Teratologic study of rats exposed to ultrasound

Intrauterine effects of ultrasound: animal studies

During the past several decades, the use of ultrasound technology in the clinical setting has greatly increased. Because nearly every pregnant woman receives at least one sonographic procedure today, there has been developing concern about the safety of such procedures. Since ultrasound exposure can result in hyperthermia and other physiological effects, the determination of a threshold or no-effect exposure has become a high-priority goal. Animal research has been important to the study of the effects of various exposures at all stages of pregnancy, since the clinical use of ultrasonography can occur during the preimplantation, organogenic, and fetal stages. Animal experiments using various mammalian species have been able to determine no-effect exposure levels for embryonic loss, congenital malformations and neurobehavioral effects. The preponderance of evidence from these studies indicates that, in the absence of a thermal effect, ultrasonography represents no measurable risk when used at recommended intensity levels.

Behavioral teratologic effects of prenatal exposure to continuous-wave ultrasound in unanesthetized rats

While there are no known risks associated with diagnostic ultrasound, uncertainty about the safety of prenatal ultrasound exposure remains. The purpose of the present experiment was to evaluate the behavioral teratogenic potential of continuous-wave (cw) ultrasound in rats, in the absence of maternal anesthesia or restraint. Pregnant CD rats, trained to remain immobile in a water-filled ultrasound exposure tank, were scanned with 3 MHz cw ultrasound at levels of 0, 2, 10, 20, or 30 W/cm2 ISPTA (spatial peak, temporal average intensity) on gestational days 4-20 for approximately 10 min/day. Offspring were examined postnatally for survival, growth, physical landmarks of development, behavioral development, and the adult functions of locomotor activity, learning and memory, and startle reactivity. No effects of prenatal ultrasound were found on maternal characteristics, offspring survival or growth, physical or behavioral landmarks of development, or adult tests of passive avoidance or startle. Effects at the highest intensity were obtained on corner and side locomotor activity and in a multiple-T water maze on measures of errors of commission and time spent finding the goal. The results showed that prenatal cw ultrasound in rats can induce effects on some postnatal neurobehavioral functions at high exposure intensities (30 W/cm2), but at lower intensities (2-20 W/cm2) no consistent evidence of neurobehavioral effects was observed.

Teratologic evaluation of rats prenatally exposed to pulsed-wave ultrasound

While there are no known risks associated with diagnostic ultrasound, uncertainty about the safety of prenatal ultrasound exposure remains. The purpose of the present experiment was to evaluate the teratogenic potential of pulsed-wave (pw) ultrasound in rats, in the absence of maternal anesthesia or restraint. Pregnant CD rats, trained to remain immobile in a water-filled ultrasound exposure tank, were scanned with 3-MHz pw ultrasound at levels of 0, 2, 20 or 30 W/cm2 ISPTA (spatial peak, temporal average intensity) on gestational days 4-19 for approximately 10 min/day. Examination of fetuses on E20 revealed no increase in skeletal or visceral malformations in groups exposed to pw ultrasound in utero. The number of implantations/dam was significantly increased in all pw ultrasound exposure groups compared to sham-exposed animals and, in a possibly related finding, resorptions were increased in the two highest exposure groups. Although significantly increased compared to controls, resorption frequencies in these groups were low (< 10%). No exposure-related changes in fetal weights were observed in offspring of rats scanned with pw ultrasound during gestation. The results indicate that, under the conditions described, no overt embryotoxicity is associated with gestational exposure to pw ultrasound intensities of up to 30 W/cm2.

A teratologic evaluation of continuous-wave, daily ultrasound exposure in unanesthetized pregnant rats

Pregnant Sprague-Dawley rats were trained to remain immobile when placed in water in an ultrasound exposure tank and exposed to 0, 0.1, 2.0, or 30.0 W/cm2 ISPTA (spatial peak, temporal average), 3.0-MHz continuous wave (cw) ultrasound on embryonic (E) days 4-19 for approximately 15 min/day. On E20 fetuses were removed; weighed; examined for external, skeletal, and visceral malformations; and uteri were examined for resorptions. Analyses revealed no increase in pre-implantation loss and no effects on maternal body weight, food, or water consumption. No increase in skeletal or visceral malformations was found, in fact exposed groups had a lower incidence of defects than controls. A significant increase in resorptions in the lowest exposure group (0.1 W/cm2) was obtained, but the effect was isolated, non-dose dependent and not credible as a treatment-related effect. No reduction in fetal weight was obtained, in fact the lowest (0.1-W/cm2) and middle (2.0-W/cm2) exposure level groups weighed slightly more than controls. The immobility procedure succeeded in avoiding anesthetization or forced restraint of the dams, thereby eliminating these factors as potential confounders. The results demonstrated that in unanesthetized, unrestrained rats in utero exposure to incident intensities of ultrasound of up to 30.0 W/cm2 cw ultrasound (or estimated internal exposures of 4-21 W/cm2, depending on body orientation to the incident beam) produced no evidence of embryotoxicity based on fetal necropsy data.

Medical sonography: reproductive effects and risks

While it is clear that the levels and types of medical sonography that have been used in the past have no measurable risks, it would be inaccurate to label the modality of ultrasound as totally safe regardless of exposure. Most agents have reproductive risks and even teratogenic risks if the exposure is raised sufficiently. Thus the prudent use of sonography means that clinicians and designers of equipment have to maintain exposures far below the risks that have been demonstrated in animal studies and from the knowledge obtained about the physical changes that can be produced in humans as the absorbed dose is elevated. The reproductive risks were evaluated using five criteria: 1) human epidemiology, 2) secular trend data, 3) animal experiments, 4) dose response relationships, and 5) biologic plausibility. The analysis reveals that the human epidemiology does not indicate that diagnostic ultrasound presents a measurable risk to the developing embryo or fetus. Animal studies also indicate that diagnostic levels of ultrasound are safe and do not elevate the fetal temperature into the region where deleterious embryonic and fetal effects will occur. Because higher exposures of ultrasound can elevate the temperature of the embryo, the use of diagnostic procedures and the design of sonographic equipment should take into consideration the hyperthermic potential of higher exposures of ultrasound and the hypothetical additional risk of performing sonography on pregnant patients who are febrile. It would appear that if the embryonic temperature never exceeds 39 degrees C, then there is no measurable risk. We suggest that sonography (the field) and sonogram (the procedure) are the most appropriate and least anxiety provoking terms.

Multiple malformations and exposure to therapeutic ultrasound during organogenesis

We present a case of sacral agenesis, microcephaly, and developmental delay. The pregnancy with this child was complicated by left psoas bursitis that was treated by 18 applications of ultrasound between days 6 and 29 of gestation.

Developmental exposure of mice to pulsed ultrasound

Exposure of pregnant mice on gestation day (gd) 8 to 1 MHz continuous-wave ultrasound (0, 0.05, 0.50, or 1.00 W/cm2) was reported previously to result in a slight (nonsignificant) increase in malformations. The present study was conducted in a similar fashion using pulsed ultrasound but was designed to maximize the likelihood of finding effects of gd 8 ultrasound exposure on prenatal development. Pregnant ICR mice (approximately 60 animals/group) were exposed on gd 8 to pulsed ultrasound with a center frequency of 1 MHz at levels of 0 (sham control), 0.05, 0.50, or 1.00 W/cm2 (spatial average, temporal average intensities; ISATA) with a spatial peak, pulse average intensity (ISPPA) of 90 W/cm2 and pulse duration of 6.5 microseconds. Anesthetized animals were placed in a degassed water bath (30 degrees C) and exposed for two 10 min intervals during which the beam was centered 1 cm on either side of the abdominal midline. On gd 17, dams were killed; the uterus and its contents were weighed and examined; and live fetuses were weighed and examined for external, visceral, and skeletal malformations. Although one female in the 0.50 W/cm2 group and seven animals in the 1.00 W/cm2 group died following exposure, no other significant change from controls was seen in any maternal or fetal parameter evaluated. Thus the results of this study indicate that there was no detectable effect on prenatal development of mice following exposure to ultrasound on gd 8 (a time of maximal sensitivity), even at exposure intensities that were lethal to some maternal animals.

Evaluation of the bioeffects of prenatal ultrasound exposure in the cynomolgus macaque (Macaca fascicularis): I. Neonatal/infant observations

The frequency of use of ultrasonography for evaluating the developing embryo/fetus has continued to rise although the possible risks from exposure still remain uncertain. The cynomolgus macaque (Macaca fascicularis) is currently being used in our laboratory as a model to assess these risks. In utero exposure was performed utilizing a commercial real-time mechanical sector scanner with a 7.5 MHz scanhead (ATL, MK 600). Maximum acoustic power output for this unit is as follows: I(SPTA) = 12.0 mW/cm2, I(SPPA) = 98 W/cm2, and Im = 137 W/cm2. Animals exposed to ultrasound (N = 16) were scanned five times weekly on gestational days (GD) 21-35 +/- 2 for 10 minutes/exam (m/e), three times weekly on GD 36-60 +/- 2 for 10 m/e, and once weekly on GD 61-150 +/- 2 for 20 m/e. Controls (N = 14) were "scanned" with the unit placed on standby. Assessment of simian Apgar scores at 1, 5, and 10 minutes of life revealed higher scores for treated animals at 10 minutes (P less than or equal to 0.045); greater scores in muscle tone (P less than or equal to 0.013) and color (P less than or equal to 0.016) were observed. Evaluation of morphometrics at birth including weight, biparietal diameter, occipitofrontal diameter, head circumference, hand and foot lengths, humerus and femur lengths, arm circumference, chest circumference, tail length, skinfold thickness, and crown-rump length (CRL) indicated a significant reduction in only two parameters, birth weight (P less than or equal to 0.027) and CRL (P less than or equal to 0.033). Hematologic analysis at 2 +/- 1, 9 +/- 1, and 16 +/- 1 days of life revealed a significant difference in white blood cell counts (WBCs). Treated animals displayed lower WBCs with reductions in numbers of segmented neutrophils and monocytes at all ages observed. Hematologic differences were not significant by 5-6 months of age. No abortions, gross malformations, or stillbirths were observed in the exposed animals.

The embryotoxic effects of ultrasound exposure in pregnant ICR mice

The embryotoxicity of ultrasound exposure during pregnancy was investigated in DUB:(ICR) mice. On day 0 of gestation (day of plug), pregnant mice were assigned to one of five groups: cage control, sham exposed (0 W/cm2), 0.05 W/cm2, 0.50 W/cm2. or 1.00 W/cm2. Females were anesthetized on day 8 of gestation and their abdomens were shaved to assure good acoustic coupling. The animals were strapped on a lucite board and placed vertically into a distilled degassed water bath (30 degrees C) so that the abdomen was fully submerged and centered in the axis of the ultrasonic beam. Insonation was carried out using a PZT transducer with a radius of 1.27 cm and a frequency of 1 MHz under continuous wave conditions. Each animal was placed at a distance of 25 cm from the transducer and exposed to the appropriate intensity for 120 seconds. On day 17 of gestation, the maternal animals were killed, the uterine contents were examined, and live fetuses were weighed and then shipped in cold lactated Ringer's solution from Maryland to Arkansas. Fetuses were examined on the day following maternal sacrifice for external and visceral defects and skeletons were prepared and examined subsequently. Slight but significant differences were detected between the cage control and sham-exposed groups. No statistically significant changes were seen that could be attributed to ultrasound exposure, although there was a slight increase in the incidence of malformed fetuses and the occurrence of multiple malformations in individual fetuses as intensity of the ultrasonic exposure increased.

Influence of low-intensity ultrasonic irradiation on prenatal development of two inbred mouse strains

Effects of low-intensity ultrasonic irradiation on prenatal development of DHS and A/HeMk mice were studied. On day 8 of gestation (VP day = 0) pregnant females were exposed to ultrasonic waves with a frequency of 2.25 MHz and power of 40 mW/cm2 for 5 h. A low frequency of severe cranial and facial anomalies occurred that was attributable to the irradiation in both strains. The difference in frequency of malformed fetuses was marked between irradiated and untreated control A/HeMk mice, but not DHS mice. Fetal growth inhibition and death were also produced in both strains, although the possible effect of binding the pregnant mice for irradiation cannot be discounted.