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L A Crum

Publications and source records attributed to L A Crum.

At least 37 records · Page 2Linked to original sources
PubMed1998

In vivo pressure measurements of lithotripsy shock waves in pigs.

Stone comminution and tissue damage in lithotripsy are sensitive to the acoustic field within the kidney, yet knowledge of shock waves in vivo is limited. We have made measurements of lithotripsy shock waves inside pigs with small hydrophones constructed of a 25-microm PVDF membrane stretched over a 21-mm diameter ring. A thin layer of silicone rubber was used to isolate the membrane electrically from pig fluid. A hydrophone was positioned around the pig kidney following a flank incision. Hydrophones were placed on either the anterior (shock wave entrance) or the posterior (shock wave exit) surface of the left kidney. Fluoroscopic imaging was used to orient the hydrophone perpendicular to the shock wave. For each pig, the voltage settings (12-24 kV) and the position of the shock wave focus within the kidney were varied. Waveforms measured within the pig had a shape very similar to those measured in water, but the peak pressure was about 70% of that in water. The focal region in vivo was 82 mm x 20 mm, larger than that measured in vitro (57 mm x 12 mm). It appeared that a combination of nonlinear effects and inhomogeneities in the tissue broadened the focus of the lithotripter. The shock rise time was on the order of 100 ns, substantially more than the rise time measured in water, and was attributed to higher absorption in tissue.

Acoustics↗
PubMed1998

Comparison of electrohydraulic lithotripters with rigid and pressure-release ellipsoidal reflectors. I. Acoustic fields.

The most common lithotripter, a Dornier HM-3, utilizes an underwater spark to generate an acoustic pulse and a rigid ellipsoidal reflector to focus the pulse on the kidney stone to be comminuted. The pulse measured in water with a PVDF membrane hydrophone at the external focus of the ellipsoid was a 1-microsecond positive-pressure spike followed by a 3-microsecond negative-pressure trough. When we replaced the rigid reflector in our experimental lithotripter with a pressure-release reflector, the pulse was a 1.6-microsecond trough followed by a 0.6-microsecond positive spike. The waveforms are nearly time inverses (i.e., their spikes and troughs are reversed). The frequency spectra, the maximum peak positive pressures P+ (42 MPa, rigid and 43 MPa, pressure-release), and the maximum peak negative pressures P- (-12 MPa and -14 MPa) are comparable. The maximum P- occurred 20 mm closer to the reflector than did the maximum P+, for both reflectors. However, the spatial maxima of the peak pressures (P+ and P-) produced by the pressure-release reflector were located 20 mm nearer to the reflector than those produced by the rigid reflector. Qualitative explanation of the waveforms and the location of pressure maxima as well as comparison to previous theoretical and experimental results is given. The alternate waveform produced by the pressure-release reflector may be a tool in determining the role of cavitation in lithotripsy because cavitation is highly sensitive to waveform.

Acoustics↗
PubMed1997

The effect of polypropylene vials on lithotripter shock waves.

In studies to understand the mechanisms responsible for shock wave lithotripsy (SWL) cell injury, we observed that shock waves (SWs) are influenced by the shape of the specimen vial. Lytic injury to kidney cells treated in a Dornier HM3 lithotripter was higher (p < 0.0001) when SWs entered the vial through the flat end (cap end) compared to the round end. Measurements of the acoustic field within polypropylene vials were carried out using both lithotripter SWs and pulsed ultrasound (US) in the megahertz frequency range. We compared pressure amplitudes inside the round and flat vials and found significant differences. When SWs entered through the round end, the average peak positive pressure was 40% of free-field pressure, due mostly to a dramatic reduction in pressure off axis. The average peak pressure inside the flat vial was twice that of the round vial. Experiments with US demonstrated that sound field focusing was induced by the curved interface of the round vial. Ray analysis for the round vial indicates the presence of "hot spots" on axis and "cold spots" off axis, in qualitative agreement with pressure profiles. We conclude that the shape of the specimen vial is an important factor that should be considered in model systems of SWL cell injury.

Animals↗
PubMed1996

Acoustically enhanced bubble growth at low frequencies and its implications for human diver and marine mammal safety.

Computations are made of the conditions necessary to obtain bubble growth by rectified diffusion under a variety of conditions associated with low-frequency sonar propagation in the ocean. The complex issue of microbubble nuclei stabilization is treated by assuming either a sufficient level of supersaturation to stabilize the initial bubble size, or by examining a microbubble nucleus with zero surface tension. The bubble growth rates and thresholds are obtained for a ranged of sound-pressure levels (re: 1 microPa) from 150-220 dB, for initial bubble radii from 1-10 microns, and for levels of the dissolved gas concentration from 100% to 223% of saturation. It was determined that for the range of conditions examined, it was necessary to utilize three different formulations of the equations for bubble growth. The results of these calculations (and assumptions concerning nuclei stabilization) indicate that for SPL's in excess of 210 dB, significant bubble growth can be expected to occur, and divers and marine mammals exposed to these conditions could be at risk. For SPL's below about 190 dB, however, except under relatively extreme conditions of supersaturation, significant bubble growth is unexpected.

Acoustics↗
PubMed1994

Calcium and the effects of ultrasound on frog skin.

Therapeutic ultrasound is used to enhance the repair of soft tissue, muscle, etc., and because many of the cellular reactions involved in these processes are dependent on the intracellular availability of free calcium ions, it becomes important to study the effects of ultrasound in the presence and the absence of calcium ions. Using frog skin as a biological model, the effect of therapeutic ultrasound (300 mW/cm2 1 MHz CW) was investigated. Sonication for two minutes caused a significantly larger increase in total ionic conductance (Gt) in the presence of calcium ions (140% vs. 27%). However, the time constant for Gt to return to steady state was significantly longer in calcium-free solutions (122 vs. 18 min.). This study demonstrates that the biological effects of ultrasound are influenced by calcium ions. Furthermore, the recovery time constants confirm recent findings regarding the function of calcium ions in the formation of tight junctions. The role of free radicals produced by cavitation and calcium potentiated lipid and protein peroxidation is discussed.

Animals↗
PubMed1994

Physical mechanisms governing the hydrodynamic response of an oscillating ultrasonic file.

Ultrasonically driven vibrating files are known to enhance the efficiency of root canal debridement. This paper presents a phenomenological view of the hydrodynamic response of an oscillating ultrasonic file and the relationship between the file response and various physical factors such as file size and curvature, file surface properties, file velocity amplitude, root canal geometry, and the type of irrigant. Relevant hydrodynamic properties include the propensity of a file to produce stable and transient cavitation, steady streaming, and cavitation microstreaming. These relationships were explored by experiment. Sonoluminescence was employed as an indicator of transient cavitation activity and photographic analysis was utilized as a means for detecting steady streaming, microstreaming, and stable cavitation. Measurements failed to indicate any strong correlation between registered driving power and the propensity to produce transient cavitation. Files that were pitted or possessed salient edges were very effective at generating transient cavitation. When observed, transient cavitation activity generally occurred near the tip of the straight file, provided the wall-loading did not inhibit file motion. In all cases studied, steady streaming and stable cavitation were observed to varying degrees, depending on the amount of file to wall contact. Stable cavitation was probably enhanced by the addition of moderate amounts of dissolved gas into the irrigant. Although the imposition of file-wall contact served to inhibit the production of transient cavitation, this action had relatively little effect on the ability of a file to produce a nominal level of streaming, microstreaming, and stable cavitation. The relationship between these hydrodynamic properties and the process of root canal debridement is addressed. Observations suggest that it is not prudent to ascribe enhanced cleaning effects to any one phenomenon, for it is likely that several factors are involved to varying degrees depending on the local conditions of application.

Luminescent Measurements↗
PubMed1992

Acoustic cavitation produced by microsecond pulses of ultrasound: a discussion of some selected results.

Because of its extensive utilization in clinical practice, and because the subjects examined are often fragile and sensitive to trauma, the safety of diagnostic ultrasound has always been of concern. Of the various mechanisms through which ultrasound could act in a manner deleterious to a patient, acoustic cavitation, should it occur, appears to possess significant potential for biological damage. This paper reviews several recent reports of progress by our two groups and demonstrates the conditions under which cavitation has been observed by microsecond pulses of ultrasound. Although these results give no indications that diagnostic ultrasound may pose a true risk to a patient, they do indicate that in vivo cavitation may occur under certain conditions.

Animals↗
PubMed1990

Effectiveness of ultrasonic files in the disruption of root canal bacteria.

The physical mechanisms of ultrasound, namely cavitation and acoustic streaming, generated by the Enac-Osada ultrasonic unit were investigated for effectiveness in disrupting Streptococcus mitis. In addition, the bactericidal effect of ultrasound in the presence of 2.5% sodium hypochlorite was examined. Bacterial suspensions were irradiated directly with ultrasound in simulated root canals, and the viability of bacteria was examined after growth on a blood agar medium under anaerobic conditions at 37 degrees C for 5 days. The results indicated that ultrasound per se failed to disrupt bacteria but resulted in increases in the viable counts; the former was considered to be because of the lack of cavitation and the latter because of the dispersal effects of acoustic streaming. The 2.5% sodium hypochlorite solution demonstrated powerful bactericidal activity.

Colony Count, Microbial↗
PubMed1989

The effect of therapeutic ultrasound on electrophysiological parameters of frog skin.

There are two groups of mechanisms through which ultrasound can affect biological systems, those of thermal origin and others of nonthermal origin. Since in almost every therapeutic application of ultrasound, movement of ions across cellular membranes is involved, it becomes important to study the effect of ultrasound on active and passive ionic conductance. In order to differentiate between thermal and nonthermal effects, a study was conducted on model systems in which the effect of temperature is known. The well-known sodium transporting epithelium, the epidermis of abdominal frog skin, was investigated and the effect of therapeutic ultrasound on its electrophysiological properties was determined. It was found that under open circuit conditions, irradiation of the skin with 1 MHz cw (60-480 mW/cm2) ultrasound caused a significant decrease (5-50%, depending on the applied power) in the transepithelial potential and resistance at room temperature (20-22 degrees C). Under short circuit conditions, also at room temperature, there was an increase in total ionic conductance (20-250%, depending on the applied power) and a decrease in the net actively transported current, measured as the short circuit current. These effects are reversible within the range of powers used. Furthermore, it was found that the magnitude of the observed changes was strongly dependent on the perfusion rate and the gas content of the bathing medium. The effect of ultrasound diminished in the presence of CO2 and was enhanced with faster perfusion rates. Pulsed ultrasound delivered at the same energy (Isata) as that of cw caused a significantly larger effect. At lower temperatures (12-14 degrees C) the effect of ultrasound was reduced. Analysis of the data reveals that the effects of ultrasound on ion transport reported here are not primarily of thermal origin but are probably due to cavitation and related effects, such as microsteaming.

Animals↗
PubMed1989

The significance of membrane changes in the safe and effective use of therapeutic and diagnostic ultrasound.

The cellular changes, such as alterations in motility and the stimulation of synthesis and secretion, induced by relatively low intensities of therapeutic ultrasound (e.g. 500 mW cm-2, SAPA; 100 mW cm-2 SATA) are primarily non-thermal in origin. They appear to be associated with changes in the permeability of the cell (plasma) membrane and in the transport of ions and molecules across it, effects which have been demonstrated in cells irradiated in suspension. In epithelial tissues, both in vitro and in vivo, it has been demonstrated that not only the cellular membrane transport pathways but also the paracellular or intercellular pathways are affected. Although membrane-mediated effects can be of value therapeutically, they could produce adverse effects if they were to occur during development, for the reception and transmission by the membrane of environmental signals are involved in determination of the fate of each cell. Determination is followed by selective gene expression and differentiation, that is, by the progressive increase in structural complexity brought about by the acquisition of specialised characteristics by various cell groups. Most cells of early embryos are ionically coupled via gap junctions which provide an intercellular pathway for electrochemical signalling and the maintenance of the concentration gradients which provide the cells with positional information. Differentiation of the cells varies according to their location with respect to these gradients. Increase in the intracellular concentration of calcium ions, which has been shown to occur after exposure to therapeutic levels of ultrasound, can decrease the permeability of gap junctions and uncouple cells, in the manner which occurs when they differentiate. Ultrasonically induced increases in calcium ion concentration are thus of considerable clinical significance, since they could affect differentiation and consequently histogenesis. Modification of plasma membrane permeability and transport properties, resulting in changes in the availability and activity of second messengers such as free calcium ions, can have profound effects on cell behaviour. Calcium channels appear to be the first channels to develop in the cell membranes of embryos, and internal calcium ion concentration is known to affect the synthesis of fetal proteins. Although generally reversible at intensities of less than 500 mW cm-2, changes in membrane permeability, particularly to calcium ions, could, if prolonged, have undesirable side effects not only on embryogenesis but on late prenatal and postnatal development. It is therefore recommended that the environmental conditions, thresholds, and mechanisms involved in the production of such changes be determined, so that they can be avoided when ultrasound is used diagnostically on sensitive targets such as embryos and fetuses.

Animals↗
PubMed1988

Cavitation microjets as a contributory mechanism for renal calculi disintegration in ESWL.

The rarefaction shock wave produced by an extracorporeal shockwave lithotripter can result in liquid failure at numerous discrete sites near the second focus. When the liquid fails, vapor-filled cavities can grow to relatively large sizes and subsequently collapse with enormous violence. This phenomenon, called acoustic cavitation, has been shown to cause severe erosion in materials exposed to cavitation fields. It is proposed in this paper that ESWL devices generate acoustic cavitation in vivo and that the high speed liquid microjets produced during cavitation bubble collapse play an important role in renal calculi disintegration.

Biophysical Phenomena↗
PubMed1988

Cavitation threshold measurements for microsecond length pulses of ultrasound.

The acoustic cavitation threshold of an aqueous solution has been measured at megahertz frequencies as a function of pulse width and pulse repetition frequency for various combinations of these quantities. The fluid tested was a 0.1M KOH-H3BO3 buffer solution with pH 10.9, which contained luminol, was saturated with argon, and filtered to 25 mu. The presence of cavitation was detected by a photomultiplier tube that required the emission of visible light that was both larger in magnitude and longer in duration than a preset criterion. It was observed that the cavitation threshold of water under pulse conditions decreases both when the pulse width is fixed and the pulse repetition frequency is increased, and when the pulse repetition frequency is fixed and the pulse width is increased. Acoustic cavitation thresholds measured in aqueous solutions are significantly less than those acoustic pressures associated with instruments that are currently in widespread use in medicine.

Acoustic Stimulation↗
PubMed1987

Acoustic cavitation generated by an extracorporeal shockwave lithotripter.

Evidence is presented of acoustic cavitation generated by a Dornier extracorporeal shockwave lithotripter. Using x-ray film, thin aluminum sheets, and relatively thick metal plates as targets, evidence of liquid jet impacts associated with cavitation bubble collapse was observed. The jet impact was violent enough to puncture thin foils and deform metal plates. Furthermore, numerous jet impacts were generated over a volume of greater than 200 cm3. It is likely that such violent cavitation will also occur in tissue, and observed biological effects (e.g. renal calculus disintegration and tissue trauma) may be related to cavitation damage.

Lithotripsy↗
PubMed1987

Ultrasonically induced gas bubble production in agar based gels: Part I. Experimental investigation.

Macroscopically visible gas bubbles can be produced in an agar based gel by irradiation with either continuous or pulsed ultrasound at frequencies from 0.75 to 3.0 MHz. The variation in the number of bubbles formed with frequency, acoustic pressure, pulse length, duty cycle, and temperature closely resembles that seen in vivo. Furthermore, the acoustic pressure required to initiate bubble formation is also close to that required in vivo. It has been observed that alterations in the concentration and pH of the gels can have a profound effect on the nature and quantity of bubbles. This suggests that not only is this gel model suitable for the representation of the macroscopic features of bubble formation in vivo, but can be used to gain information about the preexisting bubble nuclei. Based on the experimental results obtained it can be suggested that for peak negative acoustic pressures of up 1 MPa (equivalent, for a plane travelling sinusoidal wave, to a time averaged intensity of 30 W/cm2) bubble formation can be avoided by the use of high frequencies, short pulse lengths and long duty cycles.

Agar↗
PubMed1987

Ultrasonically induced gas bubble production in agar based gels: Part II. Theoretical analysis.

Visible size gas bubbles can be produced in an agar based gel when irradiated with either continuous wave (CW) or pulsed ultrasound. It is shown that many aspects of the production of these bubbles can be explained in a qualitative manner by a theoretical model based upon growth of a cavitation nucleus by rectified diffusion. Quantitative predictions for the number of bubbles produced as a function of various parameters tend to be different from measured values by less than an order of magnitude. The results given here provide a useful theoretical basis for the explanation of earlier measurements of ultrasonically induced bubbles in vivo.

Agar↗