Extracorporeal Shock Wave Lithotripter Research Articles

On the increase of disintegration efficiency of an acoustic lithotripter

Currently, disintegration of renal calculi, noninvasively, with the use of lithotripters, is in common use. In the hospitals, commercial lithotripters like extra-corporeal shock wave lithotripters (ESWL) are in routine use. These systems are very cumbersome and expensive. Treatment of the patients also takes comparatively more time, due to more number of sittings. The high ultrasonic intensity used in such systems has side effects, and causes damage to delicate nerves and fibers in the surroundings of the stones present in the kidney. Therefore, the present lithotripters are required to be improved by increasing the efficiency of disintegration and other factors. In the present work, the increase in the disintegration efficiency of the ultrasonic lithotripters, in this case Siemens ESWL, is made by stimulating externally the main shock wave with an acoustic stress wave. The cavitation bubbles are found to implode faster, with more disintegration efficiency of the lithotripter, giving in turn, better treatment to the patient, with less sitting time and less ultrasound dosage level.

IN VITRO STUDY OF ULTRASOUND BASED REAL-TIME TRACKING OF RENAL STONES FOR SHOCK WAVE LITHOTRIPSY: PART 1

We developed a real-time tracking system for renal stones that decreases the number of shock waves and treatment time of shock wave lithotripsy. Ultrasound images were analyzed to identify the renal stones. A computer software module for ultrasound image processing was developed to monitor stone location instantaneously. Another computer software module controlled generator movement in real time for tracking the stone. We used 13 ultrasound stone trajectories recorded from patients to test the system in vitro. Two tests were established to verify tracking system reliability. One test focused on improvement in the coincidence ratio, which denotes the matching extent of the stone within the effective focal area. The other test focused on improvement in the efficiency ratio, that is a decrease in the number of shocks for stone fragmentation. For each recorded stone trajectory 2 experiments were done under tracking and nontracking conditions. The average coincidence and efficiency ratios plus or minus standard deviation were 79.6% +/- 9.8% and 45.0% +/- 12.7% without tracking, and 97.0% +/- 3.0% and 85.5 +/- 6.8% with tracking, respectively. All tests were statistically significant (paired t test p <0.01). An ultrasound based real-time tracking system proved to be significantly helpful for in vitro lithotripsy. It appears that the tracking system may greatly decrease the number of shocks and treatment time for renal stones.

Design and characterization of a research electrohydraulic lithotripter patterned after the Dornier HM3

An electrohydraulic lithotripter has been designed that mimics the behavior of the Dornier HM3 extracorporeal shock wave lithotripter. The key mechanical and electrical properties of a clinical HM3 were measured and a design implemented to replicate these parameters. Three research lithotripters have been constructed on this design and are being used in a multi-institutional, multidisciplinary research program to determine the physical mechanisms of stone fragmentation and tissue damage in shock wave lithotripsy. The acoustic fields of the three research lithotripters and of two clinical Dornier HM3 lithotripters were measured with a PVDF membrane hydrophone. The peak positive pressure, peak negative pressure, pulse duration, and shock rise time of the focal waveforms were compared. Peak positive pressures varied from 25 MPa at a voltage setting of 12 kV to 40 MPa at 24 kV. The magnitude of the peak negative pressure varied from −7 to −12 MPa over the same voltage range. The spatial variations of the peak positive pressure and peak negative pressure were also compared. The focal region, as defined by the full width half maximum of the peak positive pressure, was 60 mm long in the axial direction and 10 mm wide in the lateral direction. The performance of the research lithotripters was found to be consistent at clinical firing rates (up to 3 Hz). The results indicated that pressure fields in the research lithotripters are equivalent to those generated by a clinical HM3 lithotripter.

Full-wave modeling of lithotripter fields

The propagation of extracorporeal shock wave lithotripter pressure pulses is considered. A computational modeling must account for the effects of diffraction, nonlinear steepening and generation, and propagation of shock waves. Some commercial lithotripters are reflector focusing systems. Therefore, reflections at curved boundaries have to be considered. For specific interests in the propagation through and the interaction with human tissue structures, refraction, absorption, and scattering must be included. A full-wave model based on a set of nonlinear acoustic time domain equations is applied. This system is discretized to obtain a finite difference representation of broad bandwidth and low dispersive character. To enable an accurate shock wave computation an algorithm that modifies the temporal and spatial resolution adaptively is developed. Exemplary a reflector focusing lithotripter is modeled. To validate the model resulting computed wave profiles are compared with measured ones. The measurements are performed using a fiber-optic probe hydrophone. Excellent agreements show the method to provide an accurate and flexible tool for field predictions of complex devices. Characterizing parameters like −6-dB focal regions and energy quantities can be obtained, too. A human tissue model is applied to obtain predictions of the relevant parameters inside the human body.

Cardiac Arrhythmias with External Fixed-Rate Signal Generators in Shock Wave Lithotripsy with the Medstone Lithotripter

Objectives. To evaluate the incidence and outcome of cardiac arrhythmias associated with an external fixed-rate generator and the Medstone lithotripter with regard to stone location, anesthesia type, patient age, and gender. Methods. Data were obtained on 933 patients who underwent shock wave lithotripsy (SWL) with the Medstone lithotripter. Patients were randomly divided into two study groups, including those with pre-existing cardiac disease, hypertension, and cardiac medications. Group 1 (576 patients) were treated with the external fixed-rate signal generator (cardiac simulator). Group 2 (357 patients) underwent electrocardiogram-gated SWL (no simulator). All patients were monitored for arrhythmias during SWL. Patients who developed arrhythmias on the simulator were converted to gated SWL. Results. In group 1, 463 patients (80%) had no arrhythmias during treatment, whereas 113 patients (20%) developed cardiac arrhythmias (primarily premature ventricular contractions [PVCs]) and were successfully converted to gated SWL. Only 1 patient in group 2 (0.3%) had a documented arrhythmia (PVC). Group 1 patients who developed arrhythmias with a cardiac simulator were more likely to have renal rather than ureteral stones. Chi-square analysis showed a significant correlation between patients with and without arrhythmias in group 1 with regard to stone location ( P <0.0001). In group 1, regional anesthesia, intravenous anesthesia, and general anesthesia had an arrhythmia rate of 14%, 27%, and 38%, respectively. The only patient in group 2 who developed arrhythmias was under general anesthesia. The differences between groups 1 and 2, with regard to anesthesia type and arrhythmia, were statistically significant ( P <0.001). No patients required additional medical therapy or hospitalization for cardiac events, and all patients safely completed the SWL treatment. Conclusions. With careful monitoring and adequate precautions, external fixed-rate signal generators are safe, effective methods for pacing the Medstone shock wave lithotripter and probably other extracorporeal shock wave lithotripters.

Cardiac Effects of Water Immersion in Healthy Volunteers.

To evaluate cardiac chamber size and ejection fraction during head-out immersion of patients in the semisitting position in neutral temperature water bath, 12 volunteers were studied using transthoracic echocardiography (TTE). In this open, controlled, prospective clinical study, left ventricular end-systolic, end-diastolic volumes, and ejection fraction were measured in 12 adult awake healthy volunteers using TTE with the patient in a semisitting position pre-, during, and postimmersion in the Dornier extracorporeal shock wave lithotriptor water bath at 35 degrees C. Left ventricular end-diastolic volume significantly increased from an average of 96.0 +/- 10.8 to 130 +/- 19 cm(3), P < 0.0001 and left ventricular end-systolic volume increased from an average 38.9 +/- 6.9 to 45.2 +/- 8.0 cm(3), P < 0.01. Left ventricular ejection fraction increased significantly from control to water immersion from 0.59 +/- 0.1% to 0.65 +/- 0.1% (P < 0.001). The results of this study demonstrate that water immersion in normal healthy volunteers significantly increases both left ventricular diastolic and systolic volume, and that there is a significant increase in left ventricular ejection fraction after water immersion, (e.g., bathing, lithotripsy). While this increased cardiac filling is well-tolerated in most patients, it may not be well-tolerated in some patients who have limited cardiac reserve, including patients with heart failure.

Localized detection of cavitation generated by lithotripsy shock waves

Extracorporeal shock wave lithotripters generate intense cavitation which plays a role in both kidney stone destruction and tissue damage. The cavitation can be detected by listening for the characteristic ‘‘double-bang‘‘ acoustic emission, radiated by the bubbles during their rapid expansion and collapse phases. Previous workers have used a single focused transducer to detect the double-bang signature. However, the focal spot of the transducers is typcially 5 cm long and individual cavitation events can not be localized. A system has been developed that uses two confocal transducers (1 MHz, 20 cm radius of curvature, 10 cm diameter) to detect cavitation events in a localized volume. The system can be run in two modes: passive or active. In the passive mode both transducers listen to acoustic emissions generated by the cavitation field. In the active mode one transducer sends a tone and the second transducer listens to the sound scattered by any bubbles present in the focal region. In both cases the region to which the detectors are sensitive is approximately a sphere 5 mm in diameter. The passive system shows bubble rebound times far longer than predicted by theory for a single bubble. Scattering indicates that bubbles persist in the focal region for at least 6 ms. [Work supported by NIH.]

Measurement and Analysis of Vibrations on Surface of Phantom Induced by Piezoelectric Extracorporeal Shock Wave Lithotripter

We investigated the sound radiated from an object, induced by a piezoelectric extracorporeal shock wave lithotripter (ESWL). However, it was found that more direct measurement is necessary to analyze the sounds in piezoelectric ESWL. Therefore, we investigate the vibrations of a phantom, which are directly measured using laser Doppler velocimetry. The results show that the peak frequency in the power spectrum shifts as the number of shots increases from high frequency to low frequency. The previous results are confirmed by detecting the characteristic peaks obtained from the vibrations of bronze models. The bronze models are used to simulate the phantom during the breaking process. It is found that it is more difficult to make the models vibrate exactly at the focal point than above or below the focal region. These results will be applied to the monitoring of the breaking process and the choice of the optimum focal point on the calculus.

Economic Incentives and the Distribution of Extracorporeal Shock wave Lithotripters and Linear Accelerators in Spain

We studied the role of economic incentives for private providers in explaining Spain's disproportionately large number of extracorporeal shock wave lithotripters (ESWLs) and low number of linear accelerators (linacs) per million population (pmp) in comparison to other countries of the Organization for Economic Cooperation and Development (OECD). We found that the reimbursement policy for 1990 allowed an average profit per private ESWL of $732,000, but no profit for linacs. Regional per-capita income was positively correlated with ESWLs pmp in Spain (R2 = 0.49; p = .001), but not with linacs. Sixty-nine percent of ESWLs were privately held versus only 16% of linacs. To avoid these types of distortions, financial incentives should be based on a reasonable relationship between cost and charges and should be associated with the appropriateness of medical care.

Theoretical Study on Echo Trigger Pulse Propagation and Scattering in Extracorporeal Shock Wave Lithotripter

Theoretical Study on Echo Trigger Pulse Propagation and Scattering in Extracorporeal Shock Wave Lithotripter

The Distribution of “Big Ticket” Medical Technologies in OECD Countries

Five "big ticket" medical technologies (BTTs) in 1990 were compared in the 24 OECD countries in relation to population, the number of physicians, gross domestic product (GDP), and health care expenditures (HCE). Wide variations were observed between and within countries for all measures. Regression analysis revealed that HCE explains part of the variation in the distribution of computed tomography scanners (excluding Japan), magnetic resonance imaging units, and radiation therapy units (R2 between 0.40 and 0.69), but not extracorporeal shock wave lithotripters. To a lesser extent, GDP was also found to correlate with the distribution of these technologies, but no correlation was found with number of physicians. Other factors affecting the diffusion of these technologies are proposed for study.

Importance of the implosion of ESWL-induced cavitation bubbles

The damage induced by an extracorporeal shock wave lithotripter is observed with a fiber optic stress sensing technique. When a stone is placed in the focus, besides the expected stress induced by the incoming shock wave emitted by the ESWL apparatus, a second delayed stress is observed some hundreds of microseconds later. The second stress is induced by a shock wave generated at the collapse of a cavitation bubble. Partial reflection of the incoming shock wave at the stone boundary is at the origin of the large cavitation bubble formation. Sensing fiber fracture results always from the second shock wave due to the collapse of the cavitation bubble. Thus the largest stress is generated at the collapse. When no target is placed in the focus of the lithotripter, no large cavitation bubble is formed and no delayed shock wave is observed. Our results demonstrate unambiguously the decisive role of cavitation in ESWL procedures.

The performance of different pressure pulse generators for extracorporeal lithotripsy: A comparison based on commercial lithotripters for kidney stones

By using needle hydrophones and a PC-controlled experimental set-up, the acoustic output of 10 commercial extracorporeal shock-wave lithotripters has been measured. The pressure field was measured in the focus, along the beam axis, in the focal plane and “at the skin level” (a plane orthogonal to the beam axis, 5 cm backward from the focus, assumed as the entrance site of the pressure pulse into the patient's body). The set of tested instruments included the three technologies nowadays in use to generate the pressure pulse, namely electrohydraulic, electromagnetic and piezoelectric. Notwithstanding large intratechnology variability, the results indicate that electrohydraulic and electromagnetic generators can provide comparable performances. Piezoelectric generators produced pressure pulses with less energy than the others. Possible implications of the experimental results on treatment safety and effectiveness of kidney stone destruction are discussed.

Modeling the Dornier HM3 lithotripter

The computational modeling of a Dornier HM3 electrohydraulic, extracorporeal shock wave lithotripter is considered. In order to produce large amplitude shock waves for the purpose of pulverizing renal and ureteric calculi (stones), the HM3 uses a hemi-ellipsoidal bowl to focus the spherical field generated by a high-voltage spark gap. The initial propagation of the bowl-focused field is accomplished using a recently developed nonplanar source algorithm. An updated version of an existing nonlinear acoustic beam propagation model is then used to consider the subsequent propagation of the field. The resulting modeling sequence accounts for the effects of diffraction, attenuation, dispersion, nonlinearity, and (planar) reflection and refraction. The water path computed predictions agree well with existing measurements. The computed in vivo predictions suggest that the Dornier HM3's clinical performance is not significantly different than its water path performance.

Piezolith Extracorporeal Shockwave Lithotripsy: The Hotel-Dieu de France Experience

A total of 1500 patients underwent treatment with the Wolf Piezolith 2300 extracorporeal shockwave lithotripter for renal, ureteral, and bladder stones. Follow-up data were available at 3 months for 1435 patients. At that time, the overall stone-free rate was 82.7%: 82.4% for patients with renal stones, 81.0% for those with ureteral stones, and 100% for those with bladder stones. The overall success rate was 92.3%: 93.8% for patients with renal stones, 87.1% for those with ureteral stones, and 100% for those with bladder stones. The auxiliary treatment rate was 14.9%, and the retreatment rate was 53%. The effectiveness quotient was 49.2%. The Wolf Piezolith 2300 is an effective treatment for most stones smaller than 30 mm.

Incidence of cavitation in the fragmentation process of extracorporeal shock wave lithotriptors

The fragmentation mechanism occurring in extracorporeal shock wave lithotripsy (ESWL) is investigated using a fiber optic stress sensing technique. With our technique, we demonstrate that cavitation is a major cause of fragmentation in ESWL procedures. When a target is placed in the operating area of the lithotriptor, two shock waves are detected. The first detected shock wave corresponds to the incoming shock wave generated by the lithotriptor. The second shock wave, detected some hundreds of microseconds later, is generated in situ. It results from the collapse of a cavitation bubble, formed by the reflection of the incoming shock wave at the target boundary. This cavitation induced shock wave generates the largest stress in the target area according to our stress sensing measurements.

Power Difference in Spectrum of Sound Radiation before and after Break of Phantom by Piezoelectric Extracorporeal Shock Wave Lithotriptor

This paper investigates the difference in the spectrum of sound radiated before and after the break of a phantom at a focal point of the piezoelectric extracorporeal shock wave lithotriptor (ESWL) in order to identify the break time or to examine whether a calculus exists exactly at the focal point or not. From the preliminary experiments using a piece of chalk as a phantom of a calculus to measure the sound radiated when impact is applied to the chalk by an impact hammer, it is found that the bending vibration component of the vibration is exhibited in the spectrum of sound. However, for small-sized chalk shorter than 3 cm, the peak frequency of the bending vibration is higher than 20 kHz. From the experiments using a piezoeletric ESWL, it is found that there is clear difference in the power spectra among the sound radiated before the break, that radiated just after the break in the breaking process, and that radiated when the chalk does not exist at the focal point of the ESWL. These characteristics will be effective for the examination of the existence of the calculus at the focal point.

Measurement of Shock Wave Pressure Distribution of Extracorporeal Shock Wave Lithotripter Using Pressure-Sensitive Paper: Practical Usefulness and Limitations

A new method for evaluating shock wave pressure distribution of extracorporeal shock wave lithotripters, using pressure-sensitive paper has been developed. It is a simple method to visualize the shock wave pressure distribution and has a better spatial resolution than the hydrophone. The pressure-sensitive paper can be used to visually detect whether the focal point is on the correct position, and to detect changes of pressure distribution. Additionally, the relationship between lithotripter power and integrated density as a new evaluating value shows linearity. Therefore, this method is useful for quality assurance of lithotripters at hospitals.

Extracorporeal Shock Wave Lithotripsy for Bladder Stones

Between December 1989 and July 1991, 29 patients with bladder stones were treated on the MPL 9000 extracorporeal shock wave lithotriptor using ultrasound for localisation. The mean stone dimensions were 22.65 x 15.17 mm (< 400 mm2 in 20 patients); 75% of patients required only 1 ESWL session but the largest stone (2025 mm2) required 3. Whilst larger stones tended to require more shock waves, linear regression analysis showed a poor fit and factors other than size (operator experience and stone hardness) may determine the number of shock waves required. Satisfactory fragmentation was obtained in all patients. All were treated under intravenous analgesia without anaesthesia. Three patients also had outflow obstruction and were treated by transurethral resection of the prostate (TURP) on the day after completing ESWL. ESWL prior to TURP reduces the morbidity and operating time associated with endoscopic therapy of bladder stones. Macroscopic fragments remaining after ESWL can be washed out at TURP.

Variation of Pressure Waveforms in Measurements of Extracorporeal Shock Wave Lithotripter

In this paper, we describe measurement of variation in pressure waveforms of the acoustic field of an extra-corporeal shock-wave lithotripter (ESWL). Variations in the measured acoustic fields and pressure waveform of an underwater spark-gap-type ESWL with an exhausted spark plug electrode have been reported by researchers using crystal sensors. If the ESWL spark plugs become exhausted, patients feel pain during kidney, biliary stone disintegration. We studied the relationship between exhaustion of electrodes and the variation of pressure waveforms and shock-wave fields of the ESWL using a newly developed hydrophone.