Cavitation In Tissue Research Articles

Physics of ultrasonic surgery using tissue fragmentation: Part II

Ultrasonic surgical aspirators typically operate at a frequency between 20 and 60 kHz. A vibrating hollow horn moves against the tissue and suction is applied. The interaction causes tissue to fragment; the fragmented material is then aspirated. However, the mechanism of interaction is poorly understood: the most common view relates it to cavitation, probably active in concert with other mechanisms, including the direct jack-hammer effect, shock-induced stress, acoustic microstreaming and shearing stress. It has also been attributed to chopping, which will produce emulsification. This article reports a study that collected and analyzed ultrasonic, high-speed photographic, visual/optical and electrical data for a 23-kHz unit operating in water and a range of fresh pig tissues. The primary mechanism for tissue fragmentation is shown to be horn-tip impact and other mechanical forces, operating in combination with hydrodynamic forces applied to the tissue on the forward stroke in each cycle. No evidence of cavitation in tissue was observed.

Detection of cavitation during clinical extracorporeal lithotripsy

A clinical cavitation detection system has been developed that monitors the amplitude-time variation of the 1-MHz component of the broadband acoustic emission expected to accompany bubble collapse during clinical extracorporeal shockwave lithotripsy [C. C. Church, J. Acoust. Soc. Am. 86, 215–227 (1989)]. The detector is based on a hand-held piezoceramic focal bowl hydrophone that is acoustically coupled to the patients’ skin. A commercial electromagnetic positioning device (Fastrak, Polhemus Inc., Vermont) is used to enable the reception zone of the hydrophone to be directed under ultrasound image guidance. The system has been tested in the clinic by monitoring acoustic emission from positions around the beam focus of a clinical shockwave lithotripter during routine lithotripsy. The detected acoustic emission shows features characteristic of those obtained from cavitation observed invitro and arises from within a region of similar dimensions to that of the hyper-echoic region that can be simultaneously observed in the B-scan image. The study provides evidence that acoustic emission from cavitation in tissue can be detected noninvasively during clinical lithotripsy.

Ultrasound and microbubbles: Their generation, detection and potential utilization in tissue and organ therapy—Experimental

Ultrasound-induced cavitation in tissue and organs has been well recognized and documented. Generally, this phenomenon has been seen as something to be avoided except in cases such as lithotripsy, where its production is considered an essential part of the treatment process or as a desirable contrast media in some areas of visualization enhancement. This article covers theree areas in which the phenomenon has been observed, and shows how the effect can or may be therapeutically beneficial. Studies in the pig show that implanted human gallstones and the gallbladder itself can be eliminated in a nonsurgical procedure using ultrasound-induced cavitation in the gallbladder. In the dog brain, relatively stable cavitation-induced microbubbles have been transported through the vascular system to regions outside a focal seeding site. These bubbles produce ablation of tissue volumes at a remote site when irradiated with appropriate ultrasound. The cavitation phenomenon has been observed in the dog and human prostate. In the human prostate, microbubbles transported from ultrasound-induced focal seeding sites can be readily visualized with ultrasound and may be potentially useful under controlled conditions in tissue debulking for the treatment of benign prostatic hyperplasia (BPH). A similar microbubble transport has not been seen in the dog prostate under similar ultrasound treatment parameters. The ability to detect cavitation-induced microbubbles, follow their transportation through the vascular system and excite them at the appropriate time and place provides interesting possibilities for therapy. Of course, the entire microbubble process can be avoided by working below the cavitation threshold, thereby using only the absorption of ultrasound in tissue to produce focal thermal lesions. The term microbubble is used here in the context of those bubbles which can be transported in the vascular system down to vessels diameters below the 100-μm range. This is the vessel size in the vascular field into which microbubbles are transported and can be both visualized as well as disrupted with ultrasound.

The biological relevance of transient cavitation measurements performed in water: The effect of shear viscosity

Recent years have seen a host of direct experimental evidence of transient microcavitation in water produced by short pulses of megahertz-frequency ultrasound [Roy etal., J. Acoust. Soc. Am. 87, 2451–2458 (1990); and others]. However, to our knowledge there have been no published accounts of direct observations of such microcavitation activity in biological media. It is useful to relate the results of the aqueous experiments to the likelihood of producing cavitation in tissue. To do this, one must assess the relevant differences between the two media. As a first step in this process, a numerical implementation of the Gilmore equation for adiabatic bubble pulsations is employed [Church, J. Acoust. Soc. Am. 83, 2210–2217 (1988)] and compare predicted thresholds in water and in biological media modeled as a viscous fluid. Comparisons are made with the analytical theory of Holland and Apfel [IEEE UFFC 36, 204–208 (1989)]. The requirement that, at the threshold pressure, the bubble collapse temperature exceed 5000 K provides the rigorous comparison criterion. Results are viewed in light of recent experimental results obtained in water [Calabrese etal., J. Acoust. Soc. Am. 95, 2856 (1994)] and in tissue-mimicking phantom materials [Zheng etal., J. Acoust. Soc. Am. 95, 2855 (1994)]. [Work supported by NIH through Grant No. RO1 CA39374.]

Cavitation bioeffects: Spin-offs from lithotripsy.

Lithotripter fields can produce cavitation in the organs of the body at pressure amplitudes significantly lower than those required to fragment stones. Because of limits on the expansion of the gaseous nuclei imposed by the surrounding tissue structures, cavitation in tissue is qualitatively different from that described by classical cavitation theory for a spherical bubble in an infinite fluid. Thresholds for structural and functional changes in kidney, liver, lung, heart, and blood, developmental effects in the embryo/fetus, and killing of Drosophilia larvae have been determined. Studies of the biological effects of shock waves generated by lithotripters have guided us in the search for effects of diagnostically relevant pulsed ultrasound. In particular, lung hemorrhage occurs with pulsed ultrasound at acoustic pressures of the order of 1 MPa and effects on the dynamic behavior of heart tissue have been seen with single 1–10 ms pulses at pressure amplitudes of the order of 10 MPa. The effects in lung occur at temporal average intensities orders of magnitude lower than previously associated with biological effects in mammals.