Irina V. Pishchalnikova

Cavitation selectively reduces the negative-pressure phase of lithotripter shock pulses.

Measurements using a fiber-optic probe hydrophone, high-speed camera, and B-mode ultrasound showed attenuation of the trailing negative-pressure phase of a lithotripter shock pulse under conditions that favor generation of cavitation bubbles, such as in water with a high content of dissolved gas or at high pulse repetition rate where more cavitation nuclei persisted between pulses. This cavitation-mediated attenuation of the acoustic pulse was also observed to increase with increasing amplitude of source discharge potential, such that the negative-pressure phase of the pulse can remain fixed in amplitude even with increasing source discharge potential.

Ultrasound-guided localized detection of cavitation during, lithotripsy in pig kidney in vivo

It is supposed that inertial cavitation plays a significant role in tissue damage during extracorporeal shock wave lithotripsy (ESWL). In this work we attempted to detect cavitation in tissue. In vivo experiments with pigs were conducted in a Dornier HM3 electrohydraulic lithotripter. Kidney alignment was made using fluoroscopy and B-mode ultrasound. Cavitation was detected by a dual passive cavitation detection (DPCD) system consisting of two confocal spherical bowl PZT transducers (1.15 MHz, focal length 10 cm, radius 10 cm). An ultrasound scanhead was placed between the transducers, an hyperechoic spots in the image indicated pockets of bubbles during ESWL. A coincidence-detection algorithm and the confocal transducers made it possible to localize cavitation to within a 4 mm diameter region. The signals from both the collecting system and kidney tissue were recorded. The targeting of the DPCD focus was confirmed by using the DPCD transducers as high intensity focused ultrasound (HIFU) sources at HIFU durations below the lesion formation threshold. In this HIFU regime, a bright spot appears in the B-mode image indicating the position of the DPCD focus. In this way we could confirm that refraction and scattering in tissue did not cause a misalignment. The tissue region interrogated was also marked with a lesion produced by HIFU. Clear cavitation signals were detected from the collecting system and from pools of blood that formed near the kidney capsule and weak signals were recorded from tissue during the ESWL treatment.

Acoustic shielding by cavitation bubbles in Shock Wave Lithotripsy (SWL)

Lithotripter pulses (∼7–10 μs) initiate the growth of cavitation bubbles, which collapse hundreds of microseconds later. Since the bubble growth‐collapse cycle trails passage of the pulse, and is ∼1000 times shorter than the pulse interval at clinically relevant firing rates, it is not expected that cavitation will affect pulse propagation. However, pressure measurements with a fiber‐optic hydrophone (FOPH‐500) indicate that bubbles generated by a pulse can, indeed, shield the propagation of the negative tail. Shielding was detected within 1 μs of arrival of the negative wave, contemporaneous with the first observation of expanding bubbles by high‐speed camera. Reduced negative pressure was observed at 2 Hz compared to 0.5 Hz firing rate, and in water with a higher content of dissolved gas. We propose that shielding of the negative tail can be attributed to loss of acoustic energy into the expansion of cavitation bubbles.

Bubble Proliferation in Shock Wave Lithotripsy Occurs during Inertial Collapse

In shock wave lithotripsy (SWL), firing shock pulses at slow pulse repetition frequency (0.5 Hz) is more effective at breaking kidney stones than firing shock waves (SWs) at fast rate (2 Hz). Since at fast rate the number of cavitation bubbles increases, it appears that bubble proliferation reduces the efficiency of SWL. The goal of this work was to determine the basis for bubble proliferation when SWs are delivered at fast rate. Bubbles were studied using a high‐speed camera (Imacon 200). Experiments were conducted in a test tank filled with nondegassed tap water at room temperature. Acoustic pulses were generated with an electromagnetic lithotripter (DoLi‐50). In the focus of the lithotripter the pulses consisted of a ∼60 MPa positive‐pressure spike followed by up to −8 MPa negative‐pressure tail, all with a total duration of about 7 μs. Nonlinear propagation steepened the shock front of the pulses to become sufficiently thin (∼0.03 μm) to impose differential pressure across even microscopic bubbles. Hi...

The Characteristics of Broad and Narrow Focal Zone Lithotripters

The focal width of a lithotripter is a measure of the diameter of its focal zone, the region where acoustic pressures are at least half the maximum positive pressure generated at a given power level. Different lithotripters have different focal widths. The Dornier HM3, for example, has a focal width of ∼10–12 mm and for many years this was the widest focal zone among clinical machines. Electromagnetic lithotripters tend to have narrower focal zones, in the range of ∼4–6 mm. Recent studies suggesting that focal width plays an important role in stone breakage prompted this assessment of two electromagnetic lithotripters. Acoustical mapping using a fiber optic probe hydrophone (FOPH‐500) and breakage of U‐30 gypsum model stones were used to compare a conventional lithotripter (Dornier DoLi‐50) and a broad focal zone device (XiXin XX‐ES). FOPH mapping characterized the focal width of the DoLi to be about 5mm and that of the XX‐ES to be much wider (∼18 mm). For stone breakage experiments the DoLi was fired at ...

Bubbles trapped at the coupling surface of the treatment head significantly reduce acoustic energy delivered in shock wave lithotripsy

The coupling efficiency of a “dry head” electromagnetic lithotripter (Dornier Compact Delta) was studied in vitro. A fiber‐optic probe hydrophone (FOPH‐500) was positioned in a test tank filled with degassed water. The tank was coupled through a semi‐transparent latex membrane to the water‐filled cushion of the lithotripter head, so that bubbles (air pockets) trapped between the two coupling surfaces could be easily observed and photographed. When gel was applied to both the latex membrane and the water cushion, numerous bubbles (some several millimeters in diameter) could be seen at the coupling interface. Hydrophone measurements in the geometric focus of the lithotripter showed that the acoustic pressure could be two times lower when bubbles were present than when they were manually removed. In our in vitro design, trapped bubbles could be easily observed and therefore removed from the acoustic path. However, during patient treatment with a dry‐head lithotripter one cannot see whether bubbles are trappe...

Potential mechanism for the effect of shock wave rate in shock wave lithotripsy

Artificial stones break significantly better when shock waves (SWs) are delivered at 0.5 Hz than at 2 Hz, and patients treated at slower rates have improved stone‐free rates. One possible explanation may be cavitation bubbles that might persist between SWs at high rate and distort subsequent SWs sufficiently to reduce their effectiveness at stone comminution. High‐speed photography gives evidence that bubble numbers are greater at higher rates. B‐mode ultrasound echo in the free field typically disappears between pulses administered at 0.5 Hz but persists at 2 Hz. Fiberoptic hydrophone measurements at 2 Hz showed, in the free field of an electrohydraulic lithotripter, that SW negative tail was truncated, and proximal to a stone, that SW waveform varied and was distorted such that often the positive pressure amplitude was reduced. Changes in waveform proximal to stone declined as the stone disintegrated and fell away. Thus, data support the persistence of cavitation bubbles at high SW rate, and consequent ...

The problem of coupling in dry‐head lithotripsy

Recent in vitro studies have shown that air pockets can get trapped at the coupling interface of the treatment head in dry‐head lithotripsy, and this can pose a significant barrier to transmission of shock wave energy to the focal zone. Breakage of model stones is very sensitive to the presence of air pockets at the coupling interface. The quality of routine coupling is highly variable, and it seems quite feasible that the way in which the coupling gel is applied may have a significant effect on the quality of coupling. Therefore, attempts to find the best coupling regime may be valuable to perform, and preliminary results of in vitro tests are presented in this report. Experiments were conducted using gel or castor oil as coupling agents. The test tank was coupled through a transparent Mylar membrane to the water‐filled cushion of the treatment head, so that pockets of air trapped between the two coupling surfaces could be observed and photographed. It is shown that the quality of coupling can be improve...

Observation of cavitation during shock wave lithotripsy

A system was built to detect cavitation in pig kidney during shock wave lithotripsy (SWL) with a Dornier HM3 lithotripter. Active detection, using echo on B‐mode ultrasound, and passive cavitation detection (PCD), using coincident signals on confocal, orthogonal receivers, were equally sensitive and were used to interrogate the renal collecting system (urine) and the kidney parenchyma (tissue). Cavitation was detected in urine immediately upon SW administration in urine or urine plus X‐ray contrast agent, but in tissue, cavitation required hundreds of SWs to initiate. Localization of cavitation was confirmed by fluoroscopy, sonography, and by thermally marking the kidney using the PCD receivers as high intensity focused ultrasound sources. Cavitation collapse times in tissue and native urine were about the same but less than in urine after injection of X‐ray contrast agent. Cavitation, especially in the urine space, was observed to evolve from a sparse field to a dense field with strong acoustic collapse ...

Efficiency of spark discharge in electrohydraulic lithotripsy.

Electrohydraulic lithotripters and SWT devices generate shock waves by discharge of a high‐voltage capacitor through submerged electrodes. As the electrodes age, the interelectrode gap widens. How this affects the efficiency of spark generation was studied using a research HM3‐clone lithotripter. Widening of the interelectrode gap (∼0.3 mm with new electrodes; ∼2.5 mm after 4000 discharges) increased the lag‐time to breakdown (∼0 to ∼30 μs, respectively). Increased lag‐time as electrodes aged was associated with partial discharge of the capacitor (leakage promoted by ∼0.6 mS conductivity of the surrounding water), such that the average energy remaining at the capacitor at the moment of breakdown was reduced four times compared to new electrodes. However, with new electrodes almost 90% of the energy was lost in the circuitry rather than in the spark, as the resistance of the spark (R∼0.03 Ω) was much smaller than the resistance of the remainder of the circuit—including the high‐voltage switch and connectiv...