Superthreshold behavior and threshold estimation of ultrasound-induced lung hemorrhage in adult mice and rats

State of the Art in Lung Ultrasound, Shifting from Qualitative to Quantitative Analyses

Lung ultrasound (LUS) has been increasingly expanding since the 1990s, when the clinical relevance of vertical artifacts was first reported. However, the massive spread of LUS is only recent and is associated with the coronavirus disease 2019 (COVID-19) pandemic, during which semi-quantitative computer-aided techniques were proposed to automatically classify LUS data. In this review, we discuss the state of the art in LUS, from semi-quantitative image analysis approaches to quantitative techniques involving the analysis of radiofrequency data. We also discuss recent in vitro and in silico studies, as well as research on LUS safety. Finally, conclusions are drawn highlighting the potential future of LUS.

How safe is it to use ultrasound in prenatal medicine? Facts and contradictions - Part 2 - Laboratory experiments regarding non-thermal effects and epidemiological studies

The first part of this CME article (issue 5/20) provided a detailed examination of the biophysical effects of ultrasound waves, the exposure values, and in particular the thermal effect. In vivo and in vitro measurements have shown that the temperature increase in tissue associated with B-mode ultrasound is far too low to pose a potential risk. Even experiments with exposure values in the range of pulsed Doppler have shown that temperature increases of over 1.5 °C can only occur in areas in direct contact with the probe, thus making a limited exposure time particularly in the case of transvaginal application advisable. The second part of this CME article describes various laboratory and animal experiments for evaluating non-thermal effects and also presents the most important epidemiological studies in the last 30 years in the form of an overview and review. In addition to direct insonation of isolated cells to examine possible mutagenic effects, the blood of patients exposed in vivo to ultrasound was also analyzed in multiple experiments. Reproducible chromosome aberrations could not be found in any of the studies. In contrast, many experiments on pregnant rodents showed some significant complications, such as abortion, deformities, and behavioral disorders. As in the case of thermal effects, the results of these experiments indicate the presence of an intensity- or pressure-dependent effect threshold. Numerous epidemiological studies examining possible short-term and long-term consequences after intrauterine ultrasound exposure are available with the most important studies being discussed in the following. In contrast to information presented incorrectly in the secondary literature and in the lay press, health problems could not be seen in the children observed in the postpartum period in any of these studies.

How Safe Is the Use of Ultrasound in Prenatal Medicine? Facts and Contradictions. Part 1 - Ultrasound-Induced Bioeffects

The "Ordinance on Protection Against the Harmful Effects of Non-Ionizing Radiation in Human Applications" will go into effect at the beginning of 2021 1. § 10 of this ordinance prohibits non-medical fetal ultrasound exposure thereby resulting in uncertainty, particularly among affected patients, with respect to the generally accepted theory regarding the lack of ultrasound side effects. Although not a single study has shown a detrimental effect on fetal or child development following exposure to ultrasound, the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety has justified the ban with the purely hypothetical possibility of an unidentified side effect. The first part of the following study shows which ultrasound-induced biophysical effects are known and which dose-dependent threshold values must be taken into consideration. In particular, the study focuses on the well-researched heat effect with some in vivo measurements in humans showing that the actual temperature increase is less than the theoretically calculated values. The planned second part of this study will discuss the non-thermal effects and present the most important epidemiological studies.

Experimental Implementation of a Pulse Compression Technique Using Coherent Plane-Wave Compounding

The axial resolution of an ultrasound imaging system is inversely proportional to the bandwidth of the emitted signal. When conventional pulsing (CP) is used, the impulse response of the transducer and the excitation signal determine together the shape of the emitted pulse and its bandwidth. A way to increase the ultrasound image resolution is to increase the transducer's limited passband. The resolution enhancement compression (REC) is a coding technique that boosts the signal energy in the transition frequency bands, where the energy transduction of the ultrasound probe is less efficient. Consequently, image quality metrics including axial resolution, signal-to-noise ratio (SNR), and contrast-to-noise ratio (CNR) can be improved. In this paper, the objective is to combine REC with coherent plane-wave compounding (CPWC) in order to achieve better image quality at an ultrafast acquisition rate. Promising results are obtained from both wire and cyst phantoms using an excitation signal designed to provide a 54% increase in bandwidth over the one obtained with a broadband pulse excitation at -6 dB. The experimental bandwidth measured from the backscattered echoes was improved by 49% for the wire phantom, when using the CPWC-REC technique compared to CPWC-CP. Furthermore, the axial resolution as derived from the modulation transfer function of the envelope of the wire target was enhanced by 29%. The CNR and SNR were improved up to 9 and up to 4 dB, respectively, in the cyst phantom. These results reveal that CPWC-REC is able to achieve higher spatial resolution, compared to CPWC-CP, with better SNR and CNR. Moreover, experimental results show that an effective implementation on a research scanner of REC using plane-wave imaging is possible. Consistent in vivo acquisition results on rabbit are presented and discussed.

Pulmonary Capillary Hemorrhage Induced by Different Imaging Modes of Diagnostic Ultrasound

The induction of pulmonary capillary hemorrhage (PCH) is a well-established non-thermal biological effect of pulsed ultrasound in animal models. Typically, research has been done using laboratory pulsed ultrasound systems with a fixed beam and, recently, by B-mode diagnostic ultrasound. In this study, a GE Vivid 7 Dimension ultrasound machine with 10 L linear array probe was used at 6.6 MHz to explore the relative PCH efficacy of B-mode imaging, M-mode (fixed beam), color angio mode Doppler imaging and pulsed Doppler mode (fixed beam). Anesthetized rats were scanned in a warmed water bath, and thresholds were determined by scanning at different power steps, 2 dB apart, in different groups of six rats. Exposures were performed for 5 min, except for a 15-s M-mode group. Peak rarefactional pressure amplitude thresholds were 1.5 MPa for B-mode and 1.1 MPa for angio Doppler mode. For the non-scanned modes, thresholds were 1.1 MPa for M-mode and 0.6 MPa for pulsed Doppler mode with its relatively high duty cycle (7.7 × 10^-3 vs. 0.27 × 10^-3 for M-mode). Reducing the duration of M-mode to 15 s (from 300 s) did not significantly reduce PCH (area, volume or depth) for some power settings, but the threshold was increased to 1.4 MPa. Pulmonary sonographers should be aware of this unique adverse bio-effect of diagnostic ultrasound and should consider reduced on-screen mechanical index settings for potentially vulnerable patients.

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.

Mechanisms for Induction of Pulmonary Capillary Hemorrhage by Diagnostic Ultrasound: Review and Consideration of Acoustical Radiation Surface Pressure

Diagnostic ultrasound can induce pulmonary capillary hemorrhage (PCH) in rats and other mammals. This phenomenon represents the only clearly demonstrated biological effect of (non-contrast enhanced) diagnostic ultrasound and thus presents a uniquely important safety issue. However, the physical mechanism responsible for PCH remains uncertain more than 25 y after its discovery. Experimental research has indicated that neither heating nor acoustic cavitation, the predominant mechanisms for bioeffects of ultrasound, is responsible for PCH. Furthermore, proposed theoretical mechanisms based on gas-body activation, on alveolar resonance and on impulsive generation of liquid droplets all appear unlikely to be responsible for PCH, owing to unrealistic model assumptions. Here, a simple model based on the acoustical radiation surface pressure (ARSP) at a tissue-air interface is hypothesized as the mechanism for PCH. The ARSP model seems to explain some features of PCH, including the approximate frequency independence of PCH thresholds and the dependence of thresholds on biological factors. However, ARSP evaluated for experimental threshold conditions appear to be too weak to fully account for stress failure of pulmonary capillaries, gauging by known stresses for injurious physiologic conditions. Furthermore, consideration of bulk properties of lung tissue suggests substantial transmission of ultrasound through the pleura, with reduced ARSP and potential involvement of additional mechanisms within the pulmonary interior. Although these recent findings advance our knowledge, only a full understanding of PCH mechanisms will allow development of science-based safety assurance for pulmonary ultrasound.

Influence of Scan Duration on Pulmonary Capillary Hemorrhage Induced by Diagnostic Ultrasound

Diagnostic ultrasound can induce pulmonary capillary hemorrhage (PCH) in rats and display this as "comet tail" artifacts (CTAs) after a time delay. To test the hypothesis that no PCH occurs for brief scans, anesthetized rats were scanned using a 6-MHz linear array for different durations. PCH was characterized by ultrasound CTAs, micro-computed tomography (μCT), and measurements of fixed lung tissue. The μCT images revealed regions of PCH, sometimes penetrating the entire depth of a lobe, which were reflected in the fixed tissue measurements. At -3 dB of power, PCH was substantial for 300-s scans, but not significant for 25-s scans. At 0 dB, PCH was not strongly dependent on scan durations of 300 to 10 s. Contrary to the hypothesis, CTAs were not evident during most 10-s scans (p > 0.05), but PCH was significant (p = 0.02), indicating that PCH could occur without evidence of the injury in the images.

Pulmonary Capillary Hemorrhage Induced by Fixed-Beam Pulsed Ultrasound

The induction of pulmonary capillary hemorrhage (PCH) by pulsed ultrasound was discovered 25 y ago, but early research used fixed-beam systems rather than actual diagnostic ultrasound machines. In this study, results of exposure of rats to fixed-beam focused ultrasound for 5 min at 1.5 and 7.5 MHz were compared with recent research on diagnostic ultrasound. One exposure condition at each frequency used 10-μs pulses delivered at 25-ms intervals. Three conditions involved Gaussian modulation of the pulse amplitudes at 25-ms intervals to simulate diagnostic scanning: 7.5 MHz with 0.3- and 1.5-μs pulses at 100- and 500-μs pulse repetition periods, respectively, and 1.5 MHz with 1.7-μs pulses at 500-μs repetition periods. Four groups were tested for each condition to assess PCH areas at different exposure levels and to determine occurrence thresholds. The conditions with identical pulse timing resulted in smaller PCH areas for the smaller 7.5-MHz beam, but both had thresholds of 0.69-0.75 MPa in situ peak rarefactional pressure amplitude. The Gaussian modulation conditions for both 7.5 MHz with 0.3-μs pulses and 1.5 MHz with 1.7-μs pulses had thresholds of 1.12-1.20 MPa peak rarefactional pressure amplitude, although the relatively long 1.5-μs pulses at 7.5 MHz yielded a threshold of 0.75 MPa. The fixed-beam pulsed ultrasound exposures produced lower thresholds than diagnostic ultrasound. There was no clear tendency for thresholds to increase with increasing ultrasonic frequency when pulse timing conditions were similar.

Contrast Ultrasound Imaging of the Aorta Does Not Affect Progression of Atherosclerosis or Cardiovascular Biomarkers in ApoE-/- Mice

OBJECTIVES

Ultrasound contrast agents (UCAs) enhance cardiovascular ultrasound imaging. Adverse biological effects have occurred after administration of UCAs, and more research is needed for a comprehensive understanding of the risks involved. We used the ApoE(-/-) mouse model of atherosclerosis to characterize the effects of ultrasound and UCAs on atherosclerosis and plasma biomarkers.

METHODS

Male ApoE(-/-) mice (8 weeks old; n = 24) were intravenously infused with a UCA (2 × 10(10) Definity microbubbles per hour; Lantheus Medical Imaging, North Billerica, MA) and exposed to 2.8-MHz center frequency ultrasound (10 Hz pulse repetition frequency, 1.4 microseconds pulse duration, 2 minutes exposure duration, and 2 sites) at 1 of 3 derated peak rarefactional pressure amplitudes (0, 1.9, or 3.8 MPa), and then consumed either a chow or Western diet for 4 weeks (n = 4 per group). Blood plasma samples were collected before ultrasound exposure and at 2 and 4 weeks after exposure and assayed for total cholesterol and von Willebrand Factor (vWF). A pathologist measured atheroma thickness in formalin-fixed, hematoxylin-eosin-stained transverse aorta sections and scored them for severity of atherosclerosis.

RESULTS

Plasma total cholesterol initially averaged 286 mg/dL in the Western diet group and increased to 861 mg/dL after 4 weeks on the diet (P < .0001). Total cholesterol did not increase significantly in the chow diet group. Plasma vWF increased after 2 weeks on the Western diet (P < .0001). Atheroma thickness was greater in animals consuming the Western diet than in chow-fed animals (P < .05). Ultrasound had no significant effect on plasma total cholesterol, plasma vWF, or atheroma thickness.

CONCLUSIONS

Contrast ultrasound did not increase the severity of atherosclerosis or alter cardiovascular biomarkers in the ApoE(-/-) mouse model.

Anesthetic techniques influence the induction of pulmonary capillary hemorrhage during diagnostic ultrasound scanning in rats

OBJECTIVES

Pulmonary capillary hemorrhage can be induced by diagnostic ultrasound (US) during direct pulmonary US scanning in rats. The influence of specific anesthetic techniques on this bioeffect was examined.

METHODS

Ketamine plus xylazine has been used previously. In this study, the influence of intraperitoneal injections of ketamine and pentobarbital, inhalational isoflurane, and the supplemental use of xylazine with ketamine and isoflurane was tested. A diagnostic US machine with a 7.6-MHz linear array was used to image the right lung of anesthetized rats in a warmed water bath at different mechanical index (MI) settings. Pulmonary capillary hemorrhage was assessed by measuring comet tail artifacts in the image and by morphometry of the hemorrhagic areas on excised lungs.

RESULTS

Pulmonary capillary hemorrhage was greatest for pentobarbital, lower for inhalational isoflurane, and lowest for ketamine anesthesia, with occurrence thresholds at MIs of about 0.44, 0.8, and 0.8, respectively. Addition of xylazine produced a substantial increase in hemorrhage and a significant proportion of hemorrhage occurrence for ketamine at an MI of 0.7 (P < .01) and for isoflurane at an MI of 0.52 (P < .01).

CONCLUSIONS

Ketamine plus xylazine and pentobarbital yield lower thresholds than ketamine or isoflurane alone by nearly a factor of 2 in MI. These results suggest that the choice of the anesthetic agent substantially modifies the relative risks of pulmonary capillary hemorrhage from pulmonary US.

Quantitative analysis of ultrasound contrast agent postexcitation collapse

An empirically based peak-detection technique is described for statistically analyzing single ultrasound contrast agent collapses. It is shown that microbubbles with postexcitation collapse initially exhibit a stronger principal response on average than those without postexcitation, and that lower insonifying frequencies lead to postexcitation signals which have greater separation from their principal response and persist through more rebounds.

Contrast ultrasound imaging of the aorta alters vascular morphology and circulating von Willebrand factor in hypercholesterolemic rabbits

OBJECTIVES

Ultrasound contrast agents (UCAs) are intravenously infused microbubbles that add definition to ultrasonic images. Ultrasound contrast agents continue to show clinical promise in cardiovascular imaging, but their biological effects are not known with confidence. We used a cholesterol-fed rabbit model to evaluate these effects when used in conjunction with ultrasound (US) to image the descending aorta.

METHODS

Male New Zealand White rabbits (n = 41) were weaned onto an atherogenic diet containing 1% cholesterol, 10% fat, and 0.11% magnesium. At 21 days, rabbits were exposed to contrast US at 1 of 4 pressure levels using either the UCA Definity (Lantheus Medical Imaging, Inc, North Billerica, MA) or a saline control (n = 5 per group). Blood samples were collected and analyzed for lipids and von Willebrand factor (vWF), a marker of endothelial function. Animals were euthanized at 42 days, and tissues were collected for histologic analysis.

RESULTS

After adjustment for pre-exposure vWF, high-level US (in situ [at the aorta] peak rarefactional pressure of 1.4 or 2.1 MPa) resulted in significantly lower vWF 1 hour post exposure (P = .0127; P(adj) < .0762). This difference disappeared within 24 hours. Atheroma thickness in the descending aorta was lower in animals receiving the UCA compared to animals receiving saline.

CONCLUSIONS

Contrast US affected the descending aorta, as evidenced by two separate outcome measures. These results may be a first step in elucidating a previously unknown biological effect of UCAs. Further research is warranted to characterize the effects of this procedure.

Design and performance of Huffman sequences in medical ultrasound coded excitation

This paper deals with coded-excitation techniques for ultrasound medical echography. Specifically, linear Huffman coding is proposed as an alternative approach to other widely established techniques, such as complementary Golay coding and linear frequency modulation. The code design is guided by an optimization procedure that boosts the signal-to-noise ratio gain (GSNR) and, interestingly, also makes the code robust in pulsed-Doppler applications. The paper capitalizes on a thorough analytical model that can be used to design any linear coded-excitation system. This model highlights that the performance in frequency-dependent attenuating media mostly depends on the pulse-shaping waveform when the codes are characterized by almost ideal (i.e., Kronecker delta) autocorrelation. In this framework, different pulse shapers and different code lengths are considered to identify coded signals that optimize the contrast resolution at the output of the receiver pulse compression. Computer simulations confirm that the proposed Huffman codes are particularly effective, and that there are scenarios in which they may be preferable to the other established approaches, both in attenuating and non-attenuating media. Specifically, for a single scatterer at 150 mm in a 0.7-dB/(MHz·cm) attenuating medium, the proposed Huffman design achieves a main-to-side lobe ratio (MSR) equal to 65 dB, whereas tapered linear frequency modulation and classical complementary Golay codes achieve 35 and 45 dB, respectively.

Frequency-dependent evaluation of the role of definity in producing sonoporation of Chinese hamster ovary cells

OBJECTIVES

Sonoporation uses ultrasound (US) and ultrasound contrast agents (UCAs) to enhance cell permeabilization, thereby allowing delivery of therapeutic compounds non-invasively into specific target cells. The objective of this study was to elucidate the biophysical mechanism of sonoporation, specifically the role of UCAs as well as exposure frequency. The inertial cavitation (IC) thresholds of the lipid-shelled octafluoropropane UCA were directly compared to the levels of generated sonoporation to determine the involvement of UCAs in producing sonoporation.

METHODS

Chinese hamster ovary cells were exposed as a monolayer in a solution of the UCA, 500,000-Da fluorescein isothiocyanate-dextran, and phosphate-buffered saline to 30 seconds of pulsed US (pulse duration, 5 cycles; pulse repetition frequency, 10 Hz) at 3 frequencies (0.92, 3.2, and 5.6 MHz). The peak rarefactional pressure (P(r)) was varied over a range from 4 kPa to 4.1 MPa, and 5 to 7 independent replicates were performed at each pressure.

RESULTS

The experimental observations demonstrated that IC was likely not the physical mechanism for sonoporation. Sonoporation activity was observed at pressure levels below the threshold for IC of the UCA (1.27 ± 0.32 MPa at 0.92 MHz, 0.84 ± 0.19 MPa at 3.2 MHz, and 2.57 ± 0.26 MPa at 5.6 MHz) for all 3 frequencies examined. The P(r) values at which the peak sonoporation activity occurred were 1.4 MPa at 0.92 MHz, 0.25 MPa at 3.2 MHz, and 2.3 MPa at 5.6 MHz. The UCA collapse thresholds followed a similar trend. A 1-way analysis of variance test confirmed that sonoporation activity differed among the 3 frequencies examined (P = 10(-8)).

CONCLUSIONS

These results thus suggest that sonoporation is related to linear and/or nonlinear oscillation of the UCA occurring at pressure levels below the IC threshold.

Determination of postexcitation thresholds for single ultrasound contrast agent microbubbles using double passive cavitation detection

This work presents experimental responses of single ultrasound contrast agents to short, large amplitude pulses, characterized using double passive cavitation detection. In this technique, two matched, focused receive transducers were aligned orthogonally to capture the acoustic response of a microbubble from within the overlapping confocal region. The microbubbles were categorized according to a classification scheme based on the presence or absence of postexcitation signals, which are secondary broadband spikes following the principle oscillatory response of the ultrasound contrast agent and are indicative of the transient collapse of the microbubble. Experiments were conducted varying insonifying frequencies (0.9, 2.8, 4.6, and 7.1 MHz) and peak rarefactional pressures (200 kPa to 6.2 MPa) for two types of contrast agents (Definity and Optison). Results were fit using logistic regression analysis to define pressure thresholds where at least 5% and 50% of the microbubble populations collapsed for each frequency. These thresholds were found to occur at lower pressures for Definity than for Optison over the range of frequencies studied; additionally, the thresholds occurred at lower pressures with lower frequencies for both microbubble types in most cases, though this trend did not follow a mechanical index scaling.

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-mus 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).

Examination of inertial cavitation of Optison in producing sonoporation of chinese hamster ovary cells

The objective of this project was to elucidate the relationship between ultrasound contrast agents (UCAs) and sonoporation. Sonoporation is an ultrasound-induced, transient cell membrane permeability change that allows for the uptake of normally impermeable macromolecules. Specifically, this study will determine the role that inertial cavitation plays in eliciting sonoporation. The inertial cavitation thresholds of the UCA, Optison, are compared directly with the results of sonoporation to determine the involvement of inertial cavitation in sonoporation. Chinese hamster ovary (CHO) cells were exposed as a monolayer in a solution of Optison, 500,000 Da fluorescein isothiocyanate-dextran (FITC-dextran), and phosphate-buffered saline (PBS) to 30 s of pulsed ultrasound at 3.15-MHz center frequency, 5-cycle pulse duration and 10-Hz pulse repetition frequency. The peak rarefactional pressure (P(r)) was varied over a range from 120 kPa-3.5 MPa, and five independent replicates were performed at each pressure. As the P(r) was increased, from 120 kPa-3.5 MPa, the fraction of sonoporated cells among the total viable population increased from 0.63-10.21%, with the maximum occurring at 2.4 MPa. The inertial cavitation threshold for Optison at these exposure conditions has previously been shown to be in the range 0.77-0.83 MPa, at which sonoporation activity was found to be 50% of its maximum level. Furthermore, significant sonoporation activity was observed at pressure levels below the threshold for inertial cavitation of Optison. Above 2.4 MPa, a significant drop in sonoporation activity occurred, corresponding to pressures where >95% of the Optison was collapsing. These results demonstrate that sonoporation is not directly a result of inertial cavitation of the UCA, rather that the effect is related to linear and/or nonlinear oscillation of the UCA occurring at pressure levels below the inertial cavitation threshold.

Observations of MI Values During Neonatal Cardiac Ultrasound Scanning

Aim: The on-screen Mechanical Index (MI), was recorded from eight diagnostic ultrasound machines used for cardiac scanning of neonates. The objective of the study was to compare values of MI used in neonatal cardiac scanning with those recommended by the British Medical Ultrasound Society safety guidelines.

Methods: The eight scanners were based at Sheffield Teaching Hospitals NHS Trust (STH), Sheffield Children's Hospitals NHS Trust (SCH) and Leeds Teaching Hospitals NHS Trust (LTH). Two methods of recording the MI value were used. At the Sheffield sites the on-screen safety index was obtained from archived patient studies saved on the Trust PACS. At the Leeds site the on-screen safety index was recorded from the scanner when it was switched on to its default neonatal or paediatric pre-sets with the appropriate transducer connected.

Results: Data from clinical examinations at the Sheffield sites show that on average the MI values, for cardiac scanning, in B-mode and colour/pulsed wave Doppler mode were 1·1±0·3 and 1·3±0·3 respectively. The maximum values of MI recorded during clinical cardiac examinations were 1·6 for B-mode and 1·7 for colour/pulsed wave Doppler mode. Data from the Leeds site recorded default MI values, using appropriate transducers with neonatal and paediatric presets. One scanner from the Leeds site defaulted to an MI value of 1·9 in colour/pulsed Doppler mode with a neonatal preset. All the MI values are well above BMUS guideline recommendations for neonates when lung tissue is present within the ultrasound beam.

Conclusions: The implication of these results is that neonates undergoing cardiac ultrasound examinations, were regularly exposed to MI values at least four to six times that recommended by BMUS guidelines (MI<0·3).

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.

Improved scatterer size estimation using backscatter coefficient measurements with coded excitation and pulse compression

Scatterer size estimates from ultrasonic backscatter coefficient measurements have been used to differentiate diseased tissue from normal. A low echo signal-to-noise ratio (eSNR) leads to increased bias and variance in scatterer size estimates. One way to improve the eSNR is to use coded excitation (CE). The normalized backscatter coefficient was measured from three tissue-mimicking phantoms by using CE and conventional pulsing (CP) techniques. The three phantoms contained randomly spaced glass beads with median diameters of 30, 45, and 82 mum, respectively. Measurements were made with two weakly focused, single-element transducers (f(0)=5 MHz and f(0)=10 MHz). For CE, a linear frequency modulated chirp with a time bandwidth product of 40 was used and pulse compression was accomplished by the use of a Wiener filter. Preliminary results indicated that improved estimation bias versus penetration depth was obtained by using CE compared to CP. The depth of penetration, where the accuracy of scatterer diameter estimates (absolute divergence <25%) were obtained with the 10 MHz transducer, was increased up to 50% by using CE versus CP techniques. In addition, for a majority of the phantoms, the increase in eSNR from CE resulted in a modest reduction in estimate variance versus depth of penetration.

Bandwidth and resolution enhancement through pulse compression

A novel pulse compression technique is developed that improves the axial resolution of an ultrasonic imaging system and provides a boost in the echo signal-to-noise ratio (eSNR). The new technique, called the resolution enhancement compression (REC) technique, was validated with simulations and experimental measurements. Image quality was examined in terms of three metrics: the eSNR, the bandwidth, and the axial resolution through the modulation transfer function (MTF). Simulations were conducted with a weakly-focused, single-element ultrasound source with a center frequency of 2.25 MHz. Experimental measurements were carried out with a single-element transducer (f/3) with a center frequency of 2.25 MHz from a planar reflector and wire targets. In simulations, axial resolution of the ultrasonic imaging system was almost doubled using the REC technique (0.29 mm) versus conventional pulsing techniques (0.60 mm). The -3 dB pulse/echo bandwidth was more than doubled from 48% to 97%, and maximum range sidelobes were -40 dB. Experimental measurements revealed an improvement in axial resolution using the REC technique (0.31 mm) versus conventional pulsing (0.44 mm). The -3 dB pulse/echo bandwidth was doubled from 56% to 113%, and maximum range sidelobes were observed at -45 dB. In addition, a significant gain in eSNR (9 to 16.2 dB) was achieved.

Ultrasound-biophysics mechanisms

Ultrasonic biophysics is the study of mechanisms responsible for how ultrasound and biological materials interact. Ultrasound-induced bioeffect or risk studies focus on issues related to the effects of ultrasound on biological materials. On the other hand, when biological materials affect the ultrasonic wave, this can be viewed as the basis for diagnostic ultrasound. Thus, an understanding of the interaction of ultrasound with tissue provides the scientific basis for image production and risk assessment. Relative to the bioeffect or risk studies, that is, the biophysical mechanisms by which ultrasound affects biological materials, ultrasound-induced bioeffects are generally separated into thermal and non-thermal mechanisms. Ultrasonic dosimetry is concerned with the quantitative determination of ultrasonic energy interaction with biological materials. Whenever ultrasonic energy is propagated into an attenuating material such as tissue, the amplitude of the wave decreases with distance. This attenuation is due to either absorption or scattering. Absorption is a mechanism that represents that portion of ultrasonic wave that is converted into heat, and scattering can be thought of as that portion of the wave, which changes direction. Because the medium can absorb energy to produce heat, a temperature rise may occur as long as the rate of heat production is greater than the rate of heat removal. Current interest with thermally mediated ultrasound-induced bioeffects has focused on the thermal isoeffect concept. The non-thermal mechanism that has received the most attention is acoustically generated cavitation wherein ultrasonic energy by cavitation bubbles is concentrated. Acoustic cavitation, in a broad sense, refers to ultrasonically induced bubble activity occurring in a biological material that contains pre-existing gaseous inclusions. Cavitation-related mechanisms include radiation force, microstreaming, shock waves, free radicals, microjets and strain. It is more challenging to deduce the causes of mechanical effects in tissues that do not contain gas bodies. These ultrasonic biophysics mechanisms will be discussed in the context of diagnostic ultrasound exposure risk concerns.

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.

Ultrasonic contrast agent shell rupture detected by inertial cavitation and rebound signals

Determining the rupture pressure threshold of ultrasound contrast agent microbubbles has significant applications for contrast imaging, development of therapeutic agents, and evaluation of potential bioeffects. Using a passive cavitation detector, this work evaluates rupture based on acoustic emissions from single, encapsulated, gas-filled microbubbles. Sinusoidal ultrasound pulses were transmitted into weak solutions of Optison at different center frequencies (0.9, 2.8, and 4.6 MHz), pulse durations (three, five, and seven cycles of the center frequencies), and peak rarefactional pressures (0.07 to 5.39 MPa). Pulse repetition frequency was 10 Hz. Signals detected with a 13-MHz, center-frequency transducer revealed postexcitation acoustic emissions (between 1 and 5 micros after excitation) with broadband spectral content. The observed acoustic emissions were consistent with the acoustic signature that would be anticipated from inertial collapse followed by "rebounds" when a microbubble ruptures and thus generates daughter/free bubbles that grow and collapse. The peak rarefactional pressure threshold for detection of these emissions increased with frequency (e.g., 0.53, 0.87, and 0.99 MPa for 0.9, 2.8, and 4.6 MHz, respectively; five-cycle pulse duration) and decreased with pulse duration. The emissions identified in this work were separated from the excitation in time and spectral content, and provide a novel determination of microbubble shell rupture.

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.

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.

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.

Attenuation coefficient estimates of mouse and rat chest wall

Attenuation coefficients of intercostal tissues were estimated from chest walls removed postmortem (pm) from 41 6-to-7-week-old female ICR mice and 27 10-to-11-week-old female Sprague-Dawley rats. These values were determined from measurements through the intercostal tissues, from the surface of the skin to the parietal pleura. Mouse chest walls were sealed in plastic wrap and stored at 4 degrees C until evaluated, and rat chest walls were sealed in Glad-Lock Zipper sandwich bags, and stored at -15 degrees C. When evaluated, chest wall storage time ranged between 1 and 2 days pm for mice and between 41 and 110 days pm for rats. All chest walls were allowed to equilibrate to 22 degrees C in a water bath prior to evaluation. For both mouse and rat intercostal tissues, the estimated frequency normalized attenuation coefficient was 1.1 dB/cm-MHz. In order to determine if there was an effect of storage time on estimates of attenuation coefficient, an independent experiment was conducted. The intercostal tissues from six mouse chest walls were evaluated at three time points (1, 22, and 144 days pm), and from six rat chest walls were evaluated at four time points (1, 22, 50, and 125 days pm). There was no difference in the estimated intercostal tissue attenuation coefficient as a function of time postmortem.

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 x 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 micros 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 x 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 x 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 x ED interaction for surface area was not significant for either species.