Comparison of mouse and rabbit lung damage exposure to 30 kHz ultrasound

Application of analyzer based X-ray imaging technique for detection of ultrasound induced cavitation bubbles from a physical therapy unit

BACKGROUND

The observation of ultrasound generated cavitation bubbles deep in tissue is very difficult. The development of an imaging method capable of investigating cavitation bubbles in tissue would improve the efficiency and application of ultrasound in the clinic. Among the previous imaging modalities capable of detecting cavitation bubbles in vivo, the acoustic detection technique has the positive aspect of in vivo application. However the size of the initial cavitation bubble and the amplitude of the ultrasound that produced the cavitation bubbles, affect the timing and amplitude of the cavitation bubbles' emissions.

METHODS

The spatial distribution of cavitation bubbles, driven by 0.8835 MHz therapeutic ultrasound system at output power of 14 Watt, was studied in water using a synchrotron X-ray imaging technique, Analyzer Based Imaging (ABI). The cavitation bubble distribution was investigated by repeated application of the ultrasound and imaging the water tank. The spatial frequency of the cavitation bubble pattern was evaluated by Fourier analysis. Acoustic cavitation was imaged at four different locations through the acoustic beam in water at a fixed power level. The pattern of cavitation bubbles in water was detected by synchrotron X-ray ABI.

RESULTS

The spatial distribution of cavitation bubbles driven by the therapeutic ultrasound system was observed using ABI X-ray imaging technique. It was observed that the cavitation bubbles appeared in a periodic pattern. The calculated distance between intervals revealed that the distance of frequent cavitation lines (intervals) is one-half of the acoustic wave length consistent with standing waves.

CONCLUSION

This set of experiments demonstrates the utility of synchrotron ABI for visualizing cavitation bubbles formed in water by clinical ultrasound systems working at high frequency and output powers as low as a therapeutic system.

Bio-effects and safety of low-intensity, low-frequency ultrasonic exposure

Low-frequency (LF) ultrasound (20-100 kHz) has a diverse set of industrial and medical applications. In fact, high power industrial applications of ultrasound mainly occupy this frequency range. This range is also used for various therapeutic medical applications including sonophoresis (ultrasonic transdermal drug delivery), dentistry, eye surgery, body contouring, the breaking of kidney stones and eliminating blood clots. While emerging LF applications such as ultrasonic drug delivery continue to be developed and undergo translation for human use, significant gaps exist in the coverage of safety standards for this frequency range. Accordingly, the need to understand the biological effects of LF ultrasound is becoming more important. This paper presents a broad overview of bio-effects and safety of LF ultrasound as an aid to minimize and control the risk of these effects. Its particular focus is at low intensities where bio-effects are initially observed. To generate a clear perspective of hazards in LF exposure, the mechanisms of bio-effects and the main differences in action at low and high frequencies are investigated and a survey of harmful effects of LF ultrasound at low intensities is presented. Mechanical and thermal indices are widely used in high frequency diagnostic applications as a means of indicating safety of ultrasonic exposure. The direct application of these indices at low frequencies needs careful investigation. In this work, using numerical simulations based on the mathematical and physical rationale behind the indices at high frequencies, it is observed that while thermal index (TI) can be used directly in the LF range, mechanical index (MI) seems to become less reliable at lower frequencies. Accordingly, an improved formulation for the MI is proposed for frequencies below 500 kHz.

An animal model for ultrasound lung imaging

In the past decade, a number of clinical investigators have used ultrasound (US) to image the lung during video-assisted thoracoscopic surgery (VATS). In contrast, animal studies have shown prohibitively high attenuation levels in the lung, incompatible with the ability to image the lung. We hypothesized that the use of anesthesia during VATS augments lung collapse upon exposure to atmospheric pressure; thus, making US lung imaging possible. To test this hypothesis, we compared the effect of two commonly used anesthetic protocols on our ability to image 200 microL of US gel injected in rabbit lungs using a pulse echo transducer at 13 MHz. The anesthetic protocol, using acepromazine, ketamine and isoflurane, allowed US lung imaging in rabbits. It is concluded that US at 13 MHz can detect 200 microL of US gel injected into the lung parenchyma in a rabbit model.

High-frequency sound transmissions under water and risk of decompression sickness

We tested the possible occurrence of a neurological insult secondary to high-frequency sound exposure. Immersed, anesthetized rats were subjected to a simulated diving profile designed to induce decompression sickness, while exposed to the transmission of an acoustic beacon. Intermittent sound at a pressure level of 184.5 dB re 1 microPa at 1 m (1.7 kPa), a frequency of 37 kHz, and with a duration of 4 ms, was transmitted in a duty cycle of 0.26%. Four groups, each containing nine animals, were included in the study as follows: group 1, immersion only, no sound exposure; group 2, immersion with sound exposure; group 3, diving simulation when immersed, no sound exposure; group 4, diving simulation when immersed, with sound exposure. Somatosensory evoked potentials (SSEPs) were recorded the day before the study, and a second recording was made 30 min after immersion. Some of the SSEP components disappeared after the dive in 3 rats from group 3 and 2 rats from group 4. SSEP components could not be identified in a significantly larger number of animals from groups 3 and 4, compared with groups 1 and 2. No differences were found in wave latency, amplitude or conduction time. Our data show that the high-frequency sound exposure employed did not contribute to the development of the neurological insult.

The effect of low-frequency ultrasound on immersed pig lungs

Acoustic models suggest that high-intensity, low-frequency ultrasound (US) at 21-31 kHz, could cause damage to divers' lungs. The purpose of the study was to investigate lung tissue changes secondary to water-borne low-frequency US produced by commonly used underwater acoustic beacons (pingers). Explanted pig lungs were immersed and exposed to four different modes of low-frequency US pinger transmission. In each trial, 5 pairs of lungs were exposed to sound and 5 pairs served as controls. One central and one peripheral section were taken from each lung and evaluated microscopically for location and extent of damage. When present, microhaemorrhages were primarily found in a patchy alveolar distribution, as well as in the septal and subpleural regions. Only rare focal microhaemorrhages could be found in the Control Group. The results demonstrate a potential hazard to the immersed lungs of large mammals on exposure to prolonged transmission by commercially available underwater pingers. The relevance of these findings to human exposure should be further evaluated.

Lung damage assessment from exposure to pulsed-wave ultrasound in the rabbit, mouse, and pig

The principal motivation of the study was to assess experimentally the question: "Is the MI (Mechanical Index) an equivalent or better indicator of nonthermal bioeffect risk than I(SPPA.3) (derated spatial peak, pulse average intensity)?" To evaluate this question, the experimental design consisted of a reproducible biological effect in order to provide a quantitative assessment of the effect. The specific biological effect used was lung damage and the species chosen was the rabbit. This work was initiated, in part, by a study in which lung hemorrhage was observed in 7-week old C3H mice for diagnostic-type, pulsed-wave ultrasound exposures, and, therefore, 6- to 7-week old C3H mice were used in this study as positive controls. Forty-seven adult New Zealand White male rabbits were exposed to a wide range of ultrasound amplitude conditions at center frequencies of 3 and 6 MHz with all temporal exposure variables held constant. A calibrated, commercial diagnostic ultrasound system was used as the ultrasound source with output levels exceeding, in some cases, permissible FDA levels. The MI was shown to be at least an equivalent, and in some cases, a better indicator of rabbit lung damage than either the I(SPPA.3) or p(r.3) (derated peak rarefactional pressure), thus answering the posed question positively. Further, in situ exposure conditions were estimated at the lung pleural surface (PS); the estimated in situ I(SPPA.PS) and p(r.PS) exposure conditions tracked lung damage no better than I(SPPA.3) and p(r.3), respectively, whereas the estimated in situ MI(PS) exposure condition was a slightly poorer predictor of lung damage than MI. Finally, the lungs of six adult crossbred pigs were exposed at the highest amplitude exposure levels permitted by a diagnostic ultrasound system (to prevent probe damage) at both frequencies; no lung damage was observed which suggests the possibility of a species dependency biological effect.

Rabbit and pig lung damage comparison from exposure to continuous wave 30-kHz ultrasound

Previous comparative studies of ultrasound-induced pulmonary hemorrhage in mice and rabbits suggested that sensitivity to damage was species dependent (O'Brien and Zachary 1994b). In order to understand better these differences in species more analogous to the human, 74 pigs and 75 rabbits were each exposed for 10 min at 1 of 6 acoustic pressure levels (0, 145, 290, 340 [rabbits only], 460 and 490 [pigs only] kPa) at an ultrasonic frequency of CW 30 kHz. Eighteen mice were used as positive controls (10-min duration at 145 kPa). Because pig lung has numerous physiological and anatomical similarities to human lung, it was selected as the appropriate animal model for these studies. Pig lung data were compared to rabbit lung data; rabbit lung data have already been compared with mouse lung data (O'Brien and Zachary 1994a). Comparative analyses and extrapolation of these experimental data are intended to provide a better scientific basis for understanding the potential biological effects of ultrasound on human lungs since such studies will probably never be conducted with humans. Under the same exposure conditions and lung assessment criteria, mouse lung was determined to be more sensitive to ultrasound-induced damage than that of the rabbit by a factor of 3.9, the rabbit lung was more sensitive to ultrasound-induced damage than that of the pig by a factor of 3.7, and the mouse lung was more sensitive to ultrasound-induced damage than that of the pig by a factor of 14.4.

Lung lesions induced by continuous- and pulsed-wave (diagnostic) ultrasound in mice, rabbits, and pigs

These studies documented the presence or absence of macroscopic and microscopic intraparenchymal hemorrhage in individual lung lobes of mice, rabbits, and pigs exposed to continuous- and pulsed-wave (diagnostic) ultrasound; we described the character of and lesions associated with the hemorrhage and compared differences in the lesions among species and exposure conditions to investigate the pathogenic mechanisms and species differences associated with ultrasound-induced lung hemorrhage. In a series of three sequential interdependent studies, 312 mice, 91 rabbits, and 74 pigs were divided at random into experimental groups and exposed to continuous-wave ultrasound (3 kHz modulated at 120 Hz) of acoustic pressure levels ranging from 0 to 490 kPa for 5, 10, or 20 minutes. In a fourth study, three mice, 43 rabbits, and six pigs were divided at random into experimental groups and exposed to pulsed-wave ultrasound (3- and 6-MHz center frequency) of peak rarefactional acoustic pressure levels ranging from 0 to 5.6 MPa for 5 minutes. Macroscopic lesions induced by continuous- and pulsed-wave ultrasound consisted of dark red to black areas of hemorrhage that extended from visceral pleural surfaces into lung parenchyma. Hemorrhage appeared spatially related to the edges of lung lobes where pleura of dorsal and ventral surfaces met, occurred in specific lung lobes in all three species, and appeared anatomically related to lung that was closest to and in contiguous alignment with the ultrasound transducer and thus the path of the sound beam. Macroscopic lesions were similar in all species under all exposure conditions for both continuous- and pulsed-wave ultrasound; however, hemorrhage was not induced in pig lung exposed to pulsed-wave ultrasound at any peak rarefactional acoustic pressure level. Eighteen mice (145 kPa exposure pressure), 60 rabbits (145-460 kPa exposure pressure), and 58 pigs (145-490 kPa exposure pressure) from study 3 were used for microscopic evaluation of lung exposed to continuous-wave ultrasound; three mice (6 MHz; 2.9 and 5.4 MPa), 39 rabbits (3 and 6 MHz; 2.3-5.4 MPa), and six pigs (3 and 6 MHz; 3.3, 5.4, and 5.6 MPa) from study 4 were used for microscopic evaluation of lung exposed to pulsed-wave ultrasound. Microscopic lesions and the character of hemorrhage induced by continuous-wave ultrasound were different from those induced by pulsed-wave ultrasound. Lesions induced by continuous-wave ultrasound under all exposure conditions were similar in all three species. Lesions induced by pulsed-wave ultrasound under all exposure conditions were similar in all three species.(ABSTRACT TRUNCATED AT 400 WORDS)

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 three 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-microns 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.

Mouse lung damage from exposure to 30 kHz ultrasound

Two hundred and seventy mice were evaluated at three exposure durations (5, 10 and 20 min) and at six peak acoustic pressure levels (0, 65, 80, 87, 100 and 145 kPa) with 15 mice per exposure condition, at an ultrasonic frequency of 30 kHz modulated at 120 Hz. Threshold acoustic pressure levels for hemorrhage in mouse lung exposed for the three exposure durations appear to be in the range of 100 kPa. There did not appear to be a strong dependency on exposure duration. When compared to a study with mice in the megahertz frequency range (Child et al. 1990), it appeared that the threshold values followed a square root of frequency dependency, suggesting that the concept of the Mechanical Index (AIUM/NEMA, 1992), although developed for pulsed ultrasound conditions, may be extended to frequencies well below the diagnostic ultrasound frequency range.