Ultrasound-induced Lung Hemorrhage Research Articles

Numerical simulations of ultrasound-induced deformation of the lung surface

Ultrasound-induced lung hemorrhage remains the only known bioeffect of non-contrast, diagnostic ultrasound (DUS) found to occur in mammals. However, a fundamental understanding of the ultrasound-lung interaction is still lacking. In this study, we numerically simulate the deformation of the lung surface when subjected to clinically relevant pressure waves. We model the lung as air and the surrounding tissue as water. The two-dimensional, compressible Navier-Stokes equations are solved using a high-order accurate finite volume scheme. We first consider an air-water interface with a small sinusoidal perturbation and study the growth of the perturbations when subjected to a planar waveforms consisting of a step change in pressure. Scaling laws relating the waveform properties to the interface deformation rate are proposed for this simple case. Simulations with DUS waveforms indicate that the interface deformation depends on the unsteady nature of the wave. Preliminary analysis suggests that the initial interface deformation is driven by baroclinic vorticity deposited along the perturbed interface, and that the resulting stresses and strains are a possible candidate causing bioeffects.

Vibration Analysis of Circular Membrane Model of Alveolar Wall in Examining Ultrasound-induced Lung Hemorrhage

Ultrasound-induced lung hemorrhage was analyzed from the perspective of alveolar resonance mechanism. The alveolar resonance mechanism was theoretically studied by modeling the alveolar wall facet as a circular membrane model vibrating in fundamental mode. Based on the vibration analysis of this model, the equations of fundamental frequency and threshold pressure for the occurrence of ultrasound-induced lung hemorrhage were derived. The validity of the circular membrane model of the alveolar resonance mechanism was demonstrated. This theoretical study predicts that ultrasound-induced lung hemorrhage does not occur when the total lung capacity is ≤20% (for mammals), and when the ultrasound frequency is >1.55 MHz at a mechanical index of ≤1.9 (for humans only). The alveolar resonance mechanism is a plausible mechanism of ultrasound-induced lung hemorrhage.

Theoretical Calculation of Resonant Frequencies of the Human Alveolar Wall and Its Implications in Ultrasound Induced Lung Hemorrhage

Theoretical Calculation of Resonant Frequencies of the Human Alveolar Wall and Its Implications in Ultrasound Induced Lung Hemorrhage

Estimation of the acoustic impedance of lung versus level of inflation for different species and ages of animals

In a previous study, it was hypothesized that ultrasound-induced lung damage was related to the transfer of ultrasonic energy into the lungs (W. D. O'Brien et al. 2002, "Ultrasound-induced lung hemorrhage: Role of acoustic boundary conditions at the pleural surface," J. Acoust. Soc. Am. 111, 1102-1109). From this study a technique was developed to: 1) estimate the impedance (Mrayl) of fresh, excised, ex vivo rat lung versus its level of inflation (cm H(2)O) and 2) predict the fraction of ultrasonic energy transmitted into the lung (M. Oelze et al. 2003, "Impedance measurements of ex vivo rat lung at different volumes of inflation." J. Acoust. Soc. Am. 114, 3384-3393). In the current study, the same technique was used to estimate the frequency-dependent impedance of lungs from rats, rabbits, and pigs of various ages. Impedance values were estimated from lungs under deflation (atmospheric pressure, 0 cm H(2)O) and three volumes of inflation pressure [7 cm H(2)O (5 cm H(2)O for pigs), 10 cm H(2)O, and 15 cm H(2)O]. Lungs were scanned in a tank of degassed 37 degrees C water. The frequency-dependent acoustic pressure reflection coefficient was determined over a frequency range of 3.5-10 MHz. From the reflection coefficient, the frequency-dependent lung impedance was calculated with values ranging from an average of 1.4 Mrayl in deflated lungs (atmospheric pressure) to 0.1 Mrayl for fully inflated lungs (15 cm H(2)O). Across all species, deflated lung (i.e., approximately 7% of the total lung capacity) had impedance values closer to tissue values, suggesting that more acoustic energy was transmitted into the lung under deflated conditions. Finally, the impedance values of deflated lungs from different species at different ages were compared with the thresholds for ultrasound-induced lung damage. The comparison revealed that increases in ultrasonic energy transmission corresponded to lower injury threshold values.

Threshold Estimation and Superthreshold Behavior of Ultrasound-Induced Lung Hemorrhage in Rats: Role of Age Dependency

Age-dependent threshold and superthreshold behaviors of ultrasound-induced lung hemorrhage were investigated with one hundred ten 12.6 ± 0.8-d-old rats, one hundred ten 22.9 ± 0.8-d-old rats, and one hundred 57.7 ± 3.9-d-old rats. Exposure conditions were: 2.8 MHz, 10-s exposure duration, 1-kHz pulse repetition frequency and 1.3-μs pulse duration. The in situ (at the pleural surface) peak rarefactional pressure (p r( in situ) ) ranged between 1.4 and 10.8 MPa for which there were either 9 or 10 acoustic pressure groups for each of the three rat ages (10 rats/exposure group). For each of the three rat ages there were also shams; there were no lesions in the shams. The p r( in situ) levels were randomized within each age group; rat age was not randomized. Individuals involved in animal handling, exposure and lesion scoring were blinded to the exposure condition. In addition, one hundred fifty-six 72-d-old rats were included from three completed studies (same experimental conditions) to provide a fourth age group for the analysis. Probit regression analysis was used to examine the dependence of the occurrence of lesions on p r( in situ) in the four age groups. Likewise, lesion depth and lesion root surface area were analyzed using Gaussian tobit regression analysis. Although p r( in situ) was a significant variable, no significant age dependence of the p r( in situ) effect was found. Furthermore, age had no significant effect on either the rate of occurrence or the depth of lesions. Given the occurrence of a lesion, a weak age dependence was found for the median surface area of the induced lesion ( p-value = 0.037). (E-mail: wdo@uiuc.edu)

Threshold estimation of ultrasound-induced lung hemorrhage in adult rabbits and comparison of thresholds in mice, rats, rabbits and pigs

The objective of this study was to assess the threshold and superthreshold behavior of ultrasound (US)-induced lung hemorrhage in adult rabbits to gain greater understanding about species dependency. A total of 99 76 ± 7.6-d-old 2.4 ± 0.14-kg New Zealand White rabbits were used. Exposure conditions were 5.6-MHz, 10-s exposure duration, 1-kHz PRF and 1.1-μs pulse duration. The in situ (at the pleural surface) peak rarefactional pressure, p r(in situ), ranged between 1.5 and 8.4 MPa, with nine acoustic US exposure groups plus a sham exposure group. Rabbits were assigned randomly to the 10 groups, each with 10 rabbits, except for one group that had nine rabbits. Rabbits were exposed bilaterally with the order of exposure (left then right lung, or right then left lung) and acoustic pressure both randomized. Individuals involved in animal handling, exposure and lesion scoring were blinded to the exposure condition. Probit regression analysis was used to examine the dependence of the lesion occurrence on in situ peak rarefactional pressure and order of exposure (first vs. second). Likewise, lesion depth and lesion root surface area were analyzed using Gaussian tobit regression analysis. Neither probability of a lesion nor lesion size measurements was found to be statistically dependent on the order of exposure after the effect of p r(in situ) was considered. Also, a significant correlation was not detected between the two exposed lung sides on the same rabbit in either lesion occurrence or size measures. The p r(in situ) threshold estimates (in MPa) were similar to each other across occurrence (3.54 ± 0.78), depth (3.36 ± 0.73) and surface area (3.43 ± 0.77) of lesions. Using the same experimental techniques and statistical approach, great consistency of thresholds was demonstrated across three species (mouse, rat and rabbit). Further, there were no differences in the biologic mechanism of injury induced by US and US-induced lesions were similar in morphology in all species and age groups studied. The extent of US-induced lung damage and the ability of the lung to heal led to the conclusion that, although US can produce lung damage at clinical levels, the degree of damage does not appear to be a significant medical problem. (E-mail: wdo@uiuc.edu)

Lesions of ultrasound-induced lung hemorrhage are not consistent with thermal injury

Thermal injury, a potential mechanism of ultrasound-induced lung hemorrhage, was studied by comparing lesions induced by an infrared laser (a tissue-heating source) with those induced by pulsed ultrasound. A 600-mW continuous-wave CO2 laser (wavelength approximately 10.6 microm) was focused (680-microm beamwidth) on the surface of the lungs of rats for a duration between 10 to 40 s; ultrasound beamwidths were between 310 and 930 microm. After exposure, lungs were examined grossly and then processed for microscopic evaluation. Grossly, lesions induced by laser were somewhat similar to those induced by ultrasound; however, microscopically, they were dissimilar. Grossly, lesions were oval, red to dark red and extended into subjacent tissue to form a cone. The surface was elevated, but the center of the laser-induced lesions was often depressed. Microscopically, the laser-induced injury consisted of coagulation of tissue, cells and fluids, whereas injury induced by ultrasound consisted solely of alveolar hemorrhage. These results suggest that ultrasound-induced lung injury is most likely not caused by a thermal mechanism.

Superthreshold Behavior of Ultrasound‐Induced Lung Hemorrhage in Adult Rats

The purpose of this study was to enhance the findings of an earlier ultrasound-induced lung hemorrhage study (Ultrasound Med Biol 2003; 29:1625-1634) that estimated pressure thresholds as a function of pulse duration (PD: 1.3, 4.4, 8.2, and 11.6 micros; 2.8 MHz; 10-s exposure duration [ED]; 1-kHz pulse repetition frequency [PRF]). In this study, the roles of PRF and PD were evaluated at 5.9 MPa, the peak rarefactional pressure threshold near that of the ED50 estimate previously determined. A 4 x 4 factorial design study (PRF: 50, 170, 500, and 1700 Hz; PD: 1.3, 4.4, 8.2, and 11.6 mus) was conducted (2.8 MHz; 10-s ED). Sprague Dawley rats (n = 175) were divided into 16 exposure groups (10 rats per group) and 1 sham group (15 rats); no lesions were produced in the sham group. Logistic regression analysis evaluated significance of effects for lesion occurrence, and Gaussian tobit analysis evaluated significance for lesion depth and surface area. For lesion occurrence and sizes, the main effect of PRF was not significant. The interaction term, PRF x PD, was highly significant, indicating a strong positive dependence of lesion occurrence on the duty factor. The main effect of PD was almost significant (P = .052) and thus was included in the analysis model for a better fit. Compared with the findings from a PRF x ED factorial study (J Ultrasound Med 2005; 24:339-348), a function that considers PRF, PD, and ED might yield a sensitive indicator for consideration of a modified mechanical index, at least for the lung.

Superthreshold Behavior of Ultrasound-Induced Lung Hemorrhage in Adult Rats

The purpose of this study was to augment and reevaluate the ultrasound-induced lung hemorrhage findings of a previous 5 x 3 factorial design study (Ultrasound Med Biol 2001; 27:267-277) that evaluated the role of pulse repetition frequency (PRF: 25, 50, 100, 250, and 500 Hz) and exposure duration (ED; 5, 10, and 20 s) on ultrasound-induced lung hemorrhage at an in situ (at the pleural surface) peak rarefactional pressure [pr(in situ)] of 12.3 MPa; only PRF was found to be significant. However, saturation (response plateau) due to the high pr(in situ) might have skewed the results. In this follow-up 3 x 3 factorial design study, a wider range of PRFs and EDs were used at a lower pr(in situ). Sprague Dawley rats (n=198) were divided into 18 ultrasonically exposed groups (10 rats per group) and 6 sham groups (3 per group). The 3 x 3 factorial design study (PRF: 17, 170, and 1700 Hz; ED: 5, 31.6, and 200 s) was conducted at 2 frequencies (2.8 and 5.6 MHz). The p(r(in situ)) was 6.1 MPa. Logistic regression analysis evaluated lesion occurrence, and Gaussian tobit analysis evaluated lesion depth and surface area. Frequency did not have a significant effect, so the analysis combined results for the 2 frequencies. For lesion occurrence and sizes, the main effects for PRF and ED were not significant. The interaction term was highly significant, indicating a strong dependence of lesion occurrence and size on the total number of pulses (PRF x ED). The results of both studies are consistent with the hypothesis that the total number of pulses is an important factor in the genesis of ultrasound-induced lung hemorrhage.

Threshold estimates and superthreshold behavior of ultrasound-induced lung hemorrhage in adult rats: role of pulse duration

The study objective was to estimate the pressure threshold (ED 05, effective dose, or in situ peak rarefactional pressure associated with 5% probability of lesions) of ultrasound (US)-induced lung hemorrhage as a function of pulse duration (PD) in adult rats. A total of 220 10- to 11-week-old 250-g female Sprague-Dawley rats (Harlan) were randomly divided into 20 ultrasonic exposure groups (10 rats/group) and one sham group (20 rats). The 20 ultrasonic exposure groups (2.8-MHz; 10-s exposure duration; 1-kHz PRF; −6-dB pulse-echo focal beam width of 470 μm) were divided into four PD groups (1.3, 4.4, 8.2 and 11.6 μs) and, for each PD group, there were five in situ peak rarefactional pressures (range between 4 and 9 MPa). Rats were weighed, anesthetized, depilated, exposed, and euthanized under anesthesia. The left lung was removed and scored for the occurrence of hemorrhage. If hemorrhage was present, the lesion surface area and depth were measured. Individuals involved in animal handling, exposure and lesion scoring were “blinded” to the exposure conditions. Logistic regression analysis was used to examine the dependence of the lesion occurrences, and Gaussian tobit regression analysis was used to examine the dependence of the lesion surface areas and depths on in situ peak rarefactional pressure and PD. Threshold results are reported in terms of ED 05. For PDs of 1.3, 4.4, 8.2 and 11.6 μs, respectively, lesion occurrence ED 05s were 3.1, 2.8, 2.3 and 2.0 MPa with standard errors around 0.6 MPa. Lesion size ED 05s showed similar values. A mechanical index (MI) of 1.9, the US Food and Drug Administration (FDA) regulatory limit of diagnostic US equipment, is equivalent to the adult rat's in situ peak rarefactional pressure of 4.0 MPa. PDs of 8.2 and 11.6 μs had ED 05s more than 2 standard errors below 4.0 MPa, indicating that the ED 05s of these two PDs are statistically significantly different from 4.0 MPa. The ED 05 threshold levels for a PD of 1.3 μs are consistent with previous US-induced lung hemorrhage studies. As the PD increases, the ED 05 levels decrease, suggesting greater likelihood of lung damage as the PD increases. All of the ED 05s are less than the FDA limit. (E-mail: wdo@uiuc.edu)

Threshold estimates of ultrasound-induced lung hemorrhage in adult rats: role of pulse duration

The pressure threshold of ultrasound-induced lung hemorrhage has been estimated as a function of pulse duration (PD) in adult rats. A total of 220 10to 11week-old 250-gram female Sprague-Dawley rats (Harlan) were randomly divided into 20 ultrasonically exposed groups (10 rats/group) and one sham group (20 rats). The 20 ultrasonically exposed groups (2.8 MHz; 10-s exposure duration; 1-kHz PRF) were divided into four PD groups, and for each PD, there were five in situ (at the lung surface) peak rarefactional pressures. For PDs of 1.3, 4.4, 8.2, and 11.6 μs, respectively, lesion occurrence thresholds were 3.1, 2.8, 2.3 and 2.0 MPa. Lesion size thresholds showed similar values, thus suggesting greater likelihood of lung damage as the PD increases. A Mechanical Index of 1.9, the FDA regulatory limit of diagnostic ultrasound equipment, is equivalent to the adult rat’s in situ peak rarefactional pressure of 4.0 MPa. All of the ED05s are less than the FDA limit. Introduction The effect of exposure timing quantities (e.g., pulse duration, exposure duration, total on-time, and pulse repetition frequency) on the threshold for ultrasoundinduced lung hemorrhage has been examined to a limited extent. Our study examines the role of pulse duration (PD) in producing ultrasound-induced lung hemorrhage when pulse repetition frequency (PRF) and exposure duration (ED) are held constant. Specifically, the threshold estimates of ultrasoundinduced lung hemorrhage in adult rats are examined at four pulse durations. There appears to be only one study that has reported a dependency of ultrasound-induced lung hemorrhage on PD [1]. However, there have been numerous studies that have demonstrated that ultrasoundinduced lung hemorrhage can occur in mice, rats, rabbits, monkeys and pigs [2,3,4]. While the principal goal of the one PD study [1] was not aimed at assessing the dependency of ultrasound-induced lung hemorrhage on PD, its observations strongly suggested that PD influenced the pressure threshold value. Therefore, a directed study focused on whether ultrasound-induced lung hemorrhage is influenced by pulse duration is warranted. Methods Exposimetry Ultrasonic exposures were conducted using one focused, 19-mm-diameter, lithium niobate ultrasonic transducer. Water-based (highly degassed water, 22 C) pulse-echo ultrasonic field distribution measurements were performed and yielded a center frequency of 2.8 MHz, a fractional bandwidth of 12%, a focal length of 19 mm, a -6-dB focal beamwidth of 470 μm, and a -6-dB depth of focus of 2.7 mm. An automated procedure was developed to routinely calibrate the ultrasound fields that was based on established standards [3,5,6]. A calibrated PVDF membrane hydrophone (Marconi Model Y-34-6543, Chelmsford, UK) was used. Off-line processing yielded the water-based peak rarefactional pressure. The in situ (at the pleural surface) peak rarefactional pressure was determined by computing the attenuation of the water-based pressure using the rat’s chest wall thickness and the measured intercostal tissue attenuation coefficient at 2.8 MHz of 2.8 dB/cm. The rats in the 20 ultrasonically exposed groups were exposed as follows: 2.8-MHz center frequency, 10-s exposure duration, 1-kHz pulse repetition frequency. The two variables were pulse duration and in situ (at the lung surface) peak rarefactional pressure. Animals A total of 220 10to 11-week-old 250±12-g female Sprague-Dawley rats (Harlan) were randomly divided into 20 ultrasonic exposure groups (10 rats per group) and one sham group (20 rats); no lesions were produced in the sham group. The 20 ultrasonic exposure groups were divided into four PD groups, and for each PD group, there were five in situ peak rarefactional pressures. Each of the four PD groups were designed to have the same five in situ peak rarefactional pressures. The individuals involved in animal handling, exposure, and lesion scoring were blinded to the exposure condition. The exposure conditions for each animal were revealed only after the final results were tabulated. The rat exposure and analysis procedures have been described previously in detail [3]. Rats were weighed and anesthetized [ketamine hydrochloride (87.0 WCU 2003, Paris, september 7-10, 2003

Effect of pulse polarity and energy on ultrasound-induced lung hemorrhage in adult rats

Objective: The aim of this study was to further assess the role of inertial cavitation in ultrasound-induced lung hemorrhage by examining the effect of pulse polarity at a common in situ (at the lung surface) peak rarefactional pressure (Pr) and at a common in situ pulse intensity integral (PII).

Effect of pulse polarity and energy on ultrasound-induced lung hemorrhage in adult rats.

The objective of this study was to further assess the role of inertial cavitation in ultrasound-induced lung hemorrhage by examining the effect of pulse polarity at a common in situ (at the lung surface) peak rarefactional pressure [pr(in situ)] and at a common in situ pulse intensity integral (PII(in situ)). A total of 60 rats was divided into three experimental groups of 20 animals per group and randomly exposed to pulsed ultrasound. The groups were exposed as follows: Group 1 to 0 degree polarity pulses (compression followed by rarefraction) at a pr(in situ) of 3.48 MPa and a PII(in situ) of 4.78 Ws/m2, group 2 to 180 degree polarity pulses (rarefraction followed by compression) at a pr(in situ) of 3.72 MPa and a PII(in situ) of 2.55 Ws/m2, and group 3 to 180 degree polarity pulses at a pr(in situ) of 4.97 MPa and a PII(in situ) of 4.79 Ws/m2. For all experimental groups, the frequency was 2.46 MHz, the exposure duration was 240 s, the pulse repetition frequency was 2.5 kHz, and the pulse duration was 0.42 micros. Six sham animals were also randomly distributed among the experimental animals. The lesion surface area and depth were determined for each rat as well as lesion occurrence (percentage of rats with lesions) per group. It was found that lesion occurrence and size correlated better with PII(in situ) than with pr(in situ), suggesting that a mechanism other than inertial cavitation was responsible for the damage.

Superthreshold behavior and threshold estimation of ultrasound-induced lung hemorrhage in pigs: role of age dependency.

Age-dependent threshold and superthreshold behavior of ultrasound-induced lung hemorrhage were investigated with 116 2.1 +/- 0.3-kg neonate crossbred pigs (4.9 +/- 1.6 days old), 103 10 +/- 1.1-kg crossbred pigs (39 +/-5 days old), and 104 20+/-1.2-kg crossbred pigs (58 +/- 5 days old). Exposure conditions were: 3.1 MHz, 10-s exposure duration, 1-kHz pulse repetition frequency (PRF), and 1.2-micros pulse duration. The in situ (at the pleural surface) peak rarefactional pressure ranged between 2.2 and 10.4 MPa with either eight or nine acoustic pressure groups for each of the three pig ages (12 pigs/exposure group) plus sham exposed pigs. There were no lesions in the shams. Pigs were exposed bilaterally with the order of exposure (left then right lung, or right then left lung) and acoustic pressure both randomized. Pig age was not randomized. Individuals involved in animal handling, exposure, and lesion scoring were blinded to the exposure condition. Logistic regression analysis was used to examine the dependence of the lesion incidence rates on in situ peak rarefactional pressure, left versus right lung exposure, order of exposure (first versus second), and age in three age groups. Likewise, lesion depth and lesion root surface area were analyzed using Gaussian tobit regression analysis. A significant threshold effect on lesion occurrence was observed as a function of age; younger pigs were less susceptible to lung damage given equivalent in situ exposure. Overall, the oldest pigs had a significantly lower threshold (2.87 +/- 0.29 MPa) than middle-aged pigs (5.83 +/- 0.52 MPa). The oldest pigs also had a lower threshold than neonate pigs (3.60 +/- 0.44 MPa). Also, an unexpected result was observed. The ultrasound exposures were bilateral, and the threshold results reported above were based on the lung that was first exposed. After the first lung was exposed, the pig was turned over and the other lung was exposed to the same acoustic pressure. There was a significant decrease (greater than the confidence limits) in occurrence thresholds: 3.60 to 2.68, 5.83 to 2.97, and 2.87 to 1.16 MPa for neonates, middle-aged, and oldest pigs, respectively, in the second lung exposed. Thus, a subtle effect in lung physiology resulted in a major effect on lesion thresholds.

Ultrasound-induced lung hemorrhage: role of acoustic boundary conditions at the pleural surface.

In a previous study [J. Acoust. Soc. Am. 108, 1290 (2000)] the acoustic impedance difference between intercostal tissue and lung was evaluated as a possible explanation for the enhanced lung damage with increased hydrostatic pressure, but the hydrostatic-pressure-dependent impedance difference alone could not explain the enhanced occurrence of hemorrhage. In that study, it was hypothesized that the animal's breathing pattern might be altered as a function of hydrostatic pressure, which in turn might affect the volume of air inspired and expired. The acoustic impedance difference between intercostal tissue and lung would be affected with altered lung inflation, thus altering the acoustic boundary conditions. In this study, 12 rats were exposed to 3 volumes of lung inflation (inflated: approximately tidal volume; half-deflated: half-tidal volume; deflated: lung volume at functional residual capacity), 6 rats at 8.6-MPa in situ peak rarefactional pressure (MI of 3.1) and 6 rats at 16-MPa in situ peak rarefactional pressure (MI of 5.8). Respiration was chemically inhibited and a ventilator was used to control lung volume and respiratory frequency. Superthreshold ultrasound exposures of the lungs were used (3.1-MHz, 1000-Hz PRF, 1.3-micros pulse duration, 10-s exposure duration) to produce lesions. Deflated lungs were more easily damaged than half-deflated lungs, and half-deflated lungs were more easily damaged than inflated lungs. In fact, there were no lesions observed in inflated lungs in any of the rats. The acoustic impedance difference between intercostal tissue and lung is much less for the deflated lung condition, suggesting that the extent of lung damage is related to the amount of acoustic energy that is propagated across the pleural surface boundary.

Response to “Comment on ‘Ultrasound-induced lung hemorrhage is not caused by inertial cavitation’ ” [J. Acoust. Soc. Am. 110, 1737 (2001)

We reply to the preceding letter [R. E. Apfel, J. Acoust. Soc. Am. 110, 1737 (2001)].

Superthreshold behavior of ultrasound-induced lung hemorrhage in adult mice and rats: role of pulse repetition frequency and exposure duration

Superthreshold behavior for ultrasound-induced lung hemorrhage was investigated in adult mice and rats at an ultrasound center frequency of 2.8 MHz to assess the role of pulse repetition frequency and exposure duration. One hundred fifty, 6–7-week-old female ICR mice and 150 10–11-week-old female Sprague-Dawley rats were each divided into 15 exposure groups (10 animals per group) for a 3 × 5 factorial design (3 exposure durations of 5, 10, and 20 s and 5 pulse repetition frequencies of 25, 50, 100, 250, and 500 Hz). The in situ (at the pleural surface) peak rarefactional pressure of 12.3 MPa and the pulse duration of 1.42 μs were the same for all ultrasonically-exposed animals. In addition, 15 sham exposed mice and 15 sham exposed rats were included into both studies. In each study of 165 animals, the exposure conditions were randomized. The lesion depth and surface area were measured for each animal, as well as the percentage of animals with lesions per group. The characteristics of the lesions produced in mice and rats were similar to those described in studies by our research group and others, suggesting a common pathogenesis for the initiation and propagation of the lesions at the gross and microscopic levels. The proportion of lesions in both species was related statistically to pulse repetition frequency (PRF) and exposure duration (ED), with the exception that PRF in rats was not quite significant; the PRF × ED interaction (number of pulses) for lesion production was not significant for either species. The PRF, but not ED, significantly affected lesion depth in both species; the PRF × ED interaction for depth was not significant for either species. Both PRF and ED significantly affected lesion surface area in mice, while neither affected area in rats; the PRF × ED interaction for surface area was not significant for either species. (E-mail: wdo@uiuc.edu)

Superthreshold behavior and threshold estimates of ultrasound-induced lung hemorrhage in adult rats: role of beamwidth.

It is well documented that ultrasound-induced lung hemorrhage can occur in mice, rats, rabbits, pigs, and monkeys. The objective of this study was to assess the role of the ultrasound beamwidth (beam diameter incident on the lung surface) on lesion threshold and size. A total of 144 rats were randomly exposed to pulsed ultrasound at three exposure levels and four beamwidths (12 rats per group). The three in situ peak rarefactional pressures were about 5, 7.5, and 10 MPa. The four 19-mm-diameter focused transducers had measured pulse-echo -6-dB focal beamwidths of 470 microm (2.8 MHz; f/1), 930 microm (2.8 MHz; f/2), 310 microm (5.6 MHz; f/1), and 510 microm (5.6 MHz; f/2). Exposure durations were 10 s, pulse repetition frequencies were 1 kHz, and pulse durations were 1.3 micros (2.8 MHz; f/1), 1.2 micros (2.8 MHz; f/2), 0.8 micros (5.6 MHz; f/1) and 1.1 micros (5.6 MHz; f/2). The lesion surface area and depth were measured for each rat as well as the percentage of rats with lesions per group. Logistic regression analysis and Gaussian-Tobit analysis methods were used to analyze the data. The effects of in situ peak rarefactional pressure and beamwidth were highly significant, but ultrasonic frequency was not significant. In addition, the estimated interaction between in situ peak rarefactional pressure and beamwidth was positive and highly significant. The ultrasound beamwidth incident on the lung surface was shown to strongly affect the percentage and size of ultrasound-induced lung hemorrhage lesions. Even though ultrasonic frequency was an experimental variable, it was not shown to affect the lesion percentage or size.

Ultrasound-induced lung hemorrhage is not caused by inertial cavitation.

In animal experiments, the pathogenesis of lung hemorrhage due to exposure to clinical diagnostic levels of ultrasound has been attributed to an inertial cavitation mechanism. The purpose of this article is to report the results of two experiments that directly contradict the hypothesis that ultrasound-induced lung hemorrhage is caused by inertial cavitation. Elevated hydrostatic pressure was used to suppress the involvement of inertial cavitation. In experiment one, 160 adult mice were equally divided into two hydrostatic pressure groups (0.1 or 1.1 MPa), and were randomly exposed to pulsed ultrasound (2.8-MHz center frequency, 1-kHz PRF, 1.42-micros pulse duration, 10-s exposure duration). For the two hydrostatic pressure groups (80 mice each), 8 in situ peak rarefactional pressure levels were used that ranged between 2.82 and 11.8 MPa (10 mice/group). No effect of hydrostatic pressure on the probability of hemorrhage was observed. These data lead to the conclusion that lung hemorrhage is not caused by inertial cavitation. Also, the higher hydrostatic pressure enhanced rather than inhibited the impact of ultrasonic pressure on the severity (hemorrhage area, depth, and volume) of lesions. These counterintuitive findings were confirmed in a second experiment using a 2 x 5 factorial design that consisted of two ultrasonic pressure levels and five hydrostatic pressure levels (100 mice, 10 mice/group). If inertial cavitation were the mechanism responsible for lung hemorrhage, then elevated hydrostatic pressures should have resulted in less rather than more tissue damage at each ultrasonic pressure level. This further supports the conclusion that the pathogenesis of ultrasound-induced lung hemorrhage is not caused by inertial cavitation.

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)