Donald School Textbook of Ultrasound in Obstetrics & Gynecology Asim Kurjak, Frank A Chervenak
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Section 1
1General Aspects2

Safety of Ultrasound inObstetrics and GynecologyChapter 1

Kazuo Maeda
 
INTRODUCTION
Although no adverse effects of ultrasound diagnosis have been reported, bioeffect and safety issues have been studied and discussed by various medical ultrasound organizations.1-9 It is emphasized that, for safe use, ultrasonic examinations are only performed when medically indicated. Secondly, the users are responsible for safety and should recognize that biological tissues of developing embryos and fetuses may be damaged by intense ultrasound.6 The main biological effect is the thermal effect due to the temperature rise induced by ultrasound absorption, because teratogenicity was reported in fetal animals exposed to high temperature.2 Non-thermal effects of ultrasound are inertial cavitation and other mechanical effects. Diagnostic ultrasound users are requested to know the ultrasonic intensity of their devices, the mechanisms of ultrasound bioeffects and prudent use of their ultrasound devices. No hazardous thermal effects are expected when the temperature rise in exposed tissue is less than 1.5ºC and local temperature is lower than 38.5ºC,1 the fetus was tolerable to 50 hours exposure up to 2oC rise,5 while 5 minutes at 41ºC can be hazardous to the tissue.1 No hazardous thermal effects are expected in simple B-mode imaging devices because there is minimum heat production due to low ultrasound intensity. World Federation of Ultrasound in Medicine and Biology (WFUMB) concluded that the use of simple imaging equipment is not contraindicated on thermal grounds.1 Simple transvaginal B-mode, simple three-dimensional (3D) and four-dimensional (4D) imaging are included in this category. The International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) also stated the safe use of Doppler ultrasound.7 More practical plans on the safe use of Doppler ultrasound from the user's view is discussed in this chapter. Direct subject heating with transvaginal transducer is avoided where the transducer is lower than 41ºC.
Thermal index: 1.0 thermal index (TI) is the ultrasound power to rise the temperature of exposed tissue for one degree centigrade. Soft tissue TI (TIs) is lower than bone TI (TIb) and transcranial TI (Tic). TIb is used in the safety of ultrasound, where the TI less than 1.0 is safe. Mechanical effect of ultrasound is expressed by mechanical index (MI), where MI is rarefactional pressure (Megapascal, Pr) divided by the square root of ultrasound frequency (Megahertz, MHz). Obstetric diagnostic devices are set at ultrasonic intensity lower than 1.0 TI and lower than 1.0 MI. Output intensity of simple B-mode equipment was regulated below SPTA 10 mW/cm2 in 1980 by Japanese Industrial Standard (JIS), where the level was 1/24 of non-hazardous threshold of ultrasound, which is SPTA 240 mW/cm2, and it was the results of our experimental study.11 Ultrasound of Doppler flow velocity measurements, color and power Doppler mappings are set at the level lower than 1.0 TI and 1.0 MI.
Recent two studies12,18 reported hazardous ultrasound effect, however, the effects are doubted, because the effects could not be expected because ultrasound intensity was lower than 240 mW/cm2 but can be resulted by the heating with attached ultrasound probes. As the increase of hepatic apoptosis of animal fetus by Doppler ultrasound was reported,19 ISUOG21 regulated the use of Doppler ultrasound in early pregnancy.
 
DIAGNOSTIC ULTRASOUND DEVICES AND ULTRASOUND INTENSITY
4Ultrasonic imaging devices and Doppler blood flow studies utilize pulsed wave (PW) ultrasound, while continuous wave (CW) ultrasound is applied in fetal functional tests (Table 1.1).
The ultrasound intensity differs between PW and CW devices (Figs 1.1AandB), i.e. temporal peak intensity is large in PW and weak in CW ultrasound, while temporal average intensities are nearly the same to temporal peak of CW.
 
NON-HAZARDOUS EXPOSURE TIME OF THE FETUS TO THE HEAT
The revised safety statement on diagnostic ultrasound of American Institute of Ultrasound in Medicine (AIUM)5 published in 1998, is based on the NCRP report2 in 1992, where inverse relation is found between hazardous temperature level and exposure time. They stated that the fetus tolerated 1 min at 6°C temperature rise (absolute temperature is 43°C). They showed the relation of the temperature rise above 37°C and the non-hazardous exposure time was 1000 min if the temperature rise was 1°C.2
 
DIAGNOSTIC ULTRASOUND INSTRUMENTS AND ULTRASOUND INTENSITY
Ultrasonic imaging devices and Doppler blood flow studies utilize pulse wave (PW) ultrasound, while continuous wave (CW) ultrasound is applied in fetal functional tests (Table 1.1). The ultrasound intensity differs between PW and CW machines (Fig. 1.1), i.e. temporal peak intensity is large in PW and weak in CW ultrasound, while temporal average intensity is almost identical in simple PW B-mode imaging device and CW machines (Table 1.1).
Table 1.1   Diagnostic ultrasound
Index
Mortality/1000 live births
Pulsed wave (PW) for imaging and blood flow studies
Continuous wave (CW) for the functional tests
B-mode imaging, 3D/4D ultrasound, Pulsed Doppler flow velocity wave, Color/power flow mapping
Fetal heart Doppler detector, Fetal heart rate tracing, Fetal movement record (actocardiogram), CW Doppler flow velocity wave
High peak intensity and low temporal average intensity in simple B-mode and 3D/4D ultrasound.
High peak and average intensities in pulsed Doppler flow velocity wave
High peak intensity and medium average intensity in color/power Doppler flow mapping.
Low peak and average intensities
zoom view
Figures 1.1A and B: Two types of diagnostic ultrasound waves. (A) Pulse wave (PW) for the imaging and Doppler: 1/t is repetition frequency; (B) Continuous wave (CW) for fetal functional studies in fetal heart detector
However, pulsed Doppler flow velocity measurement tends to use high peak and average intensity due to its long pulse and high repetition frequency (Fig. 1.1). The temporal average intensity of color and power Doppler flow mapping is lower than pulsed Doppler but tends to be higher than simple B-mode machine. Ultrasound intensity is usually represented by spatial peak (SP) temporal average (TA) intensity, i.e. by SPTA intensity, e.g. cultured cell growth curve was not injured, if ultrasound intensity was lower than SPTA 240 mW/cm2 in our experiment.11
 
ULTRASOUND INTENSITY OF DOPPLER ULTRASOUND
The maximum intensity of adult Doppler ultrasound was 1–3 W/cm2, which was as high as the ultrasonic physiotherapy with the tissue heating, where young patient's bone and pregnant women were contraindicated from the concern on ultrasound effect. The difference between therapeutic ultrasound and pulsed Doppler device was the exposure duration, which was shorter in Doppler ultrasound. Temperature rises not only at the sample volume but also in all tissues passed by the Doppler ultrasound beam. Ultrasound intensity is lower in color/power Doppler flow mapping than pulsed Doppler flow wave because of scanning ultrasound beam in the region of interest (ROI) of flow mapping. Ultrasound intensity of color Doppler is lower than pulsed Doppler and within the limit of nonhazard wwous FDA intensity, which was 720 mW/cm2. Thermal effect was discussed firstly in pulsed Doppler, where the safety was determined by ultrasound intensity and exposure duration, where thermal index (TI) and mechanical index (MI)<1.0 is safe in the use of Doppler ultrasound.
5
 
THE EFFECT OF DIRECT HEATING ON MAMMAL FETUSES
The presence of teratogenicity was reported by biologists in the direct exposure of mammal animal embryos and fetuses to experimental high temperature of 39–50oC in various mammals, avoiding the heating of pregnant small animals, to eliminate the influence of maternal animal reaction to the heat. The results were fetal animal head and neck anomalies,2 where a discriminal line clearly separated hazardous and non-hazardous areas. Non-hazardous exposure time was as short as one min in 43°C and infinite in physiological body temperature, 37°C. Ultrasound intensity is very low to be regulated in autocorrelation fetal heart rate meter, actocardiography and CW ultrasound Doppler devices.
 
THE ULTRASOUND INTENSITY OF NO BIOEFFECT
The effect of ultrasound exposure was studied using JTC-3 human amniotic cell origin cultured cell strain. Our experiments aimed the suppression of cultured cell growth curves with CW and PW ultrasound, in the study group of Japan, Ministry of Health and Welfare in 1973–1975, where the exposure experiment was performed avoiding the heat of ultrasound probe by inserting at 37°C stabilized water between the probe and exposure subjects.
The cultured cell growth curve was suppressed by exposure to PW ultrasound at 2 MHz SPTA 240 or more mW/cm2, 3 μs pulse duration and 1 kHz repetition PW ultrasound, and 2 MHz, 1000 or more mW/cm2 CW ultrasound. No suppression was detected when the ultrasound intensity was lower than 240 mW/cm2 in PW and 1,000 mW/cm2 in CW.
Official bioeffect threshold to suppress the cultured cells was 240 mW/cm2, where ultrasound intensity was determined by the manufacturer's data and furthermore by our steel-ball moving method. Exposure duration was 30–60 min.
The 1.0 TI intensity was determined deducting standard propagation attenuation and standard perfusion from the threshold intensity, and the 1.0 TI intensity was around SPTA 210 mW/cm2.
The most important conclusion on the results of or experiments is “the ultrasound is totally safe, if its intensity is lower than SPTA 240 mW/cm2 or lower than 1.0 TI”, namely, the safety of diagnostic ultrasound was objectively established, where no ultrasound bioeffect was recognized when the intensity was lower than SPTA 240 mW/cm2.
 
“ALARA” PRINCIPLE
The output intensity of commercial diagnostic ultrasound devices were regulated to be lower than 10 mW/cm2 by the Japan Industrial Standard (JIS) in 1982, which was about 1/20 of 1.0 TI intensity, therefore, the ultrasound safety of diagnostic ultrasound device was established in Japan, while international ultrasound safety is kept when obstetrical setting is lower than 1.0 in TI and MI. However, Sande et al.22 reported useful Doppler flow wave was obtained using 0.1 TI pulsed ultrasound, which was 1/10 of 1.0 TI, equal to about 20 mW/cm2, and close to JIS regulation. The ultrasound intensity of Doppler actocardiogram, invented by the author, was only 1 mW/cm2 in its TOITU commercial model.
ALARA (As Low AS Reasonably Achievable) principle, which is another principle of diagnostic ultrasound safety, that is further reduction of ultrasound intensity below 1.0 TI, getting sufficient clinical results even in Doppler study with the intensity lower than 1.0 TI.
Low TI will be determined correctly using TI measurement device provided by ISUOG.
 
ABSOLUTE TEMPERATURE OF THE TISSUE EXPOSED TO ULTRASOUND
Absolute tissue temperature is determined by the addition of temperature rise estimated by TI to 37°C, e.g. absolute temperature is 38°C, when TI is 1, and 39°C when TI is 2. TI is an useful value for various purposes.
 
THERML AND MECHANICAL SAFETIES OF DIAGNOSTIC ULTRASOUND BY USING THERMAL AND MECHANICAL INDICES
The thermal safety of ultrasound is shown by the thermal index (TI), which is theoretically equal to temperature rise due to ultrasound absorption of the subject tissue.
Although electrical and structural safeties of diagnostic ultrasound devices are proved by the manufacturer, the device user is responsible to the ultrasound safety,3 studying physics of medical ultrasound, mechanism of ultrasound images, ultrasound output intensity of user devices, thermal and mechanical indices, the threshold output intensity to develop bioeffect, how to reduce ultrasound device output intensity, non-medical use of ultrasonic imaging, regulation of ultrasound use in early pregnancy, etc., related diagnostic ultrasound safety. This is particularly important in obstetric application of ultrasound.
6Thermal effect of ultrasound appears along with the temperature elevation due to the absorption of ultrasound by the tissue, where immature fetal tissues are sensitive to high temperature, and thermal teratogenicity or embryonal cell damage can be concerned in some occasions. Mechanical effect of ultrasound is cavitation, which is production and rupture of microbubbles in the liquid developing high temperature, strong pressure and free radical formation. Neonatal animal lung develops hemorrhage by the cavitation. Negative pressure of ultrasound pulse is related cavitation, while positive pressure, streaming and static wave were not discussed in the mechanical effect of ultrasound.
 
THERMAL SAFETY OF ULTRASOUND
As direct heating of animal fetus developed head and neck anomaly in biological experiment,2 1°C temperature rise was selected to set an objective index, which avoid excess exposure in clinical ultrasound, i.e. the thermal index (TI) was prepared. TI is set below 1.0 for the thermal safety of ultrasound devices. 1.0 TI produces 1°C temperature rise of subject tissue, which will be less than SPTA 240 mW/cm2 which is the lowest intensity to suppress cultured cell growth. Any ultrasound device output is controlled to safe condition, if the output is less than 1.0 TI, preventing excess heating.
The thermal index (TI), mechanical index (MI), and other values related ultrasound safety are displayed on the monitor screen,3 making the users to keep the safe ultrasound diagnosis. Obstetric setting should be confirmed before ultrasound imaging and Doppler studies in pregnancy to keep the safety of ultrasound. Although ISUOG safety statement7 reported that there is no reason to withhold the use of scanners that have received FDA clearance, AIUM5 stated that in the FDA regulatory limit at 720 mW/cm2, the maximum temperature rise can exceed 2°C. As ultrasound intensity to suppress cultured cell-growth was 240 or more mW/cm2 in our studies,11 the FDA regulation intensity would not be accepted from the safety reasons.
 
Thermal Index to Prevent Thermal Effect of Ultrasound Exposure
Thermal index (TI) is a useful index to know the temperature rise by ultrasound exposure. Ultrasound intensity is estimated by the subject temperature rise due to ultrasound absorption under standard ultrasound attenuation and perfusion, where TI is 1.0, when the temperature rises 1°C above 37°C and absolute temperature is 38°C, while TI is 3.0 when temperature rises for 3°C and absolute temperature is 40°C, i.e. TI represents the ultrasound intensity to rise subject temperature for TI °C above 37°C. Since the subject temperature rises due to the absorption of ultrasound, TIs, soft tissue TI, is low and TIb of bone and TIc of cranium are higher than TIs. TIb is used to estimate ultrasound bioeffect, while TIs is applied embryo of no bone before 10 weeks of pregnancy, whereas bone TIb is applied in the fetus with bone.
No hazardous thermal effect is expected when the temperature rise of exposed tissue is less than 1.0°C and an ultrasound examination is safe when the TI is less than 1.0 in daily practice, particularly in the screening of pregnancy and research works. The output power is reduced until the TI is lower 1.0, if the displayed TI is 1.0 or more, though revised safety statement AIUM5 stated that equal or less than 2°C temperature rise above 37°C was tolerated by the fetus for short time.
 
Other Thermal Issues
Caution should be paid for the temperature of the tissue exposed to Doppler ultrasound in febrile patients, where the basic temperature is higher than 37°C. If ultrasound TI is 2 in 38°C febrile patient, the temperature rise above physiologic condition is 3°C, the situation corresponds to TI 3 in nonfebrile normal temperature case, therefore, ultrasound study will be contraindicated until the recovery of febrile condition.
Thermal effect of transvaginal ultrasound was discussed, and the surface temperature of transvaginal probe was regulated to be lower than 41°C.
 
Analysis of Ultrasonic Fetal Brain Damage Reports
Ang et al.12 reported the delay of neural cell migration after exposure to real time B-mode ultrasound for more than 30 min attaching ultrasound probe to the pregnant mice.
Ultrasound intensity maybe around SPTA 2 mW/cm2. Ping et al.17 exposed pregnant rats to 106 mW/cm2 ultrasound for 60 min in total, and found the damage of learning and memory function and hippocampus in 2 month’ infants.
They reported the damage was caused by ultrasound. However, the conclusion would be controversy, because their ultrasound intensity were lower than 240 mW/cm2, by which no bioeffect of ultasound was expected, and suggest the presence of the other cause.
There may be difference in the effects of two reports, if it is ultrasound effect, because ultrasound intensity was 50 times larger in Ping et al, but actually the effect was the same brain neuronal damage. The effects would be 7caused by the heat of ultrasound probe, because the probe suspectedly heated for 41 or more °C was attached pregnant small animal for 30–60 min, that may be enough to warm up small animal. Direct heating effect in NCRP report was head anomaly which suggested fetal brain neuronal damage by heating in their experiments. The experiments are recommended to repeat cutting off the heat of ultrasound probe, as we did in the exposure of cultured cells to detect ultrasound intensity to suppress the cultured ell growth curve. The recommendation is based on the experience in our study group in 1970s, namely, mouse fetuses did not develop anomaly by the ultrasound exposure to pregnant mice when animals were isolated from heated ultrasound probe by the 37°C thermostat water inserted between the probe and pregnant mice, while the fetuses developed head anomaly after the heating of pregnant mice with directly attached ultrasound probe at the abdomen of pregnant mice in a Japanese report in 1972.
 
MECHANICAL INDEX OF ULTRASOUND
Mechanical effect of ultrasound is prevented setting mechanical index (MI) lower than 1.0, even in Doppler ultrasound in obstetrical setting, because high MI ultrasound produced pulmonary hemorrhage of animal neonate. The MI is determined by rarefactional pressure (negative pressure) of the pulse wave (Megapascal) / Square root of ultrasound frequency (Megahert). Low MI prevents the development of cavitation, which is caused by the rupture of micro-bubbles produced by intense ultrasound, and characterized by high temperature, high pressure and free radical formation. There will be furthermore the microstreaming and blood cell stasis due to standing wave, of which bioeffects have not been reported.
 
SAFETY OF DIAGNOSTIC ULTRASOUND DEVICES
  1. Simple Abdominal Scan B-mode Imaging Device
    Simple B-mode imaging is not concerned for the thermal effect, because of its very low output intensity, e.g. the output of B-mode machine is regulated in Japan10 to be lower than SPTA 10 mW/cm2. In addition, either TI or MI should be lower than 1.0. Associated use of Doppler ultrasound should be controlled by the regulation listed in the chapter of Doppler Ultrasound, particularly the use in early pregnancy should be regulated by the opinion of ISUOG.21
  2. Transvaginal Scan Ultrasound
    Its use is the same as abdominal scan. Only difference is the regulation of transvaginal scan probe temperature, which should be lower than 41°C.
  3. Three-dimensional (3D) Ultrasound
    As the 3D ultrasound image is obtained by multiple simple B-mode scan in a few seconds, it is as safe as simple B-mode, of which TI and MI are less than 1.0. It is regulated when Doppler ultrasound is associated.
    Four-dimensional (4D) Ultrasound
    As the 4D ultrasound is the repetition of 3D ultrasound, basically 4D is as safe as simple B-mode ultrasound, however, fetal 4D image observation is recommended to be shorter than 30 min. Either TI or MI should be less than 1.0.
  4. Ultrasonic Doppler Method
    Pulsed Doppler ultrasound is concerned in fetal study. Either TI or MI should be less than 1.0, as low as possible. Pulsed Doppler ultrasound beam should not pass fetal body in Doppler examination of maternal or umbilical cord blood flow. Doppler ultrasound is recommended to perform under as low intensity as 0.1 TI, which was successfu in Sande's trial.20
 
Transient Increase of Hepatic Cell Apoptosis after Doppler Ultrasound Exposure
Pellicer et al.19 reported transient increase of hepatic cell apoptosis index after 20 or more sec exposure to 140 mW/cm2 pulsed Doppler ultrasound in fetal rat fetuses. ISUOG21 responded with a regulation of pulsed Doppler ultrasound in 11–13 weeks of pregnancy as:
  1. No pulsed Doppler ultrasound is studied in the routine ulrasound in 11–13 weeks of pregnancy.
  2. Pulsed ultrasound is used only to confirm fetal aneuploidy in 11–13 weeks of pregnancy.
  3. The ultrasound safety should be teached in obstetric ultrasound education.
Pellicer et al. used 140 mW/cm2 Doppler ultrasound, which is lower than 240 mW/cm2, it was the intensity of no ultrasound effect, and lower than 1.0 TI, therefore, the presence of any artifact is doubted but it has not been reported. ISUOG21 declared a regulation on the use of Doppler ultrasound in 11–13 weeks of pregnancy.
Two kinds of apoptosis, extrinsic and intrinsic, are classified,20 and intrinsic apoptosis will be physiologic, while it is difficult to define the apoptosis as intrinsic in Pellicer report. As Sande et al.20 reported the success of clear pulsed Doppler flow record with 0.1 TI ultrasound, Pellicer's study will be recommended to repeat using 0.1 TI ultrasound, where the usage regulation will be cleared, if no apoptosis develops after the Doppler study with 0.1 TI ultrasound. In smmary, ultrasound bioeffect will be hardly established by animal experiments due to possible presence of various archifacts.
 
NONMEDICAL USE OF DIAGNOSTIC ULTRASOUND
8Although the use of diagnostic ultrasound should be limited for medical purposes and users are responsible to the safety of ultrasound, i.e. users must keep the knowledge on possible ultrasound bioeffect and use the ultrasound under the ALARA (as low as reasonably achievable) principle. Nonmedical ultrasound in entertainment or keepsake ultrasound, fetal portrait studios or prenatal boutiques which record intrauterine fetal 3D/4D ultrasound on DVD are recent problems concerning ultrasound safety. There are also ethical concerning and false reassuring problems in the topics.13–16 WFUMB13 disapproved the use of ultrasound for the sole purpose of providing souvenir images of the fetus. Because the safety of an ultrasound examination cannot be assured, the use of ultrasound without medical benefit should be avoided. Furthermore, ultrasound should be employed only by health professionals who are well trained and updated in ultrasound clinical usage and bioeffects. The use of ultrasound to provide keepsake images or video of the fetus may be acceptable if it is undertaken as part of normal clinical diagnostic ultrasound examination, provided that it does not increase exposure to the fetus. Ultrasound imaging for nonmedical reasons is not recommended unless carried out for education, training or demonstration purposes. Live scanning of pregnant models for equipment exhibition at ultrasound congresses is considered a nonmedical practice that should be prohibited since it provides no medical benefit and afford potential risk to the fetus. When using ultrasound for nonmedical reasons, the ultrasound equipments display should be used to ensure that TI<0.5 and MI<0.3.13
 
SAFE LEVEL OF ULTRASOND INTENSITY
The safe obstetric ultrasound intensity level was reported to be one thermal index (1.0 TI) and one mechanical index (1.0 MI) in general opinions of medical ultrasound authorities. There can be possible biological hazardous effects in the ultrasound intensity above the levels.
In our detailed ultrasound radiation experiments insulating the heating of the transducer by 37°C stabilized water, the cultured fetal amniotic origin JTC-cell line floated in the culture medium held in ultrasound translucent container was exposed to quantitatively identified ultrasound for 30–60 min and the cell growth curve was compared to the sham of no radiation in the same temperature stabilized water.
The cell growth curve showed no difference to the sham below the SPTA 240 mW/cm2 (SPTP 80 W/cm2) of pulsed ultrasound, while the growth curve was suppressed after the exposure to the output intensity ultrasound above SPTA 240 mW/cm2.11 Since Japan Society of Ultrasonics in Medicine authorized the results, and Japan Industrial Standard (JIS)10 regulated medical ultrasound output intensity at the level lower than SPTA 10 mW/cm2, therefore the medical ultrasound safety was established in Japan.
Although the regulated intensity is low level, the standing wave in reflected ultrasound may increase the intensity, and deformed pulse ultrasound waves may further increase the intensity. The prudent JIS setting will contribute the safety of medical ultrasound even in its accidental increase, while possible increase of output intensity to get further clear fetal image in nonmedical entertainment will easily exceed the safe threshold intensity level. The risk should be prevented by the skilful medical staff with rich safety knowledge and prudent use of diagnostic ultrasound equipment.
In summary of opinions of ultrasound safety specialists, the non-medical use of diagnostic ultrasound for solely entertainment is not recommended or not permitted from the standpoint of diagnostic ultrasound safety.13–16
 
CONCLUSION
The strategies to keep the safety of each diagnostic ultrasound equipments depends on their system, because the thermal effect estimated by TI has been the main criteria in the safety. Simple B-mode, 3D and 4D ultrasound, fetal heart detector and fetal monitor, are not contraindicated due to thermal effect because of their low temporal average intensity. Pulsed Doppler machines are the main target in the safety due to the tendency of its high temporal average intensity. Non-hazardous exposure time of NCRP/AIUM criteria and the temperature rise estimated by TI are useful in retrospective criticism on the past examination. The principle of safe diagnostic ultrasound in daily practice is to keep the TI and MI below 1.0, where obstetrical setting is important. Research works and pregnancy screening strictly follow these principles of ultrasound safety. The ALARA principle should be remembered decreasing the intensity to 0.5–0.1 TI in necessary situations. As for the safe output intensity, the intensity level lower than 240 mW/cm2 will be safe, because no ultrasound bioeffect was found at the intensity level in the experiment, while cultured cell growth was suppresed by the exposure to 240 or higher mW/cm2 ultrasound.11
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