Ultrasound safety in early pregnancy: reduced energy setting does not compromise obstetric Doppler measurements.

OBJECTIVES: We hypothesized that first-trimester Doppler ultrasonography can be carried out at lower output energies than the currently advocated limits without compromising clinically important information. METHODS: We recruited 42 pregnant women for an ultrasound examination at 12 weeks' gestation. Twenty-one women were examined with a transvaginal transducer, the rest with a transabdominal transducer. We used pulsed Doppler to measure pulsatility index (PI) and peak systolic velocity (PSV) in five clinically relevant fetal and maternal blood vessels. The energy indicator thermal index for bone (TIb) was set at 1.0, 0.5 and 0.1. Each measurement was repeated three times. A mixed linear regression model accounting for correlation between measurements was used to assess the effect of different TIb levels and transducers. RESULTS: We were able to visualize the vessels by color Doppler and measure PI and PSV in all vessels at all energy levels in all the participants with the exception of the ductus venosus in two participants, yielding 1872 recordings for statistical analysis. A reduction in TIb from 1.0 to 0.5 and 0.1 had no effect on the PI or PSV values, nor was there any trend towards higher parameter variance with decreasing TIb. There was no difference between measured values of PI and PSV between the transducers, but the transabdominal technique was associated with a greater parameter variance. CONCLUSION: Reliable first-trimester Doppler data can be obtained with output energy reduced to a TIb of 0.5 or 0.1.

Ultrasound bioeffects in rats: quantification of cellular damage in the fetal liver after pulsed Doppler imaging.

OBJECTIVE: To determine whether pulsed Doppler examination of the ductus venosus in rat fetuses could damage exposed tissue. METHODS: On gestational day 18, the livers of a mean of approximately five fetuses per mother (n = 5.14, SD = 1.6), in a cohort of 35 pregnant female rats, were exposed individually to pulsed Doppler and these were considered the 'exposed group'. The remaining fetuses in each pregnant rat (n = 5.16, SD = 2.1) formed the 'control group'. We tested for 600, 300, 60, 20, 15, 10 and 3 s of exposure of the fetal ductus venosus and the damage was evaluated measuring a cell death index of apoptotic activity at 7 h post-exposure (n = 16). In addition, subgroups of mothers were sacrificed at 2, 4, 5, 7, 12 and 24 h post-exposure to determine when the damage appeared and disappeared and whether this depended on the exposure time. RESULTS: After exposure of 20 s or more, we observed significant damage, as assessed by caspase 3 activity (a marker of apoptotic activity related to tissue damage), in all cases; after 15 s of exposure, some samples presented damage (P = 0.4); there was no damage after 10 s or 3 s of exposure (P = 0.87 and P = 0.3, respectively). There was a positive linear correlation between apoptotic index and pulsed Doppler exposure time, (Pearson's coefficient = 0.324, P < 0.01). No liver still showed significant damage at 12 or 24 h post-exposure (P > 0.05 and P > 0.4). CONCLUSIONS: We observed reversible damage after pulsed Doppler imaging in an in-vivo fetal liver tissue rat model and found that longer exposure times produced more tissue damage. We established that 10 s was the maximum exposure time to ensure absence of damage to tissue in this model. It would appear sensible to recommend expert supervision of pulsed Doppler imaging and to have intervals between subsequent examinations.

Ultrasound probe pressure but not maternal Valsalva maneuver alters Doppler parameters during fetal middle cerebral artery Doppler ultrasonography.

OBJECTIVE: To determine the effects of increased ultrasound probe pressure and maternal Valsalva maneuver (VM) on the middle cerebral artery (MCA) Doppler ultrasonography in fetuses. METHODS: A total of 120 healthy pregnant women in second and third trimesters were enrolled in the study. MCA blood flow was measured by pulsed Doppler sonography in 60 fetuses (24 and 40 weeks' gestation) before and after the application of increased ultrasound probe pressure. In the other 60 fetuses (32 and 36 weeks' gestation), sonography was performed before and after maternal VM. Statistical analysis was performed by paired t-test. RESULTS: The pressure induced by the ultrasound probe induced a significant increase in the pulsatility index (PI), resistance index (RI), and peak systolic velocity (PSV); however, a significant decrease was found in the end-diastolic velocity (EDV) (p < 0.05). No statistically significant difference was found in the mean flow velocity (MFV). Moreover, maternal VM did not have any effect on the PI, RI, EDV, or MFV. CONCLUSION: Fetal MCA Doppler assessment is affected by increased probe pressure but not by maternal VM. Thus, the application of the MCA Doppler sonography should be undertaken in the head of fetuses without any probe pressure and without maternal VM.

Evaluation of biological effects induced by diagnostic ultrasound in the rat foetal tissues.

In recent years, there has been growing interest in estimating the degree of heating caused by the diagnostic ultrasound in clinical practice. Both theoretical and experimental methods have been suggested for estimating the heating potential, or thermal hazard, of diagnostic ultrasound. Aim of this study was to evaluate in vivo effects of ultrasound exposure of variable duration (from 10 up to 20 min) with commercially available imaging systems commonly used for diagnostic imaging. Numerical results related to the thermal effect are obtained by simulation program based on B-mode (scanning) and Doppler (non-scanning). To investigate the biological effects of the ultrasound exposure to the brain and liver tissues, the antioxidant enzyme activity and thiobarbituric acid reactive substances (TBARS) of the tissues were evaluated. In liver tissue, as a lipid peroxidation index, TBARS levels very significantly increase in Doppler group compared to control. However, in B-mode, TBARS levels are the same with the control group. Use of B-mode in foetal tissue is more reliable than Doppler mode because temperature rise is very small compared to the Doppler mode. On the other hand, the antioxidant enzyme activities tend to increase in B-mode and Doppler groups compared to the control group as a defensive mechanism. In the brain tissue, lipid peroxidation is increased slightly in B-mode compared to the control group. This situation is related to the molecular structure of the brain tissue because of its high lipid concentration. In brain tissue, the antioxidant enzyme activities and lipid peroxidation were significantly increased, such as liver tissue in Doppler groups. Doppler ultrasound may produce harmful effects in rat foetus liver and brain tissues as a result of the high temperature rises.

Thermal assessment of 40-MHz pulsed Doppler ultrasound in human eye.

Tissue exposure to diagnostic pulsed Doppler ultrasound (US) can cause significant temperature rises. Temperature rise induced by US biomicroscopy (UBM) system (VS40, VisualSonics, Toronto, ON, Canada) was measured in ex vivo human and rabbit eyes with a 26-gauge K-type needle thermocouple. The operating frequency was 40 MHz with a free field I(SPTA) of 2.6 mW/cm(2) (B-mode) and 11.9 W/cm(2) (Doppler). Peak negative pressures were 5.22 MPa (B-mode) and 7.32 MPa (Doppler), resulting in MIs of 0.83 (B-mode) and 1.05 (Doppler mode). In Doppler mode, mean temperature rises of 2.27 degrees C and 1.93 degrees C were measured for the human lens and ciliary body after a 3-min insonation, vs. 2.66 degrees C for the rabbit lens. Our results indicate that US-induced temperature rise decreases with decreasing number of cycles, decreasing pulse-repetition frequency (PRF) or increased transmit attenuation, and is consistent with simple models of heating. To limit risk of temperature rises of 1 degrees C in human ciliary body, use of the maximum settings of 16 cycles (0.400 micros pulse duration), 20-kHz PRF should include 3-dB transmit attenuation, and exposure time should be limited. For insonation of the lens, exposure settings no higher than nine cycles (0.225-micros pulse duration) and 10-kHz PRF should be employed and exposure time limited to minimize risk of temperature increases of 1 degree C.

Membrane damage thresholds for 1- to 10-MHz pulsed ultrasound exposure of phagocytic cells loaded with contrast agent gas bodies in vitro.

Monolayers of mouse macrophage-like cells provide a model system for the study of bioeffects of pulsed ultrasound (US) activation of contrast agent gas bodies. In this study, the dependence of membrane damage on ultrasonic frequency was examined for gas bodies attached to the cells. The monolayers cultured on the inside of one window of an exposure chamber were incubated with 2% Optison (Amersham Health Inc., Princeton, NJ) and then rinsed to remove unattached gas bodies. The chamber was filled with culture medium plus 20% trypan blue stain solution and was mounted at the 3.8-cm focus of an US transducer in a 37 degrees C water bath. Transducers were used with center frequencies of 1.0, 2.25, 3.5, 5.0, 7.5 and 10.0 MHz. The 1-min pulsed exposures utilized two-cycle excitation with 1% duty cycle. After exposure, cells in the focal zone were scored for trypan blue dye exclusion, with stained nuclei indicative of cell membrane damage. Exposure-response functions were approximated by performing a series of exposures with peak rarefactional pressure amplitudes differing by a factor of radical 2 (i.e., 3 dB apart). Linear regressions were performed on selected data to determine a threshold pressure amplitude at each frequency. Thresholds ranged from 0.066 MPa at 1.0 MHz to 0.62 MPa at 10 MHz and were approximately proportional to the frequency. These thresholds are less than the pressure amplitudes needed for nucleation of inertial cavitation and have a different frequency dependence than the general Mechanical Index.

Theoretical gas body pulsation in relation to empirical gas-body destabilization and to cell membrane damage thresholds.

Contrast agent gas bodies attached to phagocytic monolayer cells pulsate in response to ultrasound exposure and damage the cells above thresholds, which increase in proportion to frequency. This study considered the physical basis for the thresholds and their frequency dependence. Theory for the pulsation was evaluated using empirical pulse waveforms acquired at thresholds for 1.0, 2.25, 3.5, 5.0, 7.5, and 10 MHz. For optimum-sized gas bodies, the amplitudes calculated at the thresholds were about 11% of the initial radii. At the cell membrane damage thresholds, theoretical negative shell stresses were approximately constant with frequency at about 50 MPa. This stress appears to be sufficient to induce failure of the shell, and gas body destabilization was confirmed by increases in transmission of ultrasound pulses through the monolayer and by microscopically-observed shrinkage of the gas bodies. A model of acoustic microstreaming was used to calculate the shear stress during the pulses. The maximum shear stress increased from about 1500 to 4500 Pa from 1 to 10 MHz, sufficient for the cell membrane damage. This theoretical analysis shows that both the gas body destabilization and the cell membrane damage could be expected at similar peak rarefactional pressure amplitudes, with thresholds having the observed proportionality to frequency.

Temporal and spatial evaluation of lesion reparative responses following superthreshold exposure of rat lung to pulsed ultrasound.

This study characterized the reparative responses in rat lung. Forty-five adult female rats were exposed at two sites over the left lung to 3.1-MHz superthreshold pulsed ultrasound. The repair of lung lesions was evaluated from 0 through 44 days postexposure. Macroscopic lesions at 0 days postexposure were large bright red ellipses of hemorrhage. By 1 and 3 days postexposure, lesions were the same size and dark red to red-black, but, by 3 days postexposure, lesions had a raised surface appearance. From 5 to 10 days postexposure, lesions grew smaller in size, progressed from red-gray to yellow-brown, and retained a raised surface appearance. From 13 through 44 days postexposure, lesions gradually decreased in size, had a faint yellow-brown discoloration, and gradually lost the raised surface appearance. By 37 and 44 days postexposure, lung returned to near normal morphology, but had small areas of light yellow-brown discoloration in the areas where lung was exposed. Microscopic lesions at 0 and 1 days postexposure were areas of acute alveolar hemorrhage. By 3 days postexposure, lesions had loss of alveolar erythrocytes and the formation of hemoglobin crystals. From 5 through 44 days postexposure, iron in degraded erythrocytes was processed to hemosiderin and was negligible in quantity at 44 days postexposure. The proliferation of resident cells (likely alveolar epithelial cells, fibroblasts and endothelial cells) and the infiltration of inflammatory cells in lesions declined in intensity as the lesions aged and was minimal by 44 days postexposure. Under the superthreshold exposure conditions described, lesions induced by ultrasound do not seem to have long-term residual effects in lung.

Intracranial temperature elevation from diagnostic ultrasound.

Tissues of the central nervous system are sensitive to damage by physical agents, such as heat and ultrasound. Exposure to pulsed spectral Doppler ultrasound can significantly heat biologic tissue because of the relatively high intensities used and the need to hold the beam stationary during examinations. This has significant implications for sensitive neural tissue such as that exposed during spectral Doppler flow studies of fetal cerebral vessels. Recent changes in the FDA regulation allow delivery of almost eight times higher intensity into the fetal brain by ultrasound devices that incorporate an approved real-time output display in their design. In this situation, ultrasound users are expected to assess the risk/benefit ratio based on their interpretation of equipment output displays (including the thermal index, TI) and an understanding of the significance of biologic effects. To assist in the assessment of potential thermally mediated bioeffects, a number of conclusions can be drawn from the published scientific literature: the amount of ultrasound-induced intracranial heating increases with gestational age and the development of fetal bone; pulsed spectral Doppler ultrasound can produce biologically significant heating in the fetal brain; the rate of heating near bone is rapid, with approximately 75% of the maximum heating occurring within 30 s; blood flow has minimal cooling effect on ultrasound-induced heating of the brain when insonated with narrow focused clinical beams; the threshold for irreversible damage in the developing embryo and fetal brain is exceeded when a temperature increase of 4 degrees C is maintained for 5 min; an ultrasound exposure that produces a temperature increase of up to 1.5 degrees C in 120 s does not elicit measurable electrophysiologic responses in fetal brain; for some exposure conditions, the thermal index (TI), as used in the FDA-approved output display standard, underestimates the extent of ultrasound-induced intracranial temperature increase.

Are color and pulsed Doppler sonography safe in early pregnancy?

Diagnostic ultrasound has been used for many years with a remarkable history of safety during the standard clinical practice. Introduction of color and pulsed Doppler modes resulted with higher levels of transmitted and absorbed ultrasonic energy. This fact raised the question for the safety of its use in early pregnancy. This article presents the pros and contras regarding the safety and summarized actual guidelines and safety limits suggested and prescribed by several instances that supervise the use of ultrasound in medicine (WFUMB, ECMUS, ECURS, AIUM/NEMA). In addition, different clinical and experimental applications of Doppler ultrasound in early pregnancy are discussed regarding the safety limits. Generally, there are no strictly defined limits for the use of Doppler ultrasound in the early pregnancy. However, there is an unequivocal demand for carefulness that is best expressed by the ALARA principle. The prudent use of Doppler takes into account benefits against the possible theoretical risks, rather than prohibiting clinically useful technology or applications.

Effects of pulsed ultrasound on sphenoid bone temperature and the heart rate in guinea-pig foetuses.

Temperature increase induced by exposure to unscanned pulsed ultrasound at an intensity (I(SPTA)) 2.82 W/cm2 was measured in the brain adjacent to the sphenoid bone of foetal guinea-pigs in late gestation under in vitro and in vivo (in utero) conditions. After 120 s exposure a mean temperature increase of 2.6 degrees C was measured in vitro. Removal of the overlying parietal bones increased this value to 5.2 degrees C. Mean temperature increases at the sphenoid bone recorded in utero were 1.5 degrees C live and 2.0 degrees C post mortem. Measurement of foetal ECG showed that ultrasound-induced heating of the hypothalamic region did not significantly alter foetal heart rate.

Ultrasound-induced temperature increase in guinea-pig fetal brain in utero: third-trimester gestation.

Temperature increase was measured at various depths in the brain of living fetal guinea pigs during in utero exposure to unscanned pulsed ultrasound at ISPTA 2.8 W/cm2. Mean temperature increases of 4.9 degrees C close to parietal bone and 1.2 degrees C in the midbrain were recorded after 2-min exposures. When exposures were repeated on the same sites in each fetus after death, the corresponding mean temperature increases were 4.9 degrees C and 1.3 degrees C, respectively. Cerebral blood perfusion had little cooling effect on ultrasound-induced heating in the guinea pig fetus of 57-61 days gestational age.

Bioeffects of diagnostic ultrasound in vitro.

Biological effects induced by ultrasound were frequently reported for continuous wave (cw) mode. Thresholds for the onset of bioeffects of pulsed ultrasound, starting from diagnostic conditions, have not yet been defined by standardized in vitro models. We therefore investigated the effects of pulsed ultrasound on cultured cells using diagnostic ultrasound devices, a selfmade transducer and a sonochemical laboratory reactor tunable from pulsed diagnostic conditions to cw ultrasound. Additionally, we determined physical parameters of the ultrasonic field by different types of hydrophones. Sonochemical reactions and the effects induced by the ultrasonic fields in cultured cells indicated a threshold for bioeffects.

Direct evidence of cavitation in vivo from diagnostic ultrasound.

Recent increases in the pressure output of diagnostic ultrasound scanners have led to an interest in establishing thresholds for bioeffects in many organs including the lungs of mammals. Damage may be mediated by inertial cavitation, yet there have been no such direct observations in vivo. To explore the hypothesis of cavitation-based bioeffects from diagnostic ultrasound, research has been performed on the thresholds of damage in rat lungs exposed to 4.0-MHz pulsed Doppler and color Doppler ultrasound. A 30-MHz active cavitation detection scheme complementing these studies provides the first direct evidence of cavitation in vivo from diagnostic ultrasound pulses.

Lung lesions induced by continuous- and pulsed-wave (diagnostic) ultrasound in mice, rabbits, and pigs.

These studies documented the presence or absence of macroscopic and microscopic intraparenchymal hemorrhage in individual lung lobes of mice, rabbits, and pigs exposed to continuous- and pulsed-wave (diagnostic) ultrasound; we described the character of and lesions associated with the hemorrhage and compared differences in the lesions among species and exposure conditions to investigate the pathogenic mechanisms and species differences associated with ultrasound-induced lung hemorrhage. In a series of three sequential interdependent studies, 312 mice, 91 rabbits, and 74 pigs were divided at random into experimental groups and exposed to continuous-wave ultrasound (3 kHz modulated at 120 Hz) of acoustic pressure levels ranging from 0 to 490 kPa for 5, 10, or 20 minutes. In a fourth study, three mice, 43 rabbits, and six pigs were divided at random into experimental groups and exposed to pulsed-wave ultrasound (3- and 6-MHz center frequency) of peak rarefactional acoustic pressure levels ranging from 0 to 5.6 MPa for 5 minutes. Macroscopic lesions induced by continuous- and pulsed-wave ultrasound consisted of dark red to black areas of hemorrhage that extended from visceral pleural surfaces into lung parenchyma. Hemorrhage appeared spatially related to the edges of lung lobes where pleura of dorsal and ventral surfaces met, occurred in specific lung lobes in all three species, and appeared anatomically related to lung that was closest to and in contiguous alignment with the ultrasound transducer and thus the path of the sound beam. Macroscopic lesions were similar in all species under all exposure conditions for both continuous- and pulsed-wave ultrasound; however, hemorrhage was not induced in pig lung exposed to pulsed-wave ultrasound at any peak rarefactional acoustic pressure level. Eighteen mice (145 kPa exposure pressure), 60 rabbits (145-460 kPa exposure pressure), and 58 pigs (145-490 kPa exposure pressure) from study 3 were used for microscopic evaluation of lung exposed to continuous-wave ultrasound; three mice (6 MHz; 2.9 and 5.4 MPa), 39 rabbits (3 and 6 MHz; 2.3-5.4 MPa), and six pigs (3 and 6 MHz; 3.3, 5.4, and 5.6 MPa) from study 4 were used for microscopic evaluation of lung exposed to pulsed-wave ultrasound. Microscopic lesions and the character of hemorrhage induced by continuous-wave ultrasound were different from those induced by pulsed-wave ultrasound. Lesions induced by continuous-wave ultrasound under all exposure conditions were similar in all three species. Lesions induced by pulsed-wave ultrasound under all exposure conditions were similar in all three species.(ABSTRACT TRUNCATED AT 400 WORDS)

Intestinal hemorrhage from exposure to pulsed ultrasound.

Threshold exposures for producing intestinal hemorrhage in mice were determined using focused sources operating at 0.7, 1.1, 2.4 and 3.6 MHz. The choice of pulse length (10 microseconds) and pulse repetition frequency (100 Hz) made the exposures diagnostically relevant, while at the same time, minimized possible thermal contributions to the mechanism of action of the ultrasound. Each animal was irradiated at four to five abdominal sites for 5 min per site. Suprathreshold lesions ranged from small petechiae to hemorrhagic regions extending 4 mm or more along the intestine, depending upon the exposure levels. Higher frequencies were less effective in producing intestinal hemorrhage than lower frequencies. Thermocouple measurements of temperature rise in the intestine during ultrasound exposure revealed temperature increments between 1 degrees and 2 degrees C at the highest exposure levels. The frequency dependence of the production of intestinal hemorrhage together with the observed limited heating is consistent with a cavitation-related mechanism of action of pulsed ultrasound.