The first descriptions of ultrasound as an imaging mode date from the 19th century.123 The French engineer Paul Langevin designed an ultrasound machine, using Pierre Curie's principle of the piezoelectric effect. During World War I, he attempted to use this instrument to detect submarines through echo location (hence the later coined term SONAR: SOund Navigation And Ranging). He also demonstrated that the waves produced by his machine could kill small animals in an insonated water bath, and could cause pain to his assistants when they were required to plunge their hands in the water bath in the path of the beam. Other bioeffects observed included the searing of skin when touching a resonant quartz bar, and explosive atomization (!) of fluid drops from the end of the rod. Since that time, the question of effects and safety has been on the minds of researchers85 and has given rise to literature too extensive to review in detail.2,3,6,46,120,124,125,126,127,128,129,130,131,132,133,134,135, and 136 Initially, cell suspensions and cell and tissue cultures were employed and many reports described clear effects of the ultrasound waves on these, mostly secondary to cavitational and other nonthermal mechanisms, such as cell aggregation,137 membrane damage,138 and cell lysis,139 among others. Plants were another extensively studied organism for effects of ultrasound,140 particularly the Elodea leaf, since internal gas channels are present.141 Insects have been exposed to ultrasound with significant effects, such as death of eggs and larvae as well as abnormal development, presumably secondary to the presence of gas-filled channels.142 Additionally, alterations at the chromosomal and even DNA levels have been described.143 The above effects have been reviewed extensively elsewhere,5,29 and while of major scientific and historical importance, are not of major relevance to clinical exposure of human fetuses.
Effects of ultrasound were demonstrated in animals more than 80 years ago.85 Since then, multiple studies have been performed with ultrasound on a wide variety of species. Studies of gross effects on the brain and liver of cats were first performed with well-defined lesions and demyelination in the brain144 and tissue damage in the liver,145 resulting from ultrasound exposure of a few seconds at 1 and 3 MHz, respectively. Other observed effects include limb paralysis as a result of spinal cord injury in the rat,146,147 as well as lesions in the liver, kidney, and testicles of rabbits.148 While some effects are likely due to mechanical influences, very high temperature elevations (much higher than anything reachable with diagnostic ultrasound) have also been observed and may be more directly involved with the tissue damage. Effects in muscles have been obtained, but with outputs much higher than those usually generated in clinical studies,149 and so have intestinal78 and lung150 hemorrhages, also at acoustic pressures well above those generated by ultrasound fields. These are helpful in understanding the mechanisms involved with possible bioeffects of DUS. It should also be noted that some similar effects have also been demonstrated with acoustic fields much closer to clinically pertinent ones, in particular lung and intestinal hemorrhage.78 Several major clinical end-points for bioeffects that could have direct relevance to human studies include fetal growth and birth weight, effects on brain and CNS function, and change in hematological function, and will be considered in more detail. Decreased birth weight after prenatal exposure to ultrasound has been reported in the monkey151,152 and the mouse,153,154 but not convincingly in the rat.155 Therefore, clear species differences seem to exist,156 making it difficult to generalize, and even more difficult to extrapolate to humans.
Tarantal and Hendrickx151 evaluated 30 pregnancies in monkeys, half of which were exposed to ultrasound. The scanned fetuses had lower birth weights and were shorter than the control group. No significant differences were noted between the groups with regard to the rate of abortions, major malformations, or stillbirths. Moreover, all showed catch-up growth when examined at 3 months of age.151 It should be noted that in-situ intensities were higher than what is considered routine in clinical obstetrical imaging in humans. Hande and Devi157 evaluated the effect of prenatal exposure to diagnostic ultrasound on the development of mice. Swiss albino mice were exposed to diagnostic ultrasound for 10 minutes on day 3.5 (preimplantation period), 6.5 (early organogenesis period), or 11.5 (late organogenesis period) of gestation. Sham-exposed controls were maintained for comparison. Fetuses were dissected out on the 18th day of gestation, and changes in total mortality, body weight, body length, head length, brain weight, sex ratio, and microphthalmia were recorded. Exposure on day 3.5 of gestation resulted in a small increase in the resorption rate and a significant reduction in fetal body weight. Low fetal weight and an increase in the incidence of intrauterine growth-restriction were produced by exposure on day 6.5 postcoitus.157
Others have also demonstrated restricted growth in newborns after in-utero exposure to DUS.158 Subtler findings have also been described. Pregnant Swiss albino mice were exposed to diagnostic ultrasound (3.5 MHz, 65 mW, ISPTP = 1 W/cm2, ISATA = 240 W/cm2) for 10, 20, or 30 minutes on day 14.5 (fetal period) of gestation.159 Sham-exposed controls were studied for comparison. There were significant alterations in behavior in the exposed groups as revealed by decreased locomotor and exploratory activity, and an increase in the number of trials needed for learning. No changes were observed in physiological reflexes and postnatal survival. The authors concluded that ultrasound exposure during the early fetal period can impair brain function in the adult mouse.159 Likewise, Hande et al160 found that anxiolytic activity and latency in learning were more noticeable in ultrasound-treated animals. The authors exposed pregnant Swiss mice to diagnostic levels of ultrasound (3.5 MHz, maximum acoustic output: ISPTP = 1 W/cm2 and ISATA = 240 mW/cm2, acoustic power = 65 mW) for 10 minutes on postcoital day 11.5 or 14.5. At 3 and 6 months postpartum, offspring were subjected to behavioral tests. The effect was more pronounced in the 14.5 days postcoital group than in the 11.5 days group. They concluded that exposure to diagnostic ultrasound during late organogenesis period or early fetal period in mice may cause changes in postnatal behavior.160 Temperature elevations were induced by ultrasound in guinea pig fetal brains.43 In fact, mean temperature increases of 4.9°C close to parietal bone and 1.2°C in the midbrain were recorded after 2-minute exposures, albeit at exposure conditions higher than what is usually employed in clinical examinations.43 This greatest temperature rise recorded close to the skull correlated with both gestational age and progression in bone development.40 The skull bone becomes progressively thicker and denser between 30 and 60 days' gestational age (normal gestation for guinea pigs is 66 to 68 days). After only 2 minutes of insonation with an ISPTA of 2.9 W/cm2 (about 4 times higher than currently permitted by the FDA for diagnostic use), mean maximum temperature increases varied from 1.2°C at 30 days to 5.2°C at 60 days. It is important to note that most of the heating (80% of the mean maximum temperature increase) occurred within 40 seconds. The rate of heating is relevant to the safety of clinical examinations in which the dwell time may be an important factor. Because maximal ultrasound-induced temperature increase occurs in the fetal brain near bone, worst-case heating will occur later in pregnancy, when the ultrasound beam impinges on bone, and less will occur earlier in pregnancy, when bone is less mineralized. However, milder insults early in gestation may be as significant (or more) than more severe ones in later stages.
Neurons of the cerebral neocortex in mammals, including humans, are generated during fetal life in the brain proliferative zones and then migrate to their final destinations by following an inside-to-outside sequence. Recently, Ang et al161 evaluated the effect of ultrasound waves on neuronal positioning within the embryonic cerebral cortex in mice. Neurons generated at embryonic day 16 and destined for the superficial cortical layers were chemically labeled in over 335 animals. A small but statistically significant number of neurons failed to acquire their proper position and remained scattered within inappropriate cortical layers and/or in the subjacent white matter when exposed to ultrasound for a total of 30 minutes or longer during the period of their migration. The magnitude of dispersion of labeled neurons was variable but systematically increased with duration of exposure to ultrasound (although not linearly, with some extended exposure yielding less effect than lower ones). These investigators concluded that further research in larger and slower-developing brains of nonhuman primates and continued scrutiny of unnecessarily long prenatal ultrasound exposure is warranted. It is unclear whether a relatively small misplacement in a relatively small number of cells that retain their origin cell class is of any clinical significance. It is also important to note that there are several major differences between the experimental setup of Ang et al161 and the clinical use of ultrasound in humans.6 The most noticeable difference was the length of exposure of up to 7 hours in the setup of Ang et al. No real mechanistic explanation was given for the findings, and furthermore, there was no real dose effect with high effects at the penultimate high dose, but less so at the highest dose. Moreover, scans were performed over a small period of several days. The experimental setup was such that embryos received whole-brain exposure to the beam, which is rare in humans. In addition, brains of mice are much smaller than those in humans, and develop over days. This should not completely deter from the study, but encourages caution. It should be noted that some have described a complete lack of effects of prenatal ultrasound exposure on postnatal development and growth162 or behavior.163 Another recently published study is worth considering.164 Chick brains were exposed, in ovo, on day 19 of a 21-day incubation period to B-mode (5 or 10 min), or to pulsed Doppler (1, 2, 3, 4, or 5 min) ultrasound. After hatching, learning and memory function were assessed at day 2 post-hatch. B-mode exposure did not affect memory function. However, significant memory impairment occurred following 4 and 5 min of pulsed Doppler exposure. Short-, intermediate-, and long-term memory was equally impaired, suggesting an inability to learn. Chicks were also unable to learn with a second training session. In this study, exposure to pulsed Doppler ultrasound adversely affected cognitive function in chicks. Although some methodological issues exist and extrapolation to humans is unwarranted, these findings justify further investigations.
The hematological system is the second major system to be investigated for ultrasound effects. The following have been assessed: hemolysis, coagulation factors and platelets, and leukocyte production and function.165 Increased hemolysis has been demonstrated for ultrasound in (human) fetal cells as compared to adult cells, but only in the presence of ultrasound contrast agents, with human cells being less fragile than certain tested animals.83,166 Other alterations have been described in the hemolytic system167 but appear to be of minimal, if any, clinical significance.
Human Research and Epidemiology
In 2005, the American Institute of Ultrasound in Medicine (AIUM) published the following statement: "Based on the epidemiological data available and on current knowledge of interactive mechanisms, there is insufficient justification to warrant a conclusion of a causal relationship between diagnostic ultrasound and recognized adverse effects in humans. Some studies have reported effects of exposure to diagnostic ultrasound during pregnancy, such as low birth weight, delayed speech, dyslexia, and non–right-handedness. Other studies have not demonstrated such effects. The epidemiological evidence is based on exposure conditions prior to 1992, the year in which acoustic limits of ultrasound machines were substantially increased for fetal/obstetrical applications."168 Applied to ultrasound, epidemiology is the study of effects on human populations as a result of ultrasound scanning and, in the case of obstetrical ultrasound, this should include the pregnant patient as well as her infant. Laboratory animal experiments under similar diagnostic exposure levels have shown some effects from ultrasound, under certain conditions. Effects have also been reported in humans, but a definitive statement regarding risk should, ideally, include direct analysis of the effects in human populations. Several epidemiological studies have been published.4,46,169 For an extensive discussion, including elements of statistics, see Chapter 12 in NCRP report number 140,29 an extensive review by Newnham,132 and AIUM document, Conclusions Regarding Epidemiology for Obstetric Ultrasound.170,171. Relevant details will be summarized.
Several biological end points have been analyzed in the human fetus/neonate in an attempt to determine whether prenatal exposure to diagnostic ultrasound had observable effects: intrauterine growth restriction (IUGR) and low birth weight, delayed speech, dyslexia, neurological and mental development or behavioral issues, and, more recently, non–right-handedness. Occasional studies report an association between diagnostic ultrasound and some specific abnormalities such as lower birth weight,169 delayed speech,172 dyslexia,173 and non–right-handedness.174,175 With the exception of low birth weight (also demonstrated in monkeys,166) these findings have never been duplicated, and the majority of studies have been negative for any association. Moore et al176 examined a large number of infants (over 2000, half of them exposed to ultrasound) and found a small but statistically significant lower mean birth weight of exposed versus non-exposed infants. However, information was collected several years after exposure, no indications for the examination are known, and no exposure information is available. This is very often the major problem in analyzing these reports. In a later study, the authors concluded that the relationship of ultrasound exposure and reduced birth weight may be due to shared common risk factors, which lead to both exposure and a reduction in birth weight.177 Another retrospective study, with Moore as a coauthor, reported a 2.0 greater risk of low birth weight after 4 or more exposures to diagnostic ultrasound.133 These results were not reproduced in other retrospective studies.176 In a large study (originally 10,000 pregnancies exposed to ultrasound matched with 500 controls) with a 6-year follow-up, Lyons et al178 did not find differences in birth weight (nor increased congenital malformations, chromosomal abnormalities, infant neoplasms, speech or hearing impairment, or developmental problems).
Newnham et al171 performed a randomized control trial including more than 2800 parturients. Of these, about half received 5 ultrasound imaging and Doppler flow studies at 18, 24, 28, 34, and 38 weeks gestation, and half received a single ultrasound imaging at 18 weeks. They found an increased risk of IUGR when exposed to frequent Doppler examinations, possibly via some effects on bone growth. However, when children from the last mentioned study were examined at 1 year of age, there were no differences between the study and control groups. In addition, after examining their original subjects after 8 years, no evidence of long-term adverse impact in neurological outcome was noted by the same group.179 Similarly, no harmful effect of a single or 2 prenatal scans on growth were found in several randomized studies.180,181 In fact, in some studies, birth weight was slightly higher in the scanned group, but not significantly so, except in one.182 In conclusion, decreased birth weight has been extensively analyzed after DUS exposure in utero, and it does not appear that such exposure is associated with reduced birth weight, although Doppler exposure may have some risks.136 In a few studies that appear to favor such an effect, a major problem is that there is an important confounding factor: many studies include pregnancies at risk for IUGR due to existing maternal or fetal conditions.
A second major potential effect extensively evaluated is delayed speech. In an attempt to determine if there is an association between prenatal ultrasound exposure and delayed speech in children, Campbell et al172 studied 72 children with delayed speech and found a higher rate of ultrasound exposure in utero than the 144 control subjects. Some issues render these results less valid: there was neither a dose-response effect nor any relationship to time of exposure, and many of the records were more than 5 years old. Another study of over 1100 children exposed to ultrasound in utero and over 1000 controls found no significant differences in delayed speech, limited vocabulary, or stuttering.183
Dyslexia is another widely studied subject. In one study over 4000 children, aged 7 to 12, exposed to ultrasound in utero were used as a study group and compared to matched controls to evaluate the appearance of adverse effects.173 Seventeen outcomes measures were examined, at birth (APGAR scores, gestational age, head circumference, birth weight, length, congenital abnormalities, neonatal infection, and congenital infection) or in early infancy (hearing, visual acuity and color vision, cognitive function, and behavior). No significant differences were found, except for a significantly greater proportion of dyslexia in those children exposed to ultrasound. However, the authors indicated that this could be an incidental finding, given the design of the study and the presence of several confounding factors that could have contributed to the possible dyslexia finding. On the other hand, it should be noted that exposure conditions were probably much lower than modern ultrasound systems, given that the fetal examinations were performed from 1968 to 1972. Subsequently, a long-term follow-up study was performed on over 2100 children.180,184 End points included evaluation for dyslexia along with additional hypotheses including an examination of non–right-handedness (see below) said to be associated with dyslexia. These studies185,186 ans 187 included the specific examination of more than 600 children with various tests for dyslexia such as spelling and reading. No statistically significant differences were found between ultrasound-exposed children and controls for reading, spelling, arithmetic, or overall performance as reported by teachers. Specific dyslexia tests showed similar rates of occurrence among scanned children and controls in reading, spelling, and intelligence scores, and no discrepancy between intelligence and reading or spelling. Since the original finding of dyslexia was not confirmed in subsequent randomized controlled trials, it is considered unlikely that routine ultrasound screening exams can cause dyslexia. However, these studies did raise the issue of laterality (in terms of handedness), which is discussed below.
The issue of non–right-handedness as a result of prenatal exposure has caused much ink to be used in extensive discussions and reports. The first report of a possible link between prenatal exposure to ultrasound and subsequent non–right-handedness in insonated children was published in 1993 by Salvesen et al,185 but according to the authors, "only barely significant at the 5% level." In a later analysis of the data, they described that the association was restricted to males.186 A second group of researchers (with Salvesen, the main author of the first study, included but with a new population, in Sweden as opposed to Norway) published similar findings of a statistically significant association between ultrasound exposure in utero and non–right-handedness in males.174 Salvesen then published a meta-analysis of these 2 studies and of previously unreported results.173 No difference was found in general, but a small increase in non–right-handedness was present when analyzing boys separately. No valid mechanistic explanation is given in the studies to explain the findings. In conclusion, although there may be a small increase in the incidence of non–right-handedness in male infants, there is not enough evidence to infer a direct effect on brain structure or function or even that non–right-handedness is an adverse effect.
Other end points that have been considered but not found to be associated with ultrasound exposure include congenital malformations and malignancies.189
There has been no epidemiological study published on populations scanned after 1992, when regulations were altered and acoustic output of diagnostic instruments were permitted to reach levels many times higher than previously allowed (from 94 to 720 mW/cm2 ISPTA for fetal applications). There are no epidemiological studies related to the output display standard (thermal and mechanical indices) and clinical outcomes. The safety of new technologies such as harmonic imaging and three-dimensional (3D) ultrasound, as well as that of probe self-heating, needs to be investigated.
Unfortunately there is no way to perform actual sonographic exposure measurements in the human fetus. Pressure, intensity, and power are not measured in situ, but are estimated from laboratory obtained measurements. Several tissue models have been developed to help with this estimation, depending mostly on approximate attenuation coefficients for various tissues or beam paths.29,47 A large range of variation is expected secondary to individual patient characteristics, such as weight and thickness of tissues.190 Because of these possible variations, the reasonable worst-case scenario is usually considered. There are scarce data on instruments' acoustic output (nor patient acoustic exposure) for routine clinical ultrasound examinations. Acoustic output was recorded in several prospective observational studies investigating first-trimester ultrasound,191 Doppler studies,192 and 3D/four-dimensional (4D) studies.193 Basically, first-trimester ultrasound was associated with very low TI values (with a mean of 0.2 ± 0.1).190 The TI was significantly higher in the pulsed wave Doppler (mean 1.5 ± 0.5, range 0.9-2.8) and color flow imaging studies (mean 0.8 ± 0.1, range 0.6-1.2) as compared to B-mode ultrasound (mean 0.3 ± 0.1, range 0.1-0.7; P < .01).190 In the same study, TI was above 1.5 in 43% of the Doppler studies.192 Mean TI during the 3D (0.27 ± 0.1) and 4D examinations (0.24 ± 0.1) was comparable to the TI during the B-mode scanning (0.28 ± 0.1; P = .343).193 Figures 1-3,1-4,1-5,1-6, and 1-7 are examples of actual screen shots during clinical exams, for B-mode, color Doppler, M-mode, spectral Doppler, and 3D acquisition, respectively. Figure 1-8 demonstrates that extremely elevated TIs are easily reachable with spectral Doppler, although in manufacturer's fetal setting.
TI and MI during color Doppler exam. Please note TI = 0.6 (arrow).
TI and MI during M-mode examination. Please note TI = 0.8 (arrow).
TI and MI during spectral Doppler examination. Please note TI = 2.4 (arrow).
TI and MI during multiplanar acquisition in 3D scanning. Please note TI = 0.4 (arrow).
Second-trimester spectral Doppler. Please note TI = 3.3 (arrow). This is with the instrument on "fetal" setting.
The other side of the equation is, "What are we looking for?" Ultrasound is neither radiation nor thalidomide, and it is certain that ultrasound does not kill fetuses, does not cause limb amputations, and does not cause gross structural anomalies.194 But are we looking where we should, and have we studied enough cases in a scientific fashion, looking at subtle changes? The answer is clearly, "No." We have been looking for macroscopic, gross findings and have not found any, but is it possible that harmful effects of ultrasound have been missed because the wrong time frame reference was used? Two possible factors are described for such errors.195 If one uses a term human pregnancy (280 days [40 weeks]) to life expectancy of 70 years (25,550 days) ratio, then 7 in-utero days are comparable to about 631 ex-utero days. Therefore, it is conceivable that a much shorter time interval (1 day) should be used to group fetuses to evaluate effects, not intervals of 1 or more weeks as is usually done. Furthermore, there is also a potential "dilution error." Assuming an event has a background rate of 10% in the general population but occurs in 100% of fetuses exposed on day 35, if a large number (for instance, 1000) of fetuses exposed on that particular day are examined, the incidence will be 100%, ie, 90% increase over the control population (background rate of 10%). But if we assume 1000 fetuses are exposed per day for 12 weeks, this represents 84,000 scans, and only 11.1% will be affected (all 1000 scanned on day 30 and 10% [the background rate] of all 83,000 others , or 9300 total), an increase of only 1.1% (1.07 to be precise) over the background rate of 8400 (10% of 84,000), which is very difficult to extract and observe, but still present in 100% of the fetuses exposed at the critical time (day 35 in the above example). The actual numbers are probably even more complicated since more than 1000 fetuses are scanned every day, the background rate of major anomalies is 3% to 4% in the general population and much lower for non-gross macroscopic findings, and furthermore, the hit rate of any teratological agent is rarely 100%. This points to the need for extensive, well-planned research—a goal very difficult to accomplish, given that the majority of pregnant women who receive prenatal care will have 1 or several DUS scans during their pregnancy.
Nonmedical ultrasound refers to the performance of obstetrical ultrasound with no medical indication but to provide the mother/parents-to-be with images or video clips of the fetus (on hard copy, tape, CD, or DVD), also called "scanning for pleasure."196 There are several reasons why most official organizations are opposed to this practice, such as issues of training of the providers, quality and nature of the scans, feedback to the "customers," and risks that these customers will not have a regular, clinical exam. But perhaps the most obvious reason for the resistance to these scans is the safety issue. For instance, the FDA is strongly opposed, stating, "… ultrasound energy delivered to the fetus cannot be regarded as completely innocuous. Laboratory studies have shown that diagnostic levels of ultrasound can produce physical effects in tissue, such as mechanical vibrations and rise in temperature. Although there is no evidence that these physical effects can harm the fetus, public health experts, clinicians and industry agree that casual exposure to ultrasound, especially during pregnancy, should be avoided."197 The FDA goes further and indicates, "Persons who promote, sell or lease ultrasound equipment for making 'keepsake' fetal videos should know that the FDA views this as an unapproved use of a medical device. In addition, those who subject individuals to ultrasound exposure using a diagnostic ultrasound device (a prescription device) without a physician's order may be in violation of state or local laws or regulations regarding use of a prescription medical device."197,198 Equally opposed to the nonclinical use of DUS are the American Institute of Ultrasound in Medicine (AIUM), the American College of Obstetrics and Gynecology (ACOG), and the European Committee for Medical Ultrasound Safety (ECMUS). The AIUM's most recent statement is, "The AIUM advocates the responsible use of diagnostic ultrasound … [and] strongly discourages the non-medical use of ultrasound. … The use of either two-dimensional (2D) or three-dimensional (3D) ultrasound to only view the fetus, obtain a picture of the fetus or determine the fetal gender without a medical indication is inappropriate and contrary to responsible medical practice. Although there are no confirmed biological effects on patients caused by exposures from present diagnostic ultrasound instruments, the possibility exists that such biological effects may be identified in the future. Thus ultrasound should be used in a prudent manner to provide medical benefit to the patient."199 Similarly, the ECMUS's statement includes the following: "The embryonic period is known to be particularly sensitive to any external influences. Until further scientific information is available, investigations should be carried out with careful control of output levels and exposure times. With increasing mineralization of the fetal bone as the fetus develops the possibility of heating fetal bone increases."200 More recently the World Federation of Ultrasound in Medicine and Biology (WFUMB) and the International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) issued a joint statement with identical conclusions: "The WFUMB and ISUOG disapprove of the use of ultrasound for the sole purpose of providing souvenir images of the fetus. … Furthermore, ultrasound should be employed only by health professionals who are well trained and updated in ultrasound clinical usage and bioeffects."201
Many national and international organizations or societies have issued official statements regarding the epidemiology, bioeffects, and safety of ultrasound, as well as the nonmedical usage of ultrasound such as the AIUM, WFUMB, British Medical Ultrasound Society (BMUS), and European Committee of Medical Ultrasound Safety (ECMUS). They all state, in one way or another, that ultrasound appears safe if performed for clinical indications by appropriately trained personal, but that prudence is recommended because of the possibility of yet unknown deleterious effects. For instance, the AIUM has several statements available on its Web site for epidemiology,167 prudent use,199 and keepsake fetal imaging.201 The Keepsake Fetal Imaging statement contains a clear "safety clause" particularly addressing pulsed Doppler: "Although the general use of ultrasound for medical diagnosis is considered safe, ultrasound energy has the potential to produce biological effects. Ultrasound bioeffects may result from scanning for a prolonged period, inappropriate use of color or pulsed Doppler ultrasound without a medical indication, or excessive thermal or mechanical index settings."201