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Lung damage from exposure to pulsed ultrasound
Ultrasound in Med. & Biol. Vol. 16, No. 8, pp. 817-825, 1990 Printed in the U.S.A.
0301-5629/90 $3.00 + .00 © 1990 Pergamon Press plc
@Original Contribution LUNG
DAMAGE
FROM
EXPOSURE
TO
PULSED
ULTRASOUND
S. Z. CHILD,** C. L. HARTMAN,* L. A. SCHERY • a n d E. L. CARSTENSEN t* Departments of *Electrical Engineering, and the *Rochester Center for Biomedical Ultrasound, The University of Rochester, Rochester, NY 14627 (Received 16 April 1990; in final form 20 June 1990) Abstract--Motivated by a recent tinding that threshold pressures for hemorrhage in mouse lung exposed to the fields of an electrohydraulic lithotripter were less than 2 MPa, we extended the exposures to pulsed ultrasound. Sharply defined thresholds of the order of 1 M P a were found with l 0 tts length pulses and roughly twice that value for l ~,s pulses. The thresholds at 4 MHz are greater than at ! MHz. The thresholds are comparable for focused and unfocused fields. As would be expected for a cavitation-like phenomenon, temporal average intensity is a very poor predictor of this effect. In the extreme case, lesions were found at temporal average intensities on the order of I m W / c m 2.
Key Words: Acoustics, Ultrasonic toxicity, Mouse lung, Pulsed ultrasound, Ultrasound bioeffects, Lung hemorrhage.
INTRODUCTION
E X P E R I M E N T A L PROCEDURES
Lung and kidney damage has been observed to result from exposure of experimental animals to fields of extracorporeal shock wave lithotripters (ESWL) (Delius et al. 1987, 1988a, 1988b). Thresholds now have been determined for a variety of effects of lithotripter fields: Extravasation in kidney tissue at 3-5 MPa (Mayer et al. 1990), lung hemorrhage at 1-2 MPa (Hartman et al. 1990b), malformations in chick embryos at 5-10 MPa (Hartman et al. 1990a), induction of premature ventricular contraction in frog hearts at 5-10 MPa (Dalecki et al. 1990) and killing of Drosophila larvae at 1-3 MPa (Carstensen et al. 1990a). Since pressures greater than these are used in diagnostic ultrasound (Duck et al. 1985), it is reasonable to ask whether similar effects result from exposure to pulsed ultrasound. Mouse kidney showed no significant signs of damage even after exposure to 1 and 4 MHz pulsed ultrasound at pressure levels somewhat above the thresholds for damage by lithotripter fields (Carstensen et al. 1990b). In contrast, this study provides very strong evidence that mouse lung is damaged by exposure to pulsed ultrasound and that the thresholds for these effects are similar to those found with spark-generated shock waves.
The source transducers used in this study were piezoceramic discs either in direct contact with the water in the exposure vessel or cemented to the back of planoconcave lenses. Specifications are summarized in Table 1. Acoustic pressures were measured with a 1 mm diameter, bilaminar PVDF membrane hydrophone (Marconi Research Center, Chelmsford, England). The hydrophone was calibrated by comparison with a steel sphere radiometer (Dunn et al. 1977). Threshold waveforms and pulses are shown in Figs. 1-6. Pulse average intensities were determined by integrating the square of the measured voltages over the length of the pulse using a LeCroy Model 9400 (Chestnut Ridge, NY) digital oscilloscope. Pressure amplitude of the fundamental component of the wave was determined by low-pass filtering of the signal from the hydrophone. Male C3H mice, approximately 7 weeks old, were obtained from Jackson Labs (Bar Harbor, ME), fed ad libitum and cared for in accordance with institutional guidelines on animal care. Before exposure, the mice were anesthetized with Ketamine (200 mg/kg) and Rompun (10 mg/kg). Their backs were shaved with clippers and the remaining hair was removed with Neet ® to minimize the likelihood of en-
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Ultrasound in Medicine and Biology
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Table 1. Physical characteristics of the sources used in the study. Frequency 1.1 Unfocused Diameter (cm) Distance from source (cm)
1.2 Focused
2.5 1l
t r a p m e n t o f air at the skin-water interface. The animals were positioned on a rubber frame in a tank of degassed water with their dorsal surfaces toward the source. The frame, in turn, was supported on a threeway positioner that allowed stable, precise location of the mouse with respect to the source. The coordinates of the field point as measured by the hydrophone were transferred to the center of each mouse lung. T h r e e - m i n u t e exposures were given approximately in the middle of each lung. I m m e d i a t e l y following treatment, the animals were euthanized by cervical dislocation, the lungs were removed and the lung surface was inspected for signs of hemorrhage. Positive findings were extremely rare in sham-exposed mice. Near threshold, the d a m age was localized to the site of exposure and to the p r o x i m a l surface o f the lung. At s u p e r t h r e s h o l d levels, hemorrhage extended through the lung to the far side. In all cases, there was a distinct limit below which the lungs appeared normal. Thresholds were a s s u m e d to lie b e t w e e n the lowest levels w h i c h showed damage and the upper level which did not. N e a r the threshold levels, scoring was blind. Attenuation by the rib cage was measured by positioning a P V D F needle h y d r o p h o n e ( N e l s o n T w o m e y Research, Seattle, WA) on the axis of the sound beam, establishing a reference pressure level a n d t h e n bringing the s k i n - c o v e r e d rib cage o f a
2.5 5
2.3 Unfocused 1.2 6
3.5 Unfocused
3.7 Focused
1.2 3.5
2.5 5
mouse (lungs removed) around the end of the hydrophone. The mouse was then m o v e d vertically and the m a x i m u m and m i n i m u m transmitted pressures were recorded. T h e m e a s u r e m e n t s o f a t t e n u a t i o n were performed at such low levels that the waveform was essentially sinusoidal. All of the raw threshold data (Figs. 7-12) are presented in terms of the temporal peak, positive pressures measured in water. EXPERIMENTAL
RESULTS
The experiments determined the dependence of thresholds upon frequency, b e a m diameter, temporal average intensity and pulse length for representative subsets o f the o b s e r v a t i o n s . D a t a for u n f o c u s e d sources at 1.2, 2.3 and 3.5 M H z are presented in Fig. 7. Raw data for the threshold determinations with 1.2 and 3.7 M H z focused sources are presented in Fig. 8. Figures 9 and 10 use these data to compare focused a n d u n f o c u s e d sources at the low- a n d high-frequency limits of the study. These plots m a y d e m o n strate better than Figs. 7 and 8 the sharpness of the threshold. The lack of dependence of the threshold on pulse repetition frequency and on the total n u m b e r of pulses in the exposure is shown in Fig. 1 1. Figure 12 gives the raw data for 1 and 10 us pulse lengths. The real thresholds, of course, must be expressed in terms o f the in situ field levels. Attenuation data
im_mMump, nilllnn
Fig. 1. Threshold pulse shape and waveform in water for the 1.2 MHz, 2.5 cm diameter, unfocused source at a distance of 11 cm. Pulse length, 10 us; peak positive pressure, 0.8 MPa; pulse average intensity, 10 W/cm 2.
Lung damage from exposure to pulsed ultrasound • S. Z. CHILDel al.
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Fig. 2. Threshold pulse shape and waveform in water for the 1.2 MHz, 2.5 cm diameter, focused source at a distance of 5 cm. Pulse length, 10/~s; peak positive pressure, 0.8 MPa; pulse average intensity, 15 W/cm 2.
are s u m m a r i z e d in Table 2. Presumably because of the i n h o m o g e n e o u s c h a r a c t e r o f the rib cage, the transmitted signal varied in amplitude with deliberate small vertical displacements of the m o u s e during the a t t e n u a t i o n m e a s u r e m e n t s . A r g u m e n t s c o u l d be m a d e for using either the m a x i m u m or m i n i m u m attenuation values observed depending u p o n the intended use o f the derated data. We have chosen to use the m i n i m u m attenuation in our subsequent analysis. O u r reasoning is that the threshold for hemorrhage appears to be sharply defined and related to spatial peak, temporal peak amplitude o f the sound wave. There is a greater probability that some small region of the lung surface will be exposed to the maxi m u m signal than that the entire lung will be exposed uniformly to the m i n i m u m signal. Although we feel that this is the m o s t realistic approach, it does set upper limits for in situ threshold values. Furthermore, we have only looked at superficial damage to the lung
in this study. Future investigations could reveal m o r e subtle injuries. T a b l e 3 s u m m a r i z e s the threshold data in terms o f linearly derated (in situ) pressures and intensities. Figure 13 summarizes the results in t e r m s o f the in situ, f u n d a m e n t a l pressure in the pulse. DISCUSSION
Comparison with earlier studies U p to the present time, there has been no convincing, direct e v i d e n c e t h a t clinically r e l e v a n t , pulsed u l t r a s o u n d p r o d u c e s deleterious effects in m a m m a l i a n tissue in the absence of significant heating. In a recent study, we failed to find effects of pulsed ultrasound on kidney tissues at peak positive pressures a p p r o a c h i n g 10 M P a (Carstensen et al. 1990b) in spite o f the fact that lithotripter shocks o f that amplitude produced marked hemorrhage in the
Fig. 3. Threshold pulse shape and waveform in water for the 2.3 MHz, 1.2 cm diameter, unfocused source at a distance of 6 cm. Pulse length, 10 t~s; peak positive pressure, 1.5 MPa; pulse average intensity, 30 W/cm 2.
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Ultrasound in Medicine and Biology
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-. -,
it I I
Fig. 4. Threshold pulse shape and waveform in water for the 3.5 MHz, 1.2 cm diameter, unfocused source at a distance of 3.5 cm. Pulse length, 10 us; peak positive pressure, 2.8 MPa; pulse average intensity, 90 W/cm 2.
same biological preparation (Mayer et al. 1990). In this study, the effect o f pulsed ultrasound on lung tissue is demonstrated clearly and reproducibly, and thresholds for the effect are on the order of 1 MPa. The theoretical possibility that cavitation could occur within the body has been recognized for m a n y years. Fry and colleagues at the University o f Illinois (1970) found histological suggestions o f cavitation in brain tissue with single continuous-wave (CW) exposures as long as 10,000 #s at pressures above 10 M P a or intensities greater than 3000 W / c m 2. Lele (1977) r e p o r t e d detection o f an a b r u p t increase in white noise from brain tissue irradiated continuously at approximately the same levels. H e found a correlation between this evidence of transient cavitation and histological signs o f extravasation and tissue disintegration. The closest parallel in the literature to the effects which are seen in lung is the killing of Drosophila larvae by exposure to pulsed ultrasound (Child et al. 1981; Berg et al. 1983). The threshold values are com-
parable as shown in Fig. 13. In each case, thresholds d e p e n d only very weakly on pulse repetition frequency. Most o f the evidence for killing of larvae is consistent with a cavitation-related m e c h a n i s m although it a p p e a r s that classical cavitation theory, which assumes that a bubble is free to expand without limit in an aqueous m e d i u m , is not adequate to describe the processes that involve the interaction of acoustic fields with bubbles that are confined within tissues (Carstensen et al. 1990a). The results reported here are i m p o r t a n t not because they reveal an unexpected hazard, but because they provide, for the first time, a clearly defined, reproducible nonthermal effect of diagnostically relevant ultrasound in a m a m malian system that has m a n y of the characteristics that might be expected of cavitation.
Comparison with consensus statements These observations can be viewed from the perspective o f the American Institute of Ultrasound in
mmmmm roam inmmm mmm nmiamaimamm
Fig. 5. Threshold pulse shape and waveform in water for the 3.7 MHz, 2.5 cm diameter, focused source at a distance of 5 cm. Pulse length, 10 its; peak positive pressure, 2 MPa; pulse average intensity, 70 W/cm 2.
Lung damage from exposure to pulsed ultrasound • S. Z. CHILD et al.
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Fig. 6. T h r e s h o l d pulse s h a p e a n d w a v e f o r m in water for the 3.7 M H z , 2.5 c m diameter, focused source at a distance o f 5 cm. Pulse length, 1 us; peak positive pressure, 3.9 M P a ; pulse average intensity, 180 W / c m 2.
Medicine's (AIUM 1988) Mammalian Bioeffects S t a t e m e n t . A s r e q u e s t e d b y t h e s o u r c e , it is q u o t e d h e r e i n its e n t i r e t y . F o o t n o t e a w a s a t e m p o r a r y p a t c h a d d e d i n 1 9 8 7 t o e l i m i n a t e l i t h o t r i p t e r effects w h i c h w e r e k n o w n a t t h a t t i m e t o c a u s e b i o l o g i c a l effects but which were thought to have no direct relevance to pulsed ultrasound. A review of bioeffects data supports the following statement as an update of the AlUM Statement on I n Fivo Mammalian Bioeffects: In the low megahertz frequency range there have been (as of this date) no independently confirmed significant biological effects in mammalian tissues exposed in vivo to unfocused ultrasound with intensitiesa below 100 mW/cm 2, or to focused b ultrasound with
intensities below 1 W/cm 2. Furthermore, for exposure times c greater than 1 second and less than 500 seconds for unfocused ultrasound or 50 seconds for focused ultrasound, such effects have not been demonstrated even at higher intensities when the product of intensity and exposure time is less than 50 joules/cm 2.
"Free-field spatial peak, temporal average (SPTA) for continuous wave exposures, and for pulsed-mode exposures with pulses repeated at a frequency greater than 100 Hz.
b Quarter-power ( - 6 db) beam width smaller than four wavelengths or 4 mm, whichever is less at the exposure frequency. c Total time including off-time as well as on-time for repeated pulse exposures.
5
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Fig. 7. H e m o r r h a g e in m o u s e l u n g exposed to u n f o c u s e d sources at 1.2, 2.3 a n d 3.5 M H z . E a c h p o i n t is the assessm e n t for a single lung. Filled squares are positive; white p o i n t s are negative.
0
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1.2 M H z
3.7 M H z
Fig. 8. H e m o r r h a g e in m o u s e l u n g e x p o s e d to focused sources at 1.2 a n d 3.7 M H z . Each p o i n t is the assessment for a single lung. Filled squares are positive; white p o i n t s are negative.
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V o l u m e 16, N u m b e r 8, 1990
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First, the m i n i m u m levels at which effects had been observed in m a m m a l s at the writing o f this statement was greater than 0.1 W / c m 2. In the present study, the m i n i m u m value of the temporal average intensity which produced a lesion in lung was smaller than the A I U M limit by nearly two orders of magni-
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Fig. 9. Frequency of occurrence of hemorrhage in mouse lung exposed to an unfocused source (1.2 MHz) and a focused source (1.2 MHz).
cv ~rj
1/10,000
I
5
Fig. 10. Frequency of occurrence of hemorrhage in mouse lung exposed to an unfocused source (3.5 MHz) and a focused source (3.7 MHz).
Fig. 11. Hemorrhage in mouse lung exposed to a focused source of 1.2 MHz. Each point is the assessment for a single lung. Filled squares are positive; white points are negative. Essentially the same threshold is found for a 1/1000 duty cycle (100 Hz repetition frequency) or a 1/10,000 duty cycle (10 Hz repetition frequency).
tude. We m a d e no attempt to determine the threshold in terms o f the temporal average intensity because it is apparent that this is not the appropriate parameter to describe the kind of effect that is observed. If it is a s s u m e d that a cavitation-related process is involved, it is reasonable that some kind o f temporal p e a k p a r a m e t e r r a t h e r t h a n an average intensity would be expected to correlate with the effect. The framers of the A I U M statement were aware of this, but there was no clear evidence for the occurrence o f cavitation in m a m m a l s when the statement was formulated. Second, the A I U M statement says that no effects for focused ultrasound have been observed at intensities below 1 W / c m 2, i.e., 10 times the value for unfocused fields or 1000 times greater than the m i n i m u m temporal average intensities for lung hemorrhage in the present study. If one assumes that the p h e n o m e non is related to cavitation and involves the response o f individual microbubbles to the local pressure, the threshold should be essentially independent of the cross-section of the acoustic beam. In fact, independence of threshold u p o n b e a m diameter (at a given frequency) m a y be used as a criterion for the selection o f an appropriate physical descriptor of the sound field to be used as a predictor of lung hemorrhage as discussed below.
Lung damage from exposure to pulsed ultrasound • S. Z. CHILDet al.
considered the case of bubbles surrounded by a layer o f surface active material. The theoretical curves in Fig. 13 are for short, sinusoidal pulses of ultrasound, typically ~ 1 us. These theoretical a r g u m e n t s are based on the assumption that a bubble can expand without limit in a fluid m e d i u m . Since this is not true for the air bodies in the respiratory system of Drosophila larvae (Carstensen et al. 1990a) a n d very likely this is not the case for the bubbles in mouse lung, the theory must be applied with caution. Even so, the data show the expected qualitative dependence u p o n frequency and pulse length and order-ofmagnitude agreement with the expected level of the thresholds.
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Fig. 12. Pulse length dependence for hemorrhage in mouse lung exposed to a focused source of 3.7 MHz. Each point is the assessment for a single lung. Filled squares are positive; white points are negative.
At the time of its formulation, the A I U M (1988) statement on cavitation expressed a concern for possible effects at levels greater than 10 MPa, an order of magnitude greater than the thresholds for lung hemorrhage in the present study.
Comparison of the results with theory for transient cavitation Flynn (1982) concluded that thresholds for transient cavitation for bubbles exposed to microsecondlength pulses o f ultrasound increase as the frequency increases above 2 M H z as shown in Fig. 13. The absolute thresholds at a given frequency increase for decreasing bubble size. This kind of frequency dependence is to be expected for bubbles up to approxim a t e l y 3 ~zm in radius for frequencies c o m m o n l y used in medical diagnosis. U n f o r t u n a t e l y , a l m o s t nothing is k n o w n about the size distribution of gas bodies in m a m m a l i a n tissues. By assuming that optim u m - s i z e d bubbles are always present in the m e dium, Holland and Apfel (1989, 1990) and Apfel and Holland (1990) concluded that pressure thresholds for transient cavitation increase approximately with the square root of the frequency. R o y et al. (1990)
The s u m m a r y of threshold data in Table 3 provides some guidance in the selection of appropriate parameters for description of the fields in terms of their effectiveness in causing lung hemorrhage. Although it is quite clear that temporal average intensities have little value, temporal peak positive (compression) pressure, temporal m a x i m u m negative (rarefaction) pressure, the temporal peak pressure at the fundamental frequency or pulse average intensity are all correlated roughly with the observed damage to the lung. O f these, the fundamental pressure appears to be least sensitive to b e a m diameter. There is a rationale in theory for using the fundamental pressure in a nonlinearly distorted wave as a predictor of cavitation. T h e h i g h - f r e q u e n c y c o m p o n e n t s in a nonlinearly distorted wave are less effective in causing cavitation than the fundamental frequency. N o t only does the threshold for transient cavitation increase with frequency but, at high frequencies, the change in response is m u c h less dramatic and, in fact, the very concept o f a threshold becomes nebulous (Flynn, 1982). The usefulness o f the fundamental frequency as a predictor o f cavitation was tested for a typical nonlinearly distorted, a s y m m e t r i c wave theoretically (Aym6 and Carstensen 1989a) and experimentally in the killing o f Drosophila larvae (Aym6 and Carstensen 1989b). It turned out that the negative and fundamental pressures in these waves were sufficiently Table 2. Attenuation of mouse rib cage--Ratios of pressures within chest cavity of a mouse to the free field pressure. Measurements were made with small signal (sinusoidal) levels.
1.1 MHz 3.4 MHz
Minimum (_+SD)
Maximum (_+SD)
0.84 _ 0.04 0.75 _+0.06
0.55 _+0.05 0.45 + 0.06
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Table 3. Threshold levels for lung hemorrhage in mice exposed to pulsed ultrasound. Data in Figs. 7-12 were used to determine threshold levels as defined in the text. Exposure parameters were measured as shown in Figs. 1-6. Values for the fundamental pressure amplitude py were obtained by low-pass filtering of the data. Pulse average intensities leA were obtained by integrating the squared pressure over the pulse and dividing by the on-time of the electrical excitation of the source. These values were derated linearly using the attenuation data in Table 2 to give the in situ values for this Table. Frequency
1.1 Unfocused
1.2 Focused
1.2 Focused
2.3 Unfocused
3.5 Unfocused
3.7 Focused
3.7 Focused
10 #s 1/1000
10 #s 1/1000
10 #s 1/10,000
10 #s 1/1000
10 #s 1/1000
10 #s 1/1000
1 #s 1/1000
0.7 0.4 0.6 7 7
0.7 0.7 0.7 11 11
0.7 0.7 0.7 11 1
1.2 0.6 0.8 20 20
2.1 1.3 1.3 50 50
1.5 1.0 1.2 40 40
3.0 1.4 1.8 100 100
Pulse length Duty cycle Thresholds p+ (MPa) p_ (MPa) p/(MPa) lea (W/cm 2) ITA (mW/cm 2)
similar that they were almost equally useful predictors. The differences a m o n g the various parameters are clearer in the lung study. In Table 3, the maxi-
m u m negative pressures do not fare as well in the focused/unfocused criterion as does the fundamental pressure either at high or low frequency.
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Fig. 13. Comparison of theoretical predictions of thresholds for transient cavitation in water with thresholds for killing of Drosophila larvae and mouse lung hemorrhage. The solid curves are for 1 #s pulses with bubble radii of 0.3 #m (upper curve) and 1 #m (lower curve) (Flynn 1982; Carstensen and Flynn 1982). For a given bubble size, thresholds become less distinct as frequency increases. The dotted curve is a similar analysis under the assumption that bubbles of the optimum size for each frequency are present (Apfel and Holland 1990). Roy et al. (1990) (dashed line) considered the case of a bubble surrounded by a layer of surface active material. All of the theoretical predictions are for short (1 #s), sinusoidal pulses of ultrasound. The data for Drosophila larvae are for 1 #s pulses ofsinusoidal ultrasound. Only the mouse lung data involved exposures to fields with significant nonlinear distortion of the waveform.
There appears to be a consensus a m o n g the members of the medical ultrasound c o m m u n i t y that users should be informed of the acoustic output levels o f diagnostic equipment. Progress has been made recently in relating tissue heating to the temporal average intensity of the sound field. However, hemorrhage in the mouse lung is the first biological effect in m a m m a l s that has been related directly to a temporal peak parameter of the diagnostic ultrasound field. In addition to information on absolute magnitudes of cavitation-related thresholds and their frequency dependence, it is apparent from these studies that the thresholds are somewhat lower for the relatively long pulses used in Doppler instruments than for the short pulses characteristic of scanners. According to a recent survey o f the output levels o f diagnostic u l t r a s o u n d e q u i p m e n t ( D u c k et al. 1985), the modal pressure in water for Doppler instruments is ~ 5 MPa. On the one hand, commercial Doppler devices typically operate at higher pulse repetition frequencies and at higher temporal average intensities than were used in this study. In extreme cases, the exposures could be complicated by ultrasonic heating of the tissue. On the other hand, it must be assumed that the attenuation o f the signal before reaching the focus in h u m a n patients is somewhat greater than for the mouse. Whether h u m a q lung is acoustically and pathologically equivalent to mouse lung for these effects remains to be demonstrated. The experimental data for lung show that the threshold for hemorrhage is sharply defined. For the moment, it is reasonable to assume that, as observed with Drosophila larvae (Child et al. 1981), exposures
Lung damage from exposure to pulsed ultrasound • S. Z. CHILD et al.
with much higher temporal average intensities extending for much longer times will have no effect as long as the temporal peak amplitude does not exceed that threshold. Overall, the safety picture for medical ultrasound which emerges from this new information is encouraging. Many diagnostic procedures can be carfled out below the threshold levels for lung hemorrhage. It may be assumed that, if appropriate bubbles exist in other parts of the body, similar effects could occur. However, our studies of the effects of pulsed ultrasound on mouse kidney show that the probability of such an occurrence is extremely small even at the highest levels used in diagnosis (Carstensen et al. 1990b). In that study, peak positive pressures were almost an order of magnitude greater than the threshold for lung hemorrhage. In fact, when fetal lung was exposed to lithotripter fields with peak positive pressures of 20 MPa, only 2% of the samples showed signs of hemorrhage (Hartman et al. 1990b). All evidence available at the present time suggests that if cavitation occurs in organs other than the lung, it probably occurs rarely and its effects are highly localized. Acknowledgments--This work was supported in part by U.S.P.H.S. Grants No. DK39796 and CA39241.
REFERENCES American Institute of Ultrasound in Medicine (AIUM), Bioeffects Committee. Bioeffects considerations for the safety of diagnostic ultrasound. J. Ultrasound Med. 7:(Suppl. 9)S1-$38; 1988. Apfel, R. E.; Holland, C. K. Gauging the likelihood of cavitation from short pulse, low duty cycle diagnostic ultrasound. Ultrasound in Med. & Biol. [1990]. Aymr, E. J.; Carstensen, E. L. Cavitation induced by asymmetric, distorted pulses of ultrasound: Theoretical predictions. IEEE Trans. Ultrasonics, Ferroelectrics and Freq. Control 36:32-40; 1989a. Aym& E. J.; Carstensen, E. L. Cavitation induced by asymmetric, distorted pulses of ultrasound: A biological test. Ultrasound in Med. & Biol. 15:61-66; 1989b. Berg, R. B.; Child, S. Z.; Carstensen, E. L. The influence of carrier frequency on the killing of drosophila larvae by microsecond pulses of ultrasound. Ultrasound in Med.& Biol. 9:L448-L451 ; 1983. Carstensen, E. L.; Flynn, H. G. The potential for transient cavitation with microsecond pulses of ultrasound. Ultrasound in Med. & Biol. 8:L720-L724; 1982. Carstensen, E. L,; Campbell, D. S.; Hoffman, D.; Child, S. Z.; Aymr-Bellagarda, E. Killing of Drosophila larvae by the fields
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of an electrohydraulic lithotripter. Ultrasound in Med. & Biol. 16(7):687-698; 1990a. Carstensen, E. L.; Hartman, C.; Child, S. Z.; Cox, C. A.; Mayer, R.; Schenk, E. Test for kidney hemorrhage following exposure to intense, pulsed ultrasound. Ultrasound in Med. & Biol. 16(7):681-685; 1990b. Child, S. Z.; Carstensen, E. L.; Lam, S. K. Effects of ultrasound on Drosophila: III. Exposure of larvae to low-temporal-average-intensity, pulsed irradiation. Ultrasound in Med. & Biol. 7:167173; 1981. Delius, M.; Enders, G.; Heine, G.; Stark, J.; Remberger, K.; Brendel, W. Biological effects of shock waves: Lung haemorrhage by shock waves in dogs--Pressure dependence. Ultrasound in Med. & Biol. 13:61-67; 1987. Delius, M.; Jordan, M.; Eizenhoefer, H.; Marlinghaus, E.; Heine, G.; Liebich, H.-G.; Brendel, W. Biological effects of shock waves: Kidney haemorrhage by shock waves in dogs--Administration rate dependence. Ultrasound in Med. & Biol. 14:689694; 1988a. Delius, M.; Enders, G.; Xuan, Z.; Liebich, H.-G.; Brendel, W. Biological effects of shock waves: Kidney damage by shock waves in dogs--Dose dependence. Ultrasound in Med. & Biol. 14:117-122; 1988b. Dalecki, D.; Carstensen, E. L.; Keller, B. Thresholds for premature ventricular contractions caused by lithotripter shocks. Ultrasound in Med, & Biol. Submitted for publication; 1990. Duck, F. A.; Starritt, H. C.; Aindow, J. D.; Perkins, M. A.; Hawkins, A. J. The output of pulse-echo ultrasound equipment: A survey of power, pressure and intensities. Br. J. Radiol. 58:989-1001; 1985. Dunn, F.; Averbuch, A. J.; O'Brien, W. D., Jr. A primary method for the determination of ultrasonic intensity with the elastic sphere radiometer. Acustica 38:58-61 ; 1977. Flynn, H. G. Generation of transient cavities in liquids by microsecond pulses of ultrasound. J. Acoust. Soc. Am. 72:19261932; 1982. Fry, F. J.; Kossoff, G.; Eggieton, R. C.; Dunn, F. Threshold ultrasonic dosages for structural changes in mammalian brain. J. Acoust. Soc. Am. 48:1413-1417; 1970. Hartman, C.; Cox, C. A.; Brewer, L.; Child, S. Z.; Cox, C. F.; Carstensen, E. L. Effects of lithotripter fields on development of chick embryos. Ultrasound in Med. & Biol. 16(6):581-585; 1990a. Hartman, C.; Child, S. Z.; Mayer, R.; Schenk, E.; Carstensen, E. L. Lung damage from exposure to the fields of an electrohydraulic lithotripter. Ultrasound in Med. & Biol. 16(7):675-679; 1990b. Holland, C. K.; Apfel, R. E. An improved theory for the prediction of microcavitation thresholds. IEEE Trans. Ultrasonics, Ferroelectrics, and Freq. Control 36:204-208; 1989. Holland, C. K.; Apfel, R. E. Thresholds for transient cavitation produced by pulsed ultrasound in controlled nuclei environment. J. Acoust. Soc. Am. [1990]. Lele, P. P. Thresholds and mechanisms of ultrasonic damage to "organized" animal tissues. In: Proceedings of the Symposium on Biological Effects and Characterizations of Ultrasound Sources, Rockville, MD; 2-3 June 1977:224-238. Mayer, R.; Schenk, E.; Child, S.; Norton, S.; Cox, C. A.; Hartman, C.; Cox, C. F.; Carstensen, E. Pressure threshold for shockwave induced renal hemorrhage. J. Urology [1990]. Roy, R. A.; Church, C. C.; Calabrese, A. Cavitation produced by short pulses of ultrasound. In: Proceedings of the International Conference on Nonlinear Acoustics, Austin, TX; August, 1990 [in press].
0301-5629/90 $3.00 + .00 © 1990 Pergamon Press plc
@Original Contribution LUNG
DAMAGE
FROM
EXPOSURE
TO
PULSED
ULTRASOUND
S. Z. CHILD,** C. L. HARTMAN,* L. A. SCHERY • a n d E. L. CARSTENSEN t* Departments of *Electrical Engineering, and the *Rochester Center for Biomedical Ultrasound, The University of Rochester, Rochester, NY 14627 (Received 16 April 1990; in final form 20 June 1990) Abstract--Motivated by a recent tinding that threshold pressures for hemorrhage in mouse lung exposed to the fields of an electrohydraulic lithotripter were less than 2 MPa, we extended the exposures to pulsed ultrasound. Sharply defined thresholds of the order of 1 M P a were found with l 0 tts length pulses and roughly twice that value for l ~,s pulses. The thresholds at 4 MHz are greater than at ! MHz. The thresholds are comparable for focused and unfocused fields. As would be expected for a cavitation-like phenomenon, temporal average intensity is a very poor predictor of this effect. In the extreme case, lesions were found at temporal average intensities on the order of I m W / c m 2.
Key Words: Acoustics, Ultrasonic toxicity, Mouse lung, Pulsed ultrasound, Ultrasound bioeffects, Lung hemorrhage.
INTRODUCTION
E X P E R I M E N T A L PROCEDURES
Lung and kidney damage has been observed to result from exposure of experimental animals to fields of extracorporeal shock wave lithotripters (ESWL) (Delius et al. 1987, 1988a, 1988b). Thresholds now have been determined for a variety of effects of lithotripter fields: Extravasation in kidney tissue at 3-5 MPa (Mayer et al. 1990), lung hemorrhage at 1-2 MPa (Hartman et al. 1990b), malformations in chick embryos at 5-10 MPa (Hartman et al. 1990a), induction of premature ventricular contraction in frog hearts at 5-10 MPa (Dalecki et al. 1990) and killing of Drosophila larvae at 1-3 MPa (Carstensen et al. 1990a). Since pressures greater than these are used in diagnostic ultrasound (Duck et al. 1985), it is reasonable to ask whether similar effects result from exposure to pulsed ultrasound. Mouse kidney showed no significant signs of damage even after exposure to 1 and 4 MHz pulsed ultrasound at pressure levels somewhat above the thresholds for damage by lithotripter fields (Carstensen et al. 1990b). In contrast, this study provides very strong evidence that mouse lung is damaged by exposure to pulsed ultrasound and that the thresholds for these effects are similar to those found with spark-generated shock waves.
The source transducers used in this study were piezoceramic discs either in direct contact with the water in the exposure vessel or cemented to the back of planoconcave lenses. Specifications are summarized in Table 1. Acoustic pressures were measured with a 1 mm diameter, bilaminar PVDF membrane hydrophone (Marconi Research Center, Chelmsford, England). The hydrophone was calibrated by comparison with a steel sphere radiometer (Dunn et al. 1977). Threshold waveforms and pulses are shown in Figs. 1-6. Pulse average intensities were determined by integrating the square of the measured voltages over the length of the pulse using a LeCroy Model 9400 (Chestnut Ridge, NY) digital oscilloscope. Pressure amplitude of the fundamental component of the wave was determined by low-pass filtering of the signal from the hydrophone. Male C3H mice, approximately 7 weeks old, were obtained from Jackson Labs (Bar Harbor, ME), fed ad libitum and cared for in accordance with institutional guidelines on animal care. Before exposure, the mice were anesthetized with Ketamine (200 mg/kg) and Rompun (10 mg/kg). Their backs were shaved with clippers and the remaining hair was removed with Neet ® to minimize the likelihood of en-
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Ultrasound in Medicine and Biology
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Table 1. Physical characteristics of the sources used in the study. Frequency 1.1 Unfocused Diameter (cm) Distance from source (cm)
1.2 Focused
2.5 1l
t r a p m e n t o f air at the skin-water interface. The animals were positioned on a rubber frame in a tank of degassed water with their dorsal surfaces toward the source. The frame, in turn, was supported on a threeway positioner that allowed stable, precise location of the mouse with respect to the source. The coordinates of the field point as measured by the hydrophone were transferred to the center of each mouse lung. T h r e e - m i n u t e exposures were given approximately in the middle of each lung. I m m e d i a t e l y following treatment, the animals were euthanized by cervical dislocation, the lungs were removed and the lung surface was inspected for signs of hemorrhage. Positive findings were extremely rare in sham-exposed mice. Near threshold, the d a m age was localized to the site of exposure and to the p r o x i m a l surface o f the lung. At s u p e r t h r e s h o l d levels, hemorrhage extended through the lung to the far side. In all cases, there was a distinct limit below which the lungs appeared normal. Thresholds were a s s u m e d to lie b e t w e e n the lowest levels w h i c h showed damage and the upper level which did not. N e a r the threshold levels, scoring was blind. Attenuation by the rib cage was measured by positioning a P V D F needle h y d r o p h o n e ( N e l s o n T w o m e y Research, Seattle, WA) on the axis of the sound beam, establishing a reference pressure level a n d t h e n bringing the s k i n - c o v e r e d rib cage o f a
2.5 5
2.3 Unfocused 1.2 6
3.5 Unfocused
3.7 Focused
1.2 3.5
2.5 5
mouse (lungs removed) around the end of the hydrophone. The mouse was then m o v e d vertically and the m a x i m u m and m i n i m u m transmitted pressures were recorded. T h e m e a s u r e m e n t s o f a t t e n u a t i o n were performed at such low levels that the waveform was essentially sinusoidal. All of the raw threshold data (Figs. 7-12) are presented in terms of the temporal peak, positive pressures measured in water. EXPERIMENTAL
RESULTS
The experiments determined the dependence of thresholds upon frequency, b e a m diameter, temporal average intensity and pulse length for representative subsets o f the o b s e r v a t i o n s . D a t a for u n f o c u s e d sources at 1.2, 2.3 and 3.5 M H z are presented in Fig. 7. Raw data for the threshold determinations with 1.2 and 3.7 M H z focused sources are presented in Fig. 8. Figures 9 and 10 use these data to compare focused a n d u n f o c u s e d sources at the low- a n d high-frequency limits of the study. These plots m a y d e m o n strate better than Figs. 7 and 8 the sharpness of the threshold. The lack of dependence of the threshold on pulse repetition frequency and on the total n u m b e r of pulses in the exposure is shown in Fig. 1 1. Figure 12 gives the raw data for 1 and 10 us pulse lengths. The real thresholds, of course, must be expressed in terms o f the in situ field levels. Attenuation data
im_mMump, nilllnn
Fig. 1. Threshold pulse shape and waveform in water for the 1.2 MHz, 2.5 cm diameter, unfocused source at a distance of 11 cm. Pulse length, 10 us; peak positive pressure, 0.8 MPa; pulse average intensity, 10 W/cm 2.
Lung damage from exposure to pulsed ultrasound • S. Z. CHILDel al.
819
Fig. 2. Threshold pulse shape and waveform in water for the 1.2 MHz, 2.5 cm diameter, focused source at a distance of 5 cm. Pulse length, 10/~s; peak positive pressure, 0.8 MPa; pulse average intensity, 15 W/cm 2.
are s u m m a r i z e d in Table 2. Presumably because of the i n h o m o g e n e o u s c h a r a c t e r o f the rib cage, the transmitted signal varied in amplitude with deliberate small vertical displacements of the m o u s e during the a t t e n u a t i o n m e a s u r e m e n t s . A r g u m e n t s c o u l d be m a d e for using either the m a x i m u m or m i n i m u m attenuation values observed depending u p o n the intended use o f the derated data. We have chosen to use the m i n i m u m attenuation in our subsequent analysis. O u r reasoning is that the threshold for hemorrhage appears to be sharply defined and related to spatial peak, temporal peak amplitude o f the sound wave. There is a greater probability that some small region of the lung surface will be exposed to the maxi m u m signal than that the entire lung will be exposed uniformly to the m i n i m u m signal. Although we feel that this is the m o s t realistic approach, it does set upper limits for in situ threshold values. Furthermore, we have only looked at superficial damage to the lung
in this study. Future investigations could reveal m o r e subtle injuries. T a b l e 3 s u m m a r i z e s the threshold data in terms o f linearly derated (in situ) pressures and intensities. Figure 13 summarizes the results in t e r m s o f the in situ, f u n d a m e n t a l pressure in the pulse. DISCUSSION
Comparison with earlier studies U p to the present time, there has been no convincing, direct e v i d e n c e t h a t clinically r e l e v a n t , pulsed u l t r a s o u n d p r o d u c e s deleterious effects in m a m m a l i a n tissue in the absence of significant heating. In a recent study, we failed to find effects of pulsed ultrasound on kidney tissues at peak positive pressures a p p r o a c h i n g 10 M P a (Carstensen et al. 1990b) in spite o f the fact that lithotripter shocks o f that amplitude produced marked hemorrhage in the
Fig. 3. Threshold pulse shape and waveform in water for the 2.3 MHz, 1.2 cm diameter, unfocused source at a distance of 6 cm. Pulse length, 10 t~s; peak positive pressure, 1.5 MPa; pulse average intensity, 30 W/cm 2.
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Ultrasound in Medicine and Biology
Volume 16, Number 8, 1990
-. -,
it I I
Fig. 4. Threshold pulse shape and waveform in water for the 3.5 MHz, 1.2 cm diameter, unfocused source at a distance of 3.5 cm. Pulse length, 10 us; peak positive pressure, 2.8 MPa; pulse average intensity, 90 W/cm 2.
same biological preparation (Mayer et al. 1990). In this study, the effect o f pulsed ultrasound on lung tissue is demonstrated clearly and reproducibly, and thresholds for the effect are on the order of 1 MPa. The theoretical possibility that cavitation could occur within the body has been recognized for m a n y years. Fry and colleagues at the University o f Illinois (1970) found histological suggestions o f cavitation in brain tissue with single continuous-wave (CW) exposures as long as 10,000 #s at pressures above 10 M P a or intensities greater than 3000 W / c m 2. Lele (1977) r e p o r t e d detection o f an a b r u p t increase in white noise from brain tissue irradiated continuously at approximately the same levels. H e found a correlation between this evidence of transient cavitation and histological signs o f extravasation and tissue disintegration. The closest parallel in the literature to the effects which are seen in lung is the killing of Drosophila larvae by exposure to pulsed ultrasound (Child et al. 1981; Berg et al. 1983). The threshold values are com-
parable as shown in Fig. 13. In each case, thresholds d e p e n d only very weakly on pulse repetition frequency. Most o f the evidence for killing of larvae is consistent with a cavitation-related m e c h a n i s m although it a p p e a r s that classical cavitation theory, which assumes that a bubble is free to expand without limit in an aqueous m e d i u m , is not adequate to describe the processes that involve the interaction of acoustic fields with bubbles that are confined within tissues (Carstensen et al. 1990a). The results reported here are i m p o r t a n t not because they reveal an unexpected hazard, but because they provide, for the first time, a clearly defined, reproducible nonthermal effect of diagnostically relevant ultrasound in a m a m malian system that has m a n y of the characteristics that might be expected of cavitation.
Comparison with consensus statements These observations can be viewed from the perspective o f the American Institute of Ultrasound in
mmmmm roam inmmm mmm nmiamaimamm
Fig. 5. Threshold pulse shape and waveform in water for the 3.7 MHz, 2.5 cm diameter, focused source at a distance of 5 cm. Pulse length, 10 its; peak positive pressure, 2 MPa; pulse average intensity, 70 W/cm 2.
Lung damage from exposure to pulsed ultrasound • S. Z. CHILD et al.
821
Fig. 6. T h r e s h o l d pulse s h a p e a n d w a v e f o r m in water for the 3.7 M H z , 2.5 c m diameter, focused source at a distance o f 5 cm. Pulse length, 1 us; peak positive pressure, 3.9 M P a ; pulse average intensity, 180 W / c m 2.
Medicine's (AIUM 1988) Mammalian Bioeffects S t a t e m e n t . A s r e q u e s t e d b y t h e s o u r c e , it is q u o t e d h e r e i n its e n t i r e t y . F o o t n o t e a w a s a t e m p o r a r y p a t c h a d d e d i n 1 9 8 7 t o e l i m i n a t e l i t h o t r i p t e r effects w h i c h w e r e k n o w n a t t h a t t i m e t o c a u s e b i o l o g i c a l effects but which were thought to have no direct relevance to pulsed ultrasound. A review of bioeffects data supports the following statement as an update of the AlUM Statement on I n Fivo Mammalian Bioeffects: In the low megahertz frequency range there have been (as of this date) no independently confirmed significant biological effects in mammalian tissues exposed in vivo to unfocused ultrasound with intensitiesa below 100 mW/cm 2, or to focused b ultrasound with
intensities below 1 W/cm 2. Furthermore, for exposure times c greater than 1 second and less than 500 seconds for unfocused ultrasound or 50 seconds for focused ultrasound, such effects have not been demonstrated even at higher intensities when the product of intensity and exposure time is less than 50 joules/cm 2.
"Free-field spatial peak, temporal average (SPTA) for continuous wave exposures, and for pulsed-mode exposures with pulses repeated at a frequency greater than 100 Hz.
b Quarter-power ( - 6 db) beam width smaller than four wavelengths or 4 mm, whichever is less at the exposure frequency. c Total time including off-time as well as on-time for repeated pulse exposures.
5
5
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Fig. 7. H e m o r r h a g e in m o u s e l u n g exposed to u n f o c u s e d sources at 1.2, 2.3 a n d 3.5 M H z . E a c h p o i n t is the assessm e n t for a single lung. Filled squares are positive; white p o i n t s are negative.
0
013000000
1.2 M H z
3.7 M H z
Fig. 8. H e m o r r h a g e in m o u s e l u n g e x p o s e d to focused sources at 1.2 a n d 3.7 M H z . Each p o i n t is the assessment for a single lung. Filled squares are positive; white p o i n t s are negative.
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U l t r a s o u n d in M e d i c i n e a n d Biology
V o l u m e 16, N u m b e r 8, 1990
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First, the m i n i m u m levels at which effects had been observed in m a m m a l s at the writing o f this statement was greater than 0.1 W / c m 2. In the present study, the m i n i m u m value of the temporal average intensity which produced a lesion in lung was smaller than the A I U M limit by nearly two orders of magni-
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Fig. 9. Frequency of occurrence of hemorrhage in mouse lung exposed to an unfocused source (1.2 MHz) and a focused source (1.2 MHz).
cv ~rj
1/10,000
I
5
Fig. 10. Frequency of occurrence of hemorrhage in mouse lung exposed to an unfocused source (3.5 MHz) and a focused source (3.7 MHz).
Fig. 11. Hemorrhage in mouse lung exposed to a focused source of 1.2 MHz. Each point is the assessment for a single lung. Filled squares are positive; white points are negative. Essentially the same threshold is found for a 1/1000 duty cycle (100 Hz repetition frequency) or a 1/10,000 duty cycle (10 Hz repetition frequency).
tude. We m a d e no attempt to determine the threshold in terms o f the temporal average intensity because it is apparent that this is not the appropriate parameter to describe the kind of effect that is observed. If it is a s s u m e d that a cavitation-related process is involved, it is reasonable that some kind o f temporal p e a k p a r a m e t e r r a t h e r t h a n an average intensity would be expected to correlate with the effect. The framers of the A I U M statement were aware of this, but there was no clear evidence for the occurrence o f cavitation in m a m m a l s when the statement was formulated. Second, the A I U M statement says that no effects for focused ultrasound have been observed at intensities below 1 W / c m 2, i.e., 10 times the value for unfocused fields or 1000 times greater than the m i n i m u m temporal average intensities for lung hemorrhage in the present study. If one assumes that the p h e n o m e non is related to cavitation and involves the response o f individual microbubbles to the local pressure, the threshold should be essentially independent of the cross-section of the acoustic beam. In fact, independence of threshold u p o n b e a m diameter (at a given frequency) m a y be used as a criterion for the selection o f an appropriate physical descriptor of the sound field to be used as a predictor of lung hemorrhage as discussed below.
Lung damage from exposure to pulsed ultrasound • S. Z. CHILDet al.
considered the case of bubbles surrounded by a layer o f surface active material. The theoretical curves in Fig. 13 are for short, sinusoidal pulses of ultrasound, typically ~ 1 us. These theoretical a r g u m e n t s are based on the assumption that a bubble can expand without limit in a fluid m e d i u m . Since this is not true for the air bodies in the respiratory system of Drosophila larvae (Carstensen et al. 1990a) a n d very likely this is not the case for the bubbles in mouse lung, the theory must be applied with caution. Even so, the data show the expected qualitative dependence u p o n frequency and pulse length and order-ofmagnitude agreement with the expected level of the thresholds.
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At the time of its formulation, the A I U M (1988) statement on cavitation expressed a concern for possible effects at levels greater than 10 MPa, an order of magnitude greater than the thresholds for lung hemorrhage in the present study.
Comparison of the results with theory for transient cavitation Flynn (1982) concluded that thresholds for transient cavitation for bubbles exposed to microsecondlength pulses o f ultrasound increase as the frequency increases above 2 M H z as shown in Fig. 13. The absolute thresholds at a given frequency increase for decreasing bubble size. This kind of frequency dependence is to be expected for bubbles up to approxim a t e l y 3 ~zm in radius for frequencies c o m m o n l y used in medical diagnosis. U n f o r t u n a t e l y , a l m o s t nothing is k n o w n about the size distribution of gas bodies in m a m m a l i a n tissues. By assuming that optim u m - s i z e d bubbles are always present in the m e dium, Holland and Apfel (1989, 1990) and Apfel and Holland (1990) concluded that pressure thresholds for transient cavitation increase approximately with the square root of the frequency. R o y et al. (1990)
The s u m m a r y of threshold data in Table 3 provides some guidance in the selection of appropriate parameters for description of the fields in terms of their effectiveness in causing lung hemorrhage. Although it is quite clear that temporal average intensities have little value, temporal peak positive (compression) pressure, temporal m a x i m u m negative (rarefaction) pressure, the temporal peak pressure at the fundamental frequency or pulse average intensity are all correlated roughly with the observed damage to the lung. O f these, the fundamental pressure appears to be least sensitive to b e a m diameter. There is a rationale in theory for using the fundamental pressure in a nonlinearly distorted wave as a predictor of cavitation. T h e h i g h - f r e q u e n c y c o m p o n e n t s in a nonlinearly distorted wave are less effective in causing cavitation than the fundamental frequency. N o t only does the threshold for transient cavitation increase with frequency but, at high frequencies, the change in response is m u c h less dramatic and, in fact, the very concept o f a threshold becomes nebulous (Flynn, 1982). The usefulness o f the fundamental frequency as a predictor o f cavitation was tested for a typical nonlinearly distorted, a s y m m e t r i c wave theoretically (Aym6 and Carstensen 1989a) and experimentally in the killing o f Drosophila larvae (Aym6 and Carstensen 1989b). It turned out that the negative and fundamental pressures in these waves were sufficiently Table 2. Attenuation of mouse rib cage--Ratios of pressures within chest cavity of a mouse to the free field pressure. Measurements were made with small signal (sinusoidal) levels.
1.1 MHz 3.4 MHz
Minimum (_+SD)
Maximum (_+SD)
0.84 _ 0.04 0.75 _+0.06
0.55 _+0.05 0.45 + 0.06
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Ultrasound in Medicine and Biology
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Table 3. Threshold levels for lung hemorrhage in mice exposed to pulsed ultrasound. Data in Figs. 7-12 were used to determine threshold levels as defined in the text. Exposure parameters were measured as shown in Figs. 1-6. Values for the fundamental pressure amplitude py were obtained by low-pass filtering of the data. Pulse average intensities leA were obtained by integrating the squared pressure over the pulse and dividing by the on-time of the electrical excitation of the source. These values were derated linearly using the attenuation data in Table 2 to give the in situ values for this Table. Frequency
1.1 Unfocused
1.2 Focused
1.2 Focused
2.3 Unfocused
3.5 Unfocused
3.7 Focused
3.7 Focused
10 #s 1/1000
10 #s 1/1000
10 #s 1/10,000
10 #s 1/1000
10 #s 1/1000
10 #s 1/1000
1 #s 1/1000
0.7 0.4 0.6 7 7
0.7 0.7 0.7 11 11
0.7 0.7 0.7 11 1
1.2 0.6 0.8 20 20
2.1 1.3 1.3 50 50
1.5 1.0 1.2 40 40
3.0 1.4 1.8 100 100
Pulse length Duty cycle Thresholds p+ (MPa) p_ (MPa) p/(MPa) lea (W/cm 2) ITA (mW/cm 2)
similar that they were almost equally useful predictors. The differences a m o n g the various parameters are clearer in the lung study. In Table 3, the maxi-
m u m negative pressures do not fare as well in the focused/unfocused criterion as does the fundamental pressure either at high or low frequency.
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10
Frequency (MHz)
Fig. 13. Comparison of theoretical predictions of thresholds for transient cavitation in water with thresholds for killing of Drosophila larvae and mouse lung hemorrhage. The solid curves are for 1 #s pulses with bubble radii of 0.3 #m (upper curve) and 1 #m (lower curve) (Flynn 1982; Carstensen and Flynn 1982). For a given bubble size, thresholds become less distinct as frequency increases. The dotted curve is a similar analysis under the assumption that bubbles of the optimum size for each frequency are present (Apfel and Holland 1990). Roy et al. (1990) (dashed line) considered the case of a bubble surrounded by a layer of surface active material. All of the theoretical predictions are for short (1 #s), sinusoidal pulses of ultrasound. The data for Drosophila larvae are for 1 #s pulses ofsinusoidal ultrasound. Only the mouse lung data involved exposures to fields with significant nonlinear distortion of the waveform.
There appears to be a consensus a m o n g the members of the medical ultrasound c o m m u n i t y that users should be informed of the acoustic output levels o f diagnostic equipment. Progress has been made recently in relating tissue heating to the temporal average intensity of the sound field. However, hemorrhage in the mouse lung is the first biological effect in m a m m a l s that has been related directly to a temporal peak parameter of the diagnostic ultrasound field. In addition to information on absolute magnitudes of cavitation-related thresholds and their frequency dependence, it is apparent from these studies that the thresholds are somewhat lower for the relatively long pulses used in Doppler instruments than for the short pulses characteristic of scanners. According to a recent survey o f the output levels o f diagnostic u l t r a s o u n d e q u i p m e n t ( D u c k et al. 1985), the modal pressure in water for Doppler instruments is ~ 5 MPa. On the one hand, commercial Doppler devices typically operate at higher pulse repetition frequencies and at higher temporal average intensities than were used in this study. In extreme cases, the exposures could be complicated by ultrasonic heating of the tissue. On the other hand, it must be assumed that the attenuation o f the signal before reaching the focus in h u m a n patients is somewhat greater than for the mouse. Whether h u m a q lung is acoustically and pathologically equivalent to mouse lung for these effects remains to be demonstrated. The experimental data for lung show that the threshold for hemorrhage is sharply defined. For the moment, it is reasonable to assume that, as observed with Drosophila larvae (Child et al. 1981), exposures
Lung damage from exposure to pulsed ultrasound • S. Z. CHILD et al.
with much higher temporal average intensities extending for much longer times will have no effect as long as the temporal peak amplitude does not exceed that threshold. Overall, the safety picture for medical ultrasound which emerges from this new information is encouraging. Many diagnostic procedures can be carfled out below the threshold levels for lung hemorrhage. It may be assumed that, if appropriate bubbles exist in other parts of the body, similar effects could occur. However, our studies of the effects of pulsed ultrasound on mouse kidney show that the probability of such an occurrence is extremely small even at the highest levels used in diagnosis (Carstensen et al. 1990b). In that study, peak positive pressures were almost an order of magnitude greater than the threshold for lung hemorrhage. In fact, when fetal lung was exposed to lithotripter fields with peak positive pressures of 20 MPa, only 2% of the samples showed signs of hemorrhage (Hartman et al. 1990b). All evidence available at the present time suggests that if cavitation occurs in organs other than the lung, it probably occurs rarely and its effects are highly localized. Acknowledgments--This work was supported in part by U.S.P.H.S. Grants No. DK39796 and CA39241.
REFERENCES American Institute of Ultrasound in Medicine (AIUM), Bioeffects Committee. Bioeffects considerations for the safety of diagnostic ultrasound. J. Ultrasound Med. 7:(Suppl. 9)S1-$38; 1988. Apfel, R. E.; Holland, C. K. Gauging the likelihood of cavitation from short pulse, low duty cycle diagnostic ultrasound. Ultrasound in Med. & Biol. [1990]. Aymr, E. J.; Carstensen, E. L. Cavitation induced by asymmetric, distorted pulses of ultrasound: Theoretical predictions. IEEE Trans. Ultrasonics, Ferroelectrics and Freq. Control 36:32-40; 1989a. Aym& E. J.; Carstensen, E. L. Cavitation induced by asymmetric, distorted pulses of ultrasound: A biological test. Ultrasound in Med. & Biol. 15:61-66; 1989b. Berg, R. B.; Child, S. Z.; Carstensen, E. L. The influence of carrier frequency on the killing of drosophila larvae by microsecond pulses of ultrasound. Ultrasound in Med.& Biol. 9:L448-L451 ; 1983. Carstensen, E. L.; Flynn, H. G. The potential for transient cavitation with microsecond pulses of ultrasound. Ultrasound in Med. & Biol. 8:L720-L724; 1982. Carstensen, E. L,; Campbell, D. S.; Hoffman, D.; Child, S. Z.; Aymr-Bellagarda, E. Killing of Drosophila larvae by the fields
825
of an electrohydraulic lithotripter. Ultrasound in Med. & Biol. 16(7):687-698; 1990a. Carstensen, E. L.; Hartman, C.; Child, S. Z.; Cox, C. A.; Mayer, R.; Schenk, E. Test for kidney hemorrhage following exposure to intense, pulsed ultrasound. Ultrasound in Med. & Biol. 16(7):681-685; 1990b. Child, S. Z.; Carstensen, E. L.; Lam, S. K. Effects of ultrasound on Drosophila: III. Exposure of larvae to low-temporal-average-intensity, pulsed irradiation. Ultrasound in Med. & Biol. 7:167173; 1981. Delius, M.; Enders, G.; Heine, G.; Stark, J.; Remberger, K.; Brendel, W. Biological effects of shock waves: Lung haemorrhage by shock waves in dogs--Pressure dependence. Ultrasound in Med. & Biol. 13:61-67; 1987. Delius, M.; Jordan, M.; Eizenhoefer, H.; Marlinghaus, E.; Heine, G.; Liebich, H.-G.; Brendel, W. Biological effects of shock waves: Kidney haemorrhage by shock waves in dogs--Administration rate dependence. Ultrasound in Med. & Biol. 14:689694; 1988a. Delius, M.; Enders, G.; Xuan, Z.; Liebich, H.-G.; Brendel, W. Biological effects of shock waves: Kidney damage by shock waves in dogs--Dose dependence. Ultrasound in Med. & Biol. 14:117-122; 1988b. Dalecki, D.; Carstensen, E. L.; Keller, B. Thresholds for premature ventricular contractions caused by lithotripter shocks. Ultrasound in Med, & Biol. Submitted for publication; 1990. Duck, F. A.; Starritt, H. C.; Aindow, J. D.; Perkins, M. A.; Hawkins, A. J. The output of pulse-echo ultrasound equipment: A survey of power, pressure and intensities. Br. J. Radiol. 58:989-1001; 1985. Dunn, F.; Averbuch, A. J.; O'Brien, W. D., Jr. A primary method for the determination of ultrasonic intensity with the elastic sphere radiometer. Acustica 38:58-61 ; 1977. Flynn, H. G. Generation of transient cavities in liquids by microsecond pulses of ultrasound. J. Acoust. Soc. Am. 72:19261932; 1982. Fry, F. J.; Kossoff, G.; Eggieton, R. C.; Dunn, F. Threshold ultrasonic dosages for structural changes in mammalian brain. J. Acoust. Soc. Am. 48:1413-1417; 1970. Hartman, C.; Cox, C. A.; Brewer, L.; Child, S. Z.; Cox, C. F.; Carstensen, E. L. Effects of lithotripter fields on development of chick embryos. Ultrasound in Med. & Biol. 16(6):581-585; 1990a. Hartman, C.; Child, S. Z.; Mayer, R.; Schenk, E.; Carstensen, E. L. Lung damage from exposure to the fields of an electrohydraulic lithotripter. Ultrasound in Med. & Biol. 16(7):675-679; 1990b. Holland, C. K.; Apfel, R. E. An improved theory for the prediction of microcavitation thresholds. IEEE Trans. Ultrasonics, Ferroelectrics, and Freq. Control 36:204-208; 1989. Holland, C. K.; Apfel, R. E. Thresholds for transient cavitation produced by pulsed ultrasound in controlled nuclei environment. J. Acoust. Soc. Am. [1990]. Lele, P. P. Thresholds and mechanisms of ultrasonic damage to "organized" animal tissues. In: Proceedings of the Symposium on Biological Effects and Characterizations of Ultrasound Sources, Rockville, MD; 2-3 June 1977:224-238. Mayer, R.; Schenk, E.; Child, S.; Norton, S.; Cox, C. A.; Hartman, C.; Cox, C. F.; Carstensen, E. Pressure threshold for shockwave induced renal hemorrhage. J. Urology [1990]. Roy, R. A.; Church, C. C.; Calabrese, A. Cavitation produced by short pulses of ultrasound. In: Proceedings of the International Conference on Nonlinear Acoustics, Austin, TX; August, 1990 [in press].