A.R. Williams

Photodisruptive laser nucleation and ultrasonically-driven cavitation of tissues and materials

A method and apparatus for processing a material creates a cavitation nucleus in a portion of the material by focusing optical radiation at the portion of the material and then causing mechanical disruption in another portion of the material adjacent the cavitation nucleus by subjecting the cavitation nucleus to ultrasound waves.

INVESTIGATION OF CAVITATION IN FLOWING MEDIA BY LITHOTRIPTER SHOCK WAVES BOTH IN VITRO AND IN VIVO

Cavitation produced by lithotripter shock waves was characterized in vitro in water and blood, and in vivo in aortic blood by means of a 1.6 MHz resonant bubble detector. This system was readily able to detect bubbles resulting from shock-wave induced cavitation in both water and blood flowing through plastic tubes in vitro, and even in blood pumped by the heart through a plastic arterio-venous shunt. However, this system was unable to detect evidence of shock-wave induced cavitational activity occurring within the intact vascular systems of dogs in vivo.

A search for ultrasonic cavitation within the canine cardiovascular system

Resonant bubble detectors (RBD) were used to search for both pre-existing bubbles and bubbles created by cavitation within the cardiovascular system of 22 dogs. No pre-existing bubbles of 3.8 micron diam or larger were found, nor were any created by exposing the left ventricle to 0.51-1.61 MHz ultrasound of up to 16 W/cm2 spatial-peak intensity. Bubbles introduced into the arterial system by high speed injection were readily detected and could be held in the heart by 1 MHz ultrasound at 1-2 W/cm2 or above. A surprising observation was that gas bubbles of resonant size injected into the left ventricle and held by ultrasound did not multiply continuously as happens in saline or water in vitro. This in vivo system was designed to assess the potential for cavitation bioeffects and the essentially negative results obtained may limit the expected or potential risk of this mechanism in regard to medical applications of ultrasound.

Mechanisms for hemolysis by ultrasonic cavitation in the rotating exposure system.

The rotating chamber provides a useful system for enhancing cavitation bioeffects so that they may be more easily studied. A tube with acoustically transparent windows was rotated at 72 rpm perpendicular to its axis. The 1.61 MHz ultrasound beam was switched on and off in 42 ms bursts to expose the chamber while it was aligned with the beam. The hemolysis of 0.5% suspensions of canine erythrocytes was used as a measure of the efficacy of this rotating exposure system. Use of dialysis membrane, agar plugs, plastic sealing film or wax film for the windows produced no differences in the results for exposure intensities above an apparent threshold of 2-2.8 W/cm2, up to 16 W/cm2, at which essentially 100% lysis was observed. Changing the tube length from 1.4 cm to 4 cm, or partitioning it into three 0.5 cm segments had little influence on the results, but a short 0.5 cm chamber had reduced efficacy. Pressurizing the suspension filled chamber at 10 MPa to reduce the population of cavitation nuclei reduced the hemolysis. Separately pressurizing the suspension or the chamber produced indistinguishable, smaller reductions in hemolysis. The results lead to the hypothesis that the hemolysis occurs primarily in the bulk of the medium (rather than on surfaces). Bubbles recycled by rotation into the medium move toward the back of the chamber at estimated speeds of 10 m/s, generating hydrodynamic stresses sufficient to cause the observed hemolysis.

Phonophoresis : an in vivo evaluation using three topical anaesthetic preparations

An electrical sensory perception threshold technique has been developed for use with human volunteers. This technique has been used to reproducibly quantify the effects of three different commercially available topical anaesthetic preparations on superficial sensory cells (nociceptors) in the skin. Low intensities (0.25 W cm-2 SATA) of 1.1 MHz ultrasound had no detectable effects upon the rate of penetration of either one of the three anaesthetic preparations through human skin under conditions where temperature increases had been minimized.

Photometric detection of ATP release from human erythrocytes exposed to ultrasonically activated gas-filled pores

Abstract Suspensions of human erythrocytes in saline containing Luciferase/Luciferin (an enzyme preparation from fireflies which emits visible light in the presence of ATP) were exposed to 1.6 MHz ultrasound in the presence of stable gas bodies trapped within the holes in hydrophobic Nuclepore membranes. Ultrasonic intensities greater than 20–30 mW/cm2 (spatial peak) resulted in the emission of light, apparently indicating that some erythrocytes were being disrupted. No effects could be observed at these low intensities in the absence of the gas bodies.

Intravascular mural thrombi produced by acoustic microstreaming

Abstract A discrete portion of the wall of an intact blood vessel may be driven to oscillate at an ultrasonic frequency in vivo by the external application of a vibrating metal probe. The hydrodynamic forces generated within the local intravascular microstreaming field at the site of contact with the probe tip increase with increasing amplitude of oscillation of the ultrasonic driver. Under the appropriate experimental conditions one may produce (1) no detectable effects; (2) adhesion of platelets to apparently intact endothelium; (3) fibrin-free mural aggregates of platelets with the concomitant production of platelet micro-emboli; (4) mural thrombi of platelets permeated with fibrin and enveloped within a mixed clot and (5) obvious damage to the vessel wall and its endothelium with widespread clot formation. This technique might be of use in the in vivo screening of drugs which may interact with the haemostatic system.

Sonoporation of erythrocytes by lithotripter shockwaves in vitro

Sonoporation of red blood cells was examined in relation to cavitation-induced hemolysis. FITC-dextran at 580,000 MW was added to suspensions of canine erythrocytes and the mixture was exposed to lithotripter shockwaves. Exposure at 5% or 50% hematocrit in PBS or 50% in plasma yielded not only hemolysis but also FITC-dextran uptake in surviving cells. Hemolysis increased with increasing numbers of shockwaves. The numbers of cells with fluorescent dextran uptake remained roughly constant for 250-1000 shockwaves, but this represented an increasing percentage of the surviving cells. In addition, fluorescent microspheres formed spontaneously in samples with hemolysis. An air bubble was needed in the chamber to obtain substantial effects, implicating the cavitation mechanism. The exposure-response trends could be modeled by simple theory for random interaction of the cells with bubbles.

The effect of bovine and human serum albumins on the mechanical properties of human erythrocyte membranes

Abstract Crystalline bovine serum albumin increased the mechanical resistance of fresh human erythrocytes to lysis by hydrodynamic shear forces. A saturation effect suggests that the bovine alubmin molecules are adsorbed on to a finite number of “attachment sites” on the erythrocyte surface, possibly by displacing human proteins already occupying these sites. A heterogeneous fraction of human serum albumins does not exhibit the same marked protection effect, nor displace adsorbed bovine albumin molecules from the erythrocyte surface. The precise nature and extent of the interaction between any given concentration of either human or bovine serum albumin and the intact erythrocyte membrane depends upon the chronological age of the cell concerned.

Identification of mechanisms responsible for fetal weight reduction in mice following ultrasound exposure

This study challenges existing interpretations of the effects of ultrasound on fetal development. The results demonstrated that a lower mean fetal weight in mice observed after intrauterine exposure to 2 MHz ultrasound resulted from maternal distress, and was not simply a direct effect of ultrasound interaction with the fetuses. At day 8 of gestation continuous wave ultrasound was applied to different regions of the maternal abdomen at spatial average intensities between 0.5 and 5.5 W cm-2 for 3 min. Insonating the uterus directly resulted in resorptions, abortions and gross developmental abnormalities, with rare instances of fetal weight reduction which were apparently associated with maternal distended bladder syndrome (DBS). When the ultrasound beam deliberately avoided the uterus, by a dorsal approach centred on the vertebral column, mean fetal weight at day 18 was significantly lower in all mothers showing DBS. Fetuses of insonated mothers not affected by DBS were not significantly different from the sham-insonated group whether the uterus was directly exposed or not. The fetal weight reduction endpoint was therefore found to be more closely correlated with maternal damage (DBS) than with ultrasound intensity.