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

Mechanical and Biological Effects of Ultrasound: A Review of Present Knowledge

Ultrasound is widely used for medical diagnosis and increasingly for therapeutic purposes. An understanding of the bio-effects of sonography is important for clinicians and scientists working in the field because permanent damage to biological tissues can occur at high levels of exposure. Here the underlying principles of thermal mechanisms and the physical interactions of ultrasound with biological tissues are reviewed. Adverse health effects derived from cellular studies, animal studies and clinical reports are reviewed to provide insight into the in vitro and in vivo bio-effects of ultrasound.

Ultrasonic imaging: safety considerations

Modern ultrasound imaging for diagnostic purposes has a wide range of applications. It is used in obstetrics to monitor the progress of pregnancy, in oncology to visualize tumours and their response to treatment, and, in cardiology, contrast-enhanced studies are used to investigate heart function and physiology. An increasing use of diagnostic ultrasound is to provide the first photograph for baby's album-in the form of a souvenir or keepsake scan that might be taken as part of a routine investigation, or during a visit to an independent high-street 'boutique'. It is therefore important to ensure that any benefit accrued from these applications outweighs any accompanying risk, and to evaluate the existing ultrasound bio-effect and epidemiology literature with this in mind. This review considers the existing laboratory and epidemiological evidence about the safety of diagnostic ultrasound and puts it in the context of current clinical usage.

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.

The risk of exposure to diagnostic ultrasound in postnatal subjects: nonthermal mechanisms

This review examines the nonthermal physical mechanisms by which ultrasound can harm tissue in postnatal patients. First the physical nature of the more significant interactions between ultrasound and tissue is described, followed by an examination of the existing literature with particular emphasis on the pressure thresholds for potential adverse effects. The interaction of ultrasonic fields with tissue depends in a fundamental way on whether the tissue naturally contains undissolved gas under normal physiologic conditions. Examples of gas-containing tissues are lung and intestine. Considerable effort has been devoted to investigating the acoustic parameters relevant to the threshold and extent of lung hemorrhage. Thresholds as low as 0.4 MPa at 1 MHz have been reported. The situation for intestinal damage is similar, although the threshold appears to be somewhat higher. For other tissues, auditory stimulation or tactile perception may occur, if rarely, during exposure to diagnostic ultrasound; ultrasound at similar or lower intensities is used therapeutically to accelerate the healing of bone fractures. At the exposure levels used in diagnostic ultrasound, there is no consistent evidence for adverse effects in tissues that are not known to contain stabilized gas bodies. Although modest tissue damage may occur in certain identifiable applications, the risk for induction of an adverse biological effect by a nonthermal mechanism due to exposure to diagnostic ultrasound is extremely small.

Circumferential lesion formation around the pulmonary veins in the left atrium with focused ultrasound using a 2D-array endoesophageal device: a numerical study

Atrial fibrillation (AF) is the most frequently sustained cardiac arrhythmia affecting humans. The electrical isolation by ablation of the pulmonary veins (PVs) in the left atrium (LA) of the heart has been proven as an effective cure of AF. The ablation consists mainly in the formation of a localized circumferential thermal coagulation of the cardiac tissue surrounding the PVs. In the present numerical study, the feasibility of producing the required circumferential lesion with an endoesophageal ultrasound probe is investigated. The probe operates at 1 MHz and consists of a 2D array with enough elements (114 x 20) to steer the acoustic field electronically in a volume comparable to the LA. Realistic anatomical conditions of the thorax were considered from the segmentation of histological images of the thorax. The cardiac muscle and the blood-filled cavities in the heart were identified and considered in the sound propagation and thermal models. The influence of different conditions of the thermal sinking in the LA chamber was also studied. The circumferential ablation of the PVs was achieved by the sum of individual lesions induced with the proposed device. Different scenarios of lesion formation were considered where ultrasound exposures (1, 2, 5 and 10 s) were combined with maximal peak temperatures (60, 70 and 80 degrees C). The results of this numerical study allowed identifying the limits and best conditions for controlled lesion formation in the LA using the proposed device. A controlled situation for the lesion formation surrounding the PVs was obtained when the targets were located within a distance from the device in the range of 26 +/- 7 mm. When combined with a maximal temperature of 70 degrees C and an exposure time between 5 and 10 s, this distance ensured preservation of the esophageal structures, controlled lesion formation and delivery of an acoustic intensity at the transducer surface that is compatible with existing materials. With a peak temperature of 70 degrees C, the device and setup presented here induced highly localized lesions with a lesion volume varying from 10 +/- 4 to 18 +/- 7 mm(3) for an ultrasound exposure between 5 and 10 s, respectively, while the intensity varied from 26 +/- 7 to 20 +/- 6 W cm(-2).

Evaluation of the threshold for lung hemorrhage by diagnostic ultrasound and a proposed new safety index

In a recent report (O'Brien et al. (2006b), it was suggested that the current expression for the mechanical index (MI) was not well suited to its function of quantifying the likelihood of an adverse biological effect after exposure of the gas-filled lung to diagnostic ultrasound. The purpose of this study was to analyze the relatively large database of experimental thresholds for the induction of lung hemorrhage to: (i) determine which variable(s) best describe the data and (ii) use the resulting equation to obtain a new formulation for the MI for lung exposures. Data from 14 studies of lung hemorrhage in four common laboratory animals (mouse, rat, rabbit and pig) were tabulated with regard to five common acoustic variables: center frequency (f(c)), pulse repetition frequency (PRF), pulse duration (PD), exposure duration (ED) and the threshold in situ peak rarefactional pressure (p(r)). The 34 threshold data points were fit by linear regression to: (i) a multiplicative model of the other variables, p(r) = Af(c)(B)PRF(C)PD(D)ED(E), where A is a constant; (ii) 14 "reduced" models in which one or more variables were not included in the analysis; (iii) four models in which a multiplicative combination of variables has a common name e.g., duty factor; and (iv) the general form of the current expression for the MI. The MI was shown to provide a poor fit to the threshold data (r(2) = 0.382), as were three of the four named models. The best fits were found for the complete model and for three reduced models, all of which contain the exposure duration. Because the implementation of a time-dependent safety parameter would present significant practical difficulties, a different model, p(r) = Af(c)(B)PRF(C)PD(D), was chosen as the basis for the new MI. Thus, the expression for the lung-specific mechanical index, MI(Lung), includes several, rather than only one, of the relevant acoustic variables. This is the first potential safety index developed as a direct result of experimental measurements rather than theoretical analysis.

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-micros 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.

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.

Vascular lesions and s-thrombomodulin concentrations from auricular arteries of rabbits infused with microbubble contrast agent and exposed to pulsed ultrasound

Arterial injury resulting from the interaction of contrast agent (CA) with ultrasound (US) was studied in rabbit auricular arteries and assessed by histopathologic evaluation and s-thrombomodulin concentrations. Three sites on each artery were exposed (2.8 MHz, 5-min exposure duration, 10-Hz pulse repetition frequency, 1.4-mus pulse duration) using one of three in situ peak rarefactional pressures (0.85, 3.9 or 9.5 MPa). Saline, saline/CA, and saline/US infusion groups (n = 28) did not have histopathologic damage. The saline/CA/US infusion group (n = 10) at exposure conditions below the FDA mechanical index limit of 1.9 did not have histopathologic damage, whereas the saline/CA/US infusion group (n = 9) at exposure conditions above the FDA limit did have damage (5 of 9 arteries). Lesions were characteristic of acute coagulative necrosis. Mean s-thrombomodulin concentrations, a marker for endothelial cell injury, were highest in rabbits exposed to US at 0.85 and 3.9 MPa, suggesting that vascular injury may be physiological and not accompanied by irreversible cellular injury.

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

OBJECTIVE

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.

METHODS

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.

RESULTS

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.

CONCLUSIONS

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: role of pulse repetition frequency and exposure duration revisited

OBJECTIVE

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).

METHODS

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.

RESULTS

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).

CONCLUSIONS

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.

Effect of contrast agent on the incidence and magnitude of ultrasound-induced lung hemorrhage in rats

OBJECTIVE

To test the hypothesis that inertial cavitation in the vasculature of the lung is not the physical mechanism responsible for ultrasound-induced lung hemorrhage.

METHODS

A factorial design was used to study the effects of two types of injected agents (IA; 0.25 ml per rat of saline or Optison given intravenously) and two levels of pulsed ultrasound exposure (UE; in situ peak rarefactional pressures of 2.74 and 5.86 MPa; respective mechanical indices of 1.02 and 2.14) on the incidence and size of lung lesions. Ten 10-to-11-week-old Sprague-Dawley rats were exposed to pulsed ultrasound at each of the four combinations of IA and UE at a center frequency of 3.1 MHz, exposure duration of 10 s, pulse repetition frequency of 1,000 Hz and pulse duration of 1.2 micros. In addition, nine rats served as shams in which no lung hemorrhage occurred.

RESULTS

Rats administered contrast agent prior to exposure did not have an increase in lesion occurrence or size compared to rats that received saline with no contrast agent.

CONCLUSIONS

These results provide further evidence that the mechanism for production of lung hemorrhage is not inertial cavitation.

Excess risk thresholds in ultrasound safety studies: statistical methods for data on occurrence and size of lesions

Concerns about the safe use of clinical ultrasound (US) at diagnostic pressure levels (below a mechanical index, or MI, = 1.9) have stimulated considerable research in US risk assessment. The objective of the present study was to develop probability-based risk thresholds for US safety studies, to present statistical methods for estimating the thresholds and their standard errors and to compare these methods with the analysis based on a piecewise linear ("hockey stick") model. The excess risk at exposure level x > 0 was defined as the relative increase in the probability of a lesion at that level compared with the background probability of a lesion at exposure x = 0. The risk threshold was then defined as the exposure level at which the excess risk exceeded a specified level (e.g. 5% or 50%). Thus, given pressure-dependent estimates of the excess risk, the thresholds were estimated by solving the risk equation to obtain the pressure at which the target level of excess risk occurs. Threshold estimates of this type have been developed extensively in the literature for incidence (presence or absence) data. Only recently, however, have excess risk threshold estimates been derived for data in which lesion size (depth, surface area) is measured if present and a zero is recorded if the lesion is absent. Tobit regression was used to estimate pressure-dependent percentiles of the size distribution, and the excess risks were estimated from the tobit probability of a positive-valued response. The tobit model provides a well-established approach to modeling data constrained to be nonnegative. Solving the risk equation for the tobit model leads to risk threshold estimates that incorporate the information on size of observed lesions. Results using these probability-based risk estimates were compared with results for a piecewise linear ("hockey stick") model, which has also been used in the US safety literature, although it does not explicitly address the nonnegativity constraint in the sampling model. The comparisons were carried out for data from two previously published studies, from different laboratories, on US-induced lung hemorrhage. The thresholds derived from logistic regression of lesion occurrence and tobit regression of lesion size were quite consistent with each other and within sampling error. The hockey stick thresholds, defined as the exposure level at which the piecewise linear model for the probability of the expected size of a lesion bends upward, corresponded to quite different excess risk values for incidence (lesion occurrence) compared with size (lesion surface area or depth), although these methods have been developed previously for both types of data. The use of probability-based excess risk thresholds is recommended to obtain consistent incidence vs. size thresholds and to ensure that the thresholds are well-defined and interpretable independent of the details of the statistical model.

Mechanical Bioeffects of Ultrasound

Ultrasound is used widely in medicine as both a diagnostic and therapeutic tool. Through both thermal and nonthermal mechanisms, ultrasound can produce a variety of biological effects in tissues in vitro and in vivo. This chapter provides an overview of the fundamentals of key nonthermal mechanisms for the interaction of ultrasound with biological tissues. Several categories of mechanical bioeffects of ultrasound are then reviewed to provide insight on the range of ultrasound bioeffects in vivo, the relevance of these effects to diagnostic imaging, and the potential application of mechanical bioeffects to the design of new therapeutic applications of ultrasound in medicine.

Impedance measurements of ex vivo rat lung at different volumes of inflation

A previous study [J. Acoust. Soc. Am. 111, 1102-1109 (2002)] showed that the occurrence of ultrasonically induced lung hemorrhage in rats was directly correlated to the level of lung inflation. In that study, it was hypothesized that the lung could be modeled as two components consisting of air and parenchyma (contiguous tissue [pleura and septa]). The speed of sound and lung impedance would then depend on the fractional volume of air in the lung. According to that model, an inflated lung should act like a pressure-release surface for sound incident from tissue onto a tissue-lung boundary. A deflated lung containing less air should allow more acoustic energy into the lung tissue because the impedance was more closely matched to the contiguous tissues. In the study reported herein, a measurement technique was devised to calculate the impedance of seven rat lungs, ex vivo, under deflation (atmospheric pressure) and three volumes of inflation pressure (7-cm H2O, 10-cm H2O, and 15-cm H2O). Lungs were dissected from rats and immediately scanned in a tank of degassed 37 degrees C water. The frequency-dependent acoustic pressure reflection coefficient was measured over a frequency range of 3.5 to 10 MHz. From the reflection coefficient, the frequency-dependent lung impedance was calculated with values ranging from an average of 1 Mrayls in deflated lungs to 0.2 Mrayls for fully inflated lungs. Lung impedance calculations showed that deflated lungs had an impedance closer to water (1.52 Mrayls) than inflated lungs. At all volumes of inflation, the lungs acted as pressure-release surfaces relative to the water. The average of the four lung impedance values (deflated, 7-cm H2O, 10-cm H2O, and 15-cm H2O) at each level of inflation was statistically different (p<0.0001).

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 microm) were divided into four PD groups (1.3, 4.4, 8.2 and 11.6 micros) 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 micros, respectively, lesion occurrence ED(05)s were 3.1, 2.8, 2.3 and 2.0 MPa with standard errors around 0.6 MPa. Lesion size ED(05)s 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 micros had ED(05)s more than 2 standard errors below 4.0 MPa, indicating that the ED(05)s of these two PDs are statistically significantly different from 4.0 MPa. The ED(05) threshold levels for a PD of 1.3 micros 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(05)s are less than the FDA limit.

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.

Arrhythmias in rat hearts exposed to pulsed ultrasound after intravenous injection of a contrast agent

OBJECTIVE

To develop an animal model suitable for characterizing electrocardiographic arrhythmias in hearts exposed to ultrasound after injection of a microbubble contrast agent.

METHODS

Conduction complex and heart lesion data were recorded from 20 rats that received intravenous injections of 0.25 mL of a contrast agent and were exposed to pulsed ultrasound (frequency, 3.1 MHz; pulse duration, 1.3 microseconds; pulse repetition frequency, 1700 Hz; and in situ peak rarefactional pressure, 15.9 MPa). The volume of the contrast agent based on body weight and the mechanical index (ultrasonic pressure) exceeded those used in echocardiography by 14 to 345 and 3 to 29 times, respectively.

RESULTS

Premature atrial complexes, premature ventricular complexes, or polymorphic ventricular tachycardia occurred in 10 rats. When ultrasound exposure was halted, arrhythmias ceased but reoccurred in 4 of the 10 rats when exposure resumed. Myocardial degeneration identified by histochemical staining (hematoxylin-basic fuchsinpicric acid) was observed in 16 rats; however, only 10 rats had arrhythmias. There was no significant difference in the amount of histochemical staining in hearts from rats with arrhythmias when compared with rats without arrhythmias.

CONCLUSIONS

An animal model suitable for characterizing electrocardiographic arrhythmias in rat hearts exposed to ultrasound after injection of a microbubble contrast agent was developed. Because arrhythmias were induced principally when the contrast agent interacted with ultrasound during exposure, the presence of myocardial degeneration alone was not a sufficient explanation for ectopic electrical activity. Under these extreme exposure conditions, the data suggest that pulsed ultrasound through its biomechanical interactions with contrast agents has the potential to induce arrhythmias.

Attenuation coefficient and propagation speed estimates of rat and pig intercostal tissue as a function of temperature

Attenuation coefficient and propagation speed of intercostal tissues were estimated as functions of temperature (22, 30, and 37 degrees C) from fresh chest walls from eight 10- to 11-week-old female Sprague-Dawley (SD) rats, eight 21- to 24-week-old female Long-Evans (LE) rats, and ten 6- to 10-week-old mixed sex Yorkshire (York) pigs. The primary purpose of the study was to estimate the temperature dependence of the intercostal tissue's attenuation coefficient so that accurate estimates of the in situ (at the pleural surface) acoustic pressure levels could be made for our ultrasound-induced lung hemorrhage studies. The attenuation coefficient of intercostal tissue for both species was independent of the temperature at the discrete frequencies of 3.1 MHz (-0.0076, 0.0065, and 0.016 dB/cm/degrees C for SD rats, LE rats, and York pigs, respectively) and 6.2 MHz (-0.015, 0.014, and 0.014 dB/cm/degrees C for SD rats, LE rats, and York pigs, respectively). However, the temperature-dependent regressions yielded a significant temperature dependency of the intercostal tissue attenuation coefficients in SD and LE rats (over the 3.1 to 9.6 MHz frequency range); there was no temperature dependency in York pigs (over the 3.1 to 8.6 MHz frequency range). There was no significant temperature dependency of the intercostal tissue propagation speed in SD rats; there was a temperature dependency in LE rats and York pigs (-0.59, -1.6, and -2.9 m/s/degrees C for SD rats, LE rats, and York pigs, respectively). Even though the attenuation coefficient's temperature dependency was significant from the linear regression functions, the differences were not very great (-0.040 to -0.13, 0.011 to 0.18, and 0.055 to 0.10 dB/cm/degrees C for SD rats, LE rats, and York pigs, respectively, over the data frequency range). These findings suggest that it is not necessary to determine the attenuation coefficient of intercostal tissue at body temperature (37 degrees C), but rather it is sufficient to determine the attenuation coefficient at room temperature (22 degrees C), a much easier experimental procedure.

Attenuation coefficient and propagation speed estimates of intercostal tissue as a function of pig age

Attention coefficient and propagation speed of intercostal tissues were estimated from chest walls removed postmortem (pm) from 15 5.3+/-2.3-day-old, 19 31+/-6-day-old, and 15 61+/-3-day-old crossbred pigs. These ultrasonic propagation properties were determined from measurements through the intercostal tissues, from the surface of the skin to the parietal pleura. The chest walls were placed in a 0.9% sodium chloride solution, sealed in freezer bags, and stored at -15 degrees C prior to measurements. When evaluated, chest-wall storage time ranged between 1 and 477 days pm. All chest walls were allowed to equilibrate to 22 degrees C in a water bath prior to evaluation. There was an age dependency of the intercostal tissue propagation speed, with the speed increasing with increasing age. The attenuation coefficient of intercostal tissue was shown to be independent of the age of the pig at the discrete frequencies of 3.1 and 6.2 MHz. For pig intercostal tissues, the estimated attenuation coefficient over the 3.1-9.2 MHz frequency range was A = 1.94f(0.90) where A is in decibels per centimeter (dB/cm) and f is the ultrasonic frequency in megahertz. In order to determine if there was an effect of storage time pm on estimates of attenuation coefficient, a second experiment was conducted. Five of the youngest pig chest walls measured on day 1 pm in the first experiment were stored at 4 degrees C prior to the first evaluation then stored at -15 degrees C before being measured again at 108 days pm. There was no difference in the estimated intercostal tissue attenuation coefficient as a function of storage time pm.

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.

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.

Temporal and spatial evaluation of lesion reparative responses following superthreshold exposure of rat lung to pulsed ultrasound

This study characterized the reparative responses in rat lung. Forty-five adult female rats were exposed at two sites over the left lung to 3.1-MHz superthreshold pulsed ultrasound. The repair of lung lesions was evaluated from 0 through 44 days postexposure. Macroscopic lesions at 0 days postexposure were large bright red ellipses of hemorrhage. By 1 and 3 days postexposure, lesions were the same size and dark red to red-black, but, by 3 days postexposure, lesions had a raised surface appearance. From 5 to 10 days postexposure, lesions grew smaller in size, progressed from red-gray to yellow-brown, and retained a raised surface appearance. From 13 through 44 days postexposure, lesions gradually decreased in size, had a faint yellow-brown discoloration, and gradually lost the raised surface appearance. By 37 and 44 days postexposure, lung returned to near normal morphology, but had small areas of light yellow-brown discoloration in the areas where lung was exposed. Microscopic lesions at 0 and 1 days postexposure were areas of acute alveolar hemorrhage. By 3 days postexposure, lesions had loss of alveolar erythrocytes and the formation of hemoglobin crystals. From 5 through 44 days postexposure, iron in degraded erythrocytes was processed to hemosiderin and was negligible in quantity at 44 days postexposure. The proliferation of resident cells (likely alveolar epithelial cells, fibroblasts and endothelial cells) and the infiltration of inflammatory cells in lesions declined in intensity as the lesions aged and was minimal by 44 days postexposure. Under the superthreshold exposure conditions described, lesions induced by ultrasound do not seem to have long-term residual effects in lung.