound images showed that the system was able to locate a moving stone, re-target, and apply a new focused push pulse at that location.

The success of surgical management of lower pole stones is principally dependent on stone fragmentation and residual stone clearance. Choice of surgical method depends on stone size, yet all methods are subjected to post-surgical complications resulting from residual stone fragments. Here we present a novel method and device to reposition kidney stones using ultrasound radiation force delivered by focused ultrasound and guided by ultrasound imaging. The device couples a commercial imaging array with a focused annular array transducer.

Feasibility of repositioning stones was investigated by implanting artificial and human stones into a kidney-mimicking phantom that simulated a lower pole and collecting system. During experiment, stones were located by ultrasound imaging and repositioned by delivering short bursts of focused ultrasound. Stone motion was concurrently monitored by fluoroscopy, ultrasound imaging, and video photography, from which displacement and velocity were estimated. Stones were seen to move immediately after delivering focused ultrasound and successfully repositioned from the lower pole to the collecting system. Estimated velocities were on the order of 1 cm/s. This in vitro study demonstrates a promising modality to facilitate spontaneous clearance of kidney stones and increased clearance of residual stone fragments after surgical management.

A prototype ultrasound-based probe for use in ureteroscopy was used for in vitro measurements of stone fragments in a porcine kidney. Fifteen human stones consisting of three different compositions were placed deep in the collecting system of a porcine kidney. A 2 MHz, 1.2 mm (3.6F) needle hydrophone was used to send and receive ultrasound pulses for stone sizing. Calculated stone thicknesses were compared with caliper measurements. Correlation between ultrasound-determined thickness and caliper measurements was excellent in all three stone types (r(2) = 0.90, p < 0.0001). All 15 ultrasound measurements were accurate to within 1 mm, and 10 measurements were accurate within 0.5 mm. A 3.6F ultrasound probe can be used to accurately size stone fragments to within 1 mm in a porcine kidney.

PURPOSE: Proof-of-principle in vitro experiments evaluated a prototype ultrasound technology to size kidney stone fragments.
MATERIALS AND METHODS: Nineteen human stones were measured using manual calipers. A 10-MHz, 1/8'' (10F) ultrasound transducer probe pinged each stone on a kidney tissue phantom submerged in water using two methods. In Method 1, the instrument was aligned such that the ultrasound pulse traveled through the stone. In Method 2, the instrument was aligned partially over the stone such that the ultrasound pulse traveled through water.
RESULTS: For Method 1, the correlation between caliper- and ultrasound-determined stone size was r(2) = 0.71 (P < 0.0001). All but two stone measurements were accurate and precise to within 1 mm. For Method 2, the correlation was r(2) = 0.99 (P < 0.0001), and measurements were accurate and precise to within 0.25 mm.
CONCLUSIONS: The prototype technology and either method measured stone size with good accuracy and precision. This technology may be possible to incorporate into ureteroscopy.