Quantification of risk from fetal exposure to diagnostic ultrasound

Comparison of fetal and maternal tissue elasticity between euploid and aneuploid pregnancies by shear wave elastography

AIM

B-mode ultrasonography utilized for fetal screening of common trisomies is referred to as a genetic sonogram and includes determining major abnormalities and soft markers such as hypoplastic nasal bone and increased thickness of the nuchal fold. Elastography is a novel ultrasound technique giving information about tissue stiffness used for diagnosing cancer, transplant rejection, and organ fibrosis. This study aimed to determine via shear wave elastography (SWE) whether euploid and aneuploid fetal soft marker tissues vary in stiffness.

METHODS

The participants were all singleton pregnancies between 19 and 23 weeks of gestation; 35 euploid and 14 aneuploid fetus pregnancies were enrolled. Fetal bowel, kidney, liver, nasal bone, nuchal fold, placenta, and myometrium were investigated with SWE using acoustic radiation impulse force. Images were analyzed with a novel software calibrated and written by us using MATLAB. Statistical analysis was completed with the SPSS Program. Shapiro-Wilk normality distribution analysis, Student's t-test, and Mann-Whitney U methods were used.

RESULTS

The mean shear wave speed of fetal nasal bone was significantly lower in aneuploid fetuses. There was no difference between other tissues in mean shear wave velocity.

CONCLUSIONS

Euploid and aneuploid fetuses have different elastic properties of the nasal bone and this may have a role in differentiating aneuploid fetuses noninvasively.

Hydrophone Spatial Averaging Correction for Acoustic Exposure Measurements From Arrays-Part I: Theory and Impact on Diagnostic Safety Indexes

This article reports underestimation of mechanical index (MI) and nonscanned thermal index for bone near focus (TIB) due to hydrophone spatial averaging effects that occur during acoustic output measurements for clinical linear and phased arrays. TIB is the appropriate version of thermal index (TI) for fetal imaging after ten weeks from the last menstrual period according to the American Institute of Ultrasound in Medicine (AIUM). Spatial averaging is particularly troublesome for highly focused beams and nonlinear, nonscanned modes such as acoustic radiation force impulse (ARFI) and pulsed Doppler. MI and variants of TI (e.g., TIB), which are displayed in real-time during imaging, are often not corrected for hydrophone spatial averaging because a standardized method for doing so does not exist for linear and phased arrays. A novel analytic inverse-filter method to correct for spatial averaging for pressure waves from linear and phased arrays is derived in this article (Part I) and experimentally validated in a companion article (Part II). A simulation was developed to estimate potential spatial-averaging errors for typical clinical ultrasound imaging systems based on the theoretical inverse filter and specifications for 124 scanner/transducer combinations from the U.S. Food and Drug Administration (FDA) 510(k) database from 2015 to 2019. Specifications included center frequency, aperture size, acoustic output parameters, hydrophone geometrical sensitive element diameter, etc. Correction for hydrophone spatial averaging using the inverse filter suggests that maximally achievable values for MI, TIB, thermal dose ( t ₄₃ ), and spatial-peak-temporal-average intensity ( [Formula: see text]) for typical clinical systems are potentially higher than uncorrected values by (means ± standard deviations) 9% ± 4% (ARFI MI), 19% ± 15% (ARFI TIB), 50% ± 41% (ARFI t ₄₃ ), 43% ± 39% (ARFI [Formula: see text]), 7% ± 5% (pulsed Doppler MI), 15% ± 11% (pulsed Doppler TIB), 42% ± 31% (pulsed Doppler t ₄₃ ), and 33% ± 27% (pulsed Doppler [Formula: see text]). These values correspond to frequencies of 3.2 ± 1.3 (ARFI) and 4.1 ± 1.4 MHz (pulsed Doppler), and the model predicts that they would increase with frequency. Inverse filtering for hydrophone spatial averaging significantly improves the accuracy of estimates of MI, TIB, t ₄₃ , and [Formula: see text] for ARFI and pulsed Doppler signals.

Management of ureteric stone in pregnancy: a review

Background

Urolithiasis in pregnancy is a major health concern and is one of the most common causes for non-obstetrical abdominal pain and subsequent hospital admission during pregnancy. The incidence of urinary calculi during pregnancy varies in the range of 1/200 to 1/2000. Acute ureteric colic in pregnancy is associated with significant potential risks to both mother and fetus. Significant anatomic and functional changes occur in pregnancy which not only lead to stone formation but also create diagnostic dilemma. The diagnosis of ureteric calculi can be incorrect in about 28% of pregnant patients.

Main body

Management of ureteric stone during pregnancy is remaining to be a challenge for the treating urologist. Because of the inability to use good imaging options for the diagnosis confirmation and more invasive approach for the treatment, management continues to be difficult. The main threats are preterm labor with delivery and premature rupture of membranes. Other pregnancy complications are obstructive uropathy, gestational diabetes mellitus, recurrent abortions and pre-eclampsia. Management of diagnosed ureteric stone is unique in the pregnant population and requires multi-disciplinary care. It should be individualized for each patient and moves preferably from conservative to invasive approaches sequentially. With continued advancements in endourological techniques, few definitive treatment options are also available for such patients.

Conclusion

There are several lacunae related with the diagnostic imaging, medical expulsive therapy, reliability of ureteral stent/percutaneous nephrostomy insertions and safety of ureteroscopy during pregnancy. Herein, we review the management of ureteric stone during pregnancy, the various diagnostic modalities and treatment options with their advantages and disadvantages. We also proposed our management algorithm to deal with such clinical scenario in this particular population.

The use of elastography in placental research - A literature review

INTRODUCTION

Ultrasound elastography is a technique used to quantify biomechanical changes that occur in parenchymal tissue with disease. Recent research has applied the technique to the placenta in order to investigate changes associated with uteroplacental dysfunction. We performed a literature review to summarise the current available information regarding this novel technique.

METHODS

Pubmed, CINAHL and Embase were searched using the terms "placenta", "ultrasound" and "elastography". Only full text studies written in English and limited to placental sonoelastography were included.

RESULTS

Twenty-eight studies met the inclusion criteria and were included in this review. Publications were divided into in vivo and ex vivo groups, and further categorised into four subgroups: normal pregnancy, pregnancy-induced hypertension and pre-eclampsia, fetal growth restriction and other pregnancy complications.

CONCLUSION

Ultrasound elastography can quantitatively assess biomechanical properties of the placenta in conditions where placental function is compromised.

Temperature Rise Caused by Shear Wave Elastography, Pulse Doppler and B-Mode in Biological Tissue: An Infrared Thermographic Approach

The aim of this study was to determine the interest in and relevance of the use of infrared thermography, which is a non-invasive full-field surface temperature measurement technique, to characterize the heterogeneous heating caused by ultrasound in biological tissue. Thermal effects of shear wave elastography, pulse Doppler and B-mode were evidenced in porcine tissue. Experiments were performed using a high-frequency echography Aixplorer system (Supersonic Imagine, Aix-en-Provence, France). For all three modes, ultrasound was applied continuously for 360 s while the temperature at the sample surface was recorded with a Cedip Jade III-MWIR infrared camera (Flir, Torcy, France). Temperature changes were detected for the three modes. In particular, "heat tunnels" crossing the sample were visualized from the early stages of the experiment. Heat conduction from the transducer was also involved in the global warming of the sample. The study widens the prospects for studies on tolerability, potentially in addition to classic approaches such as those using thermocouples.

Written Feedback on Thermal Index During the First-Trimester Nuchal Translucency Examination Does Not Improve Compliance With the “as Low as Reasonably Achievable” Principle

Objective: The null hypothesis was that there would be no change in as low as reasonably achievable (ALARA) behavior based on feedback from comments on the nuchal translucency quality review (NTQR) image submissions. Methods: A review of the Perinatal Quality Foundation Database found 206 practitioners who failed their first credential attempt and received ALARA comments as feedback. The second submissions of the same subjects were reviewed to determine if compliance with ALARA improved following written feedback. Results: Seventeen percent of all second submissions addressed ALARA. Sixty-one percent of second submissions passed, however only 18.4 % of those addressed ALARA. Among those who submitted thermal index at bone values on both first and second submissions, the mean values on the second submission were 0.20 lower compared to the first submission ( P = .0288). Second submission mean thermal index at bone values were 0.68 lower among those submissions with ALARA concerns addressed compared to those not addressing these concerns ( P < .001). Conclusions: Written feedback without consequences on the ALARA principle did not improve compliance.

Adherence to Swedish national pregnancy dating guidelines and management of discrepancies between pregnancy dating methods: a survey study

Background

Swedish national guidelines for pregnancy dating were published in 2010. Follow-up is needed to assess adherence and to identify whether any clinical topics are not covered in the guidelines.

Methods

All units in Sweden that performed ultrasound-based pregnancy dating were asked to complete a web-based questionnaire comprising multiple-response questions and commentary fields. Information was collected regarding baseline information, current and previous clinical practice, and management of discrepancies between last-menstrual-period- and ultrasound-based methods for pregnancy dating.

Results

The response rate was 79%. Half of the units offered first-trimester ultrasound to all pregnant women. However, contrary to the guidelines, the crown–rump length was not used for ultrasound-based pregnancy dating in most units. Instead, ultrasound-based pregnancy dating was performed only if the biparietal diameter was between 21 and 55 mm. Management of discrepancies between methods for pregnancy dating varied widely.

Conclusions

The units reported high adherence to national guidelines, except for early pregnancy dating, for which many units followed unwritten or informal guidelines. The management of discrepancies between last-menstrual-period-based and ultrasound-based estimated day of delivery varied widely. These findings emphasize the need for regular updating of national written guidelines and efforts to improve their implementation in all units.

Frequency bands for ultrasound, suitable for the consideration of its health effects

It is proposed that the ultrasound frequency spectrum should be divided into three bands in order to facilitate a more rational assessment of its health effects. Whilst statement of the frequencies at the borders of these bands facilitates their definition, it is recognized that these observables vary continuously with frequency and consequently these border frequencies should not be used to rule out the possibility of a given effect occurring. The lowest band, US(A), lies between 17.8 and 500 kHz. In this band acoustic cavitation and its associated forces form the dominant process resulting in biological effects in liquids and soft tissues, whereas health effects from airborne ultrasound have been reported but are far less researched. In the middle band, US(B), between 500 kHz and 100 MHz, temperature rise in tissues becomes the most important biological effect of exposure. The highest band, US(C), covers frequencies above 100 MHz, for which the radiation force becomes an increasingly important biophysical mechanism. A justification for the selection of 17.8 kHz in preference to any other threshold for the lower frequency limit for ultrasound is given.

Shear wave elastography safety in fetus: A quantitative health risk assessment

PURPOSE

To identify the effects of shear wave elastography in the fetus for evaluation in widespread use.

MATERIALS AND METHODS

The Health Risk Assessment method proposed by the National Research Council was used with literature to evaluate the safety of shear wave elastography for the fetus regarding its potential effects in human tissues.

RESULTS

The experimental and epidemiologic data from 25 articles showed that shear wave elastography maintained the same thermal effect as pulsed Doppler ultrasound already authorized in obstetrics, and that cavitation effect on fetal tissue is improbable. Nonetheless, the vibratory character of shear waves could induce displacement of fetal tissue while potential effects of very short duration energy peaks of the radiation force focused wave front remain unknown.

CONCLUSION

The actual knowledge does not provide enough information to assess the effects of shear wave elastography on fetal tissues, thus these points have to be explored by further experimental studies.

Association of Prenatal Ultrasonography and Autism Spectrum Disorder

Importance

The prevalence of autism spectrum disorder (ASD) has been increasing rapidly, with current estimates of 1 in 68 children affected. Simultaneously, use of prenatal ultrasonography has increased substantially, with limited investigation into its safety and effects on brain development. Animal studies have demonstrated that prenatal ultrasonography can adversely affect neuronal migration.

Objective

To quantify prenatal ultrasound exposure by the frequency, timing, duration, and strength of ultrasonographic scans in children with later ASD, developmental delay, and typical development.

Design, Setting, and Participants

This case-control study included 107 patients with ASD, 104 control individuals with developmental delay, and 209 controls with typical development. Participants were identified from medical records based on prenatal care and delivery at Boston Medical Center, a diverse, academic, safety-net medical center, from July 1, 2006, through December 31, 2014, with a gestational age at birth of at least 37 weeks. Data were analyzed from May 1, 2015, through November 30, 2017.

Exposures

Ultrasonographic exposure was quantified by the number and timing of scans, duration of exposure, mean strength (depth, frame rate, mechanical index, and thermal index), and time of Doppler and 3- and 4-dimensional imaging.

Main Outcomes and Measures

Among participants with ASD and controls with developmental delay and typical development, ultrasound exposure was quantified and compared per trimester and for the entire pregnancy, with adjustment for infant sex, gestational age at birth, and maternal age.

Results

A total of 420 participants were included in the study (328 boys [78.1%] and 92 girls [21.9%]; mean age as of January 1, 2016, 6.6 years; 95% CI, 6.5-6.8 years). The ASD group received a mean of 5.9 scans (95% CI, 5.2-6.6), which was not significantly different from the 6.1 scans (95% CI, 5.4-6.8) in the developmental delay group or the 6.3 scans (95% CI, 5.8-6.8) in the typical development group. Compared with the typical development group, the ASD group had shorter duration of ultrasound exposure during the first (290.4 seconds [95% CI, 212.8-368.0 seconds] vs 406.4 seconds [95% CI, 349.5-463.3 seconds]) and second (1687.6 seconds [95% CI, 1493.8-1881.4 seconds] vs 2011.0 seconds [95% CI, 1868.9-2153.1 seconds]) trimesters but no difference in the number of scans. The ASD group had greater mean depth of ultrasonographic penetration than the developmental delay group in the first trimester (12.5 cm [95% CI, 12.0-13.0 cm] vs 11.6 cm [95% CI, 11.1-12.1 cm]). The ASD group had greater mean depth than the typical development group during the first (12.5 cm [95% CI, 12.0-13.0 cm] vs 11.6 cm [95% CI, 11.3-12.0 cm]) and the second (12.9 cm [95% CI, 12.6-13.3 cm] vs 12.5 cm [95% CI, 12.2-12.7 cm]) trimesters.

Conclusions and Relevance

This study found significantly greater mean depth of ultrasonographic penetration in the ASD group compared with the developmental delay group in the first trimester and compared with the typical development group in the first and second trimesters. Further research is needed to determine whether other variables of ultrasound exposure also have adverse effects on the developing fetus.

Mechanical and Biological Effects of Ultrasound: A Review of Present Knowledge

Ultrasound is widely used for medical diagnosis and increasingly for therapeutic purposes. An understanding of the bio-effects of sonography is important for clinicians and scientists working in the field because permanent damage to biological tissues can occur at high levels of exposure. Here the underlying principles of thermal mechanisms and the physical interactions of ultrasound with biological tissues are reviewed. Adverse health effects derived from cellular studies, animal studies and clinical reports are reviewed to provide insight into the in vitro and in vivo bio-effects of ultrasound.

Bioeffects of Diagnostic Dynamic 3-Dimensional (4-Dimensional) Ultrasound on Ultrastructure of Cerebral Cells of Fetal Mice in Late Pregnancy

The aim of this study was to study the bioeffects of diagnostic dynamic 3-dimensional ultrasound (4D) on ultrastructure of cerebral cells of fetal mice in late pregnancy. Thirty pregnant mice carrying 18th embryonic day fetuses were randomly allocated into 6 groups, namely, control group, sham-exposed group, 5 minute-exposed group, 10 minute-exposed group, 20 minute-exposed group, and 30 minute-exposed groups (5 in each group). In exposure groups, mice were put under the dynamic 3D ultrasound system's probe for 5 to 30 minutes. Mice in sham-exposed group did not receive ultrasound wave. At 24th hour after birth, 10 pups of each group were randomly selected (2 in each litter) and euthanized by decapitation, and the brains were immediately removed. Right parietal lobes were taken as specimen. The specimens were firstly fixed with glutaraldehyde and secondly with osmic acid, then sections were made and observed under the transmission electron microscope. There were no obvious abnormal ultrastructure changes in control group, sham-exposed group, and 5 minute-exposed group under transmission electron microscope. Ten minute-exposed group showed some enlarged mitochondria, broken crista, vacuolated endoplasmic resticulums, and a few apoptosis cells. More abnormal organelles and apoptosis cells were observed in 20 minute-exposed and 30 minute-exposed groups (P < 0.05). Dynamic 3D (4D) ultrasound exposure for more than 10 minutes may result in abnormal neuronal ultrastructure changes and apoptosis cells in fetal mouse cerebrum.

Measurement of Thermal Effects of Doppler Ultrasound: An In Vitro Study

Objective

Ultrasound is considered a safe imaging modality and is routinely applied during early pregnancy. However, reservations are expressed concerning the application of Doppler ultrasound in early pregnancy due to energy emission of the ultrasound probe and its conversion to heat. The objective of this study was to evaluate the thermal effects of emitted Doppler ultrasound of different ultrasound machines and probes by means of temperature increase of in-vitro test-media.

Methods

We investigated the energy-output of 5 vaginal and abdominal probes of 3 ultrasound machines (GE Healthcare, Siemens, Aloka). Two in-vitro test objects were developed at the Center for Medical Physics and Biomedical Engineering, Medical University Vienna (water bath and hydrogel bath). Temperature increase during Doppler ultrasound emission was measured via thermal sensors, which were placed inside the test objects or on the probes’ surface. Each probe was emitting for 5 minutes into the absorbing test object with 3 different TI/MI settings in Spectral Doppler mode.

Results

During water bath test, temperature increase varied between 0.1 and 1.0°C, depending on probe, setting and focus, and was found highest for spectral Doppler mode alone. Maximum temperature increase was found during the surface heating test, where values up to 2.4°C could be measured within 5 minutes of emission.

Conclusions

Activation of Doppler ultrasound in the waterbath model causes a significant increase of temperature within one minute. Thermally induced effects on the embryo cannot be excluded when using Doppler ultrasound in early pregnancy.

Stimulation of bone repair with ultrasound: a review of the possible mechanic effects

In vivo and in vitro studies have demonstrated the positive role that ultrasound can play in the enhancement of fracture healing or in the reactivation of a failed healing process. We review the several options available for the use of ultrasound in this context, either to induce a direct physical effect (LIPUS, shock waves), to deliver bioactive molecules such as growth factors, or to transfect cells with osteogenic plasmids; with a main focus on LIPUS (or Low Intensity Pulsed Ultrasound) as it is the most widespread and studied technique. The biological response to LIPUS is complex as numerous cell types respond to this stimulus involving several pathways. Known to-date mechanotransduction pathways involved in cell responses include MAPK and other kinases signaling pathways, gap-junctional intercellular communication, up-regulation and clustering of integrins, involvement of the COX-2/PGE2, iNOS/NO pathways and activation of ATI mechanoreceptor. The mechanisms by which ultrasound can trigger these effects remain intriguing. Possible mechanisms include direct and indirect mechanical effects like acoustic radiation force, acoustic streaming, and propagation of surface waves, fluid-flow induced circulation and redistribution of nutrients, oxygen and signaling molecules. Effects caused by the transformation of acoustic wave energy into heat can usually be neglected, but heating of the transducer may have a potential impact on the stimulation in some in-vitro systems, depending on the coupling conditions. Cavitation cannot occur at the pressure levels delivered by LIPUS. In-vitro studies, although not appropriate to identify the overall biological effects, are of great interest to study specific mechanisms of action. The diversity of current experimental set-ups however renders this analysis very complex, as phenomena such as transducer heating, inhomogeneities of the sound intensity in the near field, resonances in the transmission and reflection through the culture dish walls and the formation of standing waves will greatly affect the local type and amplitude of the stimulus exerted on the cells. A future engineering challenge is therefore the design of dedicated experimental set-ups, in which the different mechanical phenomena induced by ultrasound can be controlled. This is a prerequisite to evaluate the biological effects of the different phenomena with respect to particular parameters, like intensity, frequency, or duty cycle. By relating the variations of these parameters to the induced physical effects and to the biological responses, it will become possible to derive an 'acoustic dose' and propose a quantification and cross-calibration of the different experimental systems. Improvements in bone healing management will probably also come from a combination of ultrasound with a 'biologic' components, e.g. growth factors, scaffolds, gene therapies, or drug delivery vehicles, the effects of which being potentiated by the ultrasound.

Arrhenius thermodynamics and birth defects: chemical teratogen synergy. Untested, testable, and projected relevance

This article addresses the issue of hyperthermia-induced birth defects with an accompanying additional teratogen, be it a chemical or a physical agent (i.e., a simultaneous "combinational" exposure to two teratogens, one of which is hyperthermia). Hyperthermia per se is a recognized human and animal teratogen. An excellent example of such combinational exposures is an epileptic woman who becomes pregnant while taking valproic acid (VPA) to control seizures. VPA is a recognized chemical teratogen, and fever (hyperthermia) is not an uncommon event during pregnancy. While VPA also may occasionally induce fever as a side effect, we are concerned here with fevers arising from other, unrelated causes. There is a small but internally consistent literature on these combinational-teratogen exposures involving hyperthermia plus a chemical teratogen; in each instance, the effect level has been observed to be synergistically elevated above levels induced by the separate teratogenic components. The data were empirical. The observed synergy is, however, consistent with Arrhenius thermodynamics, a well-known chemical rate equation. The need for information about combinational teratogen exposures is acute; fever is a common occurrence during pregnancy; and there are many instances whereby there is also the simultaneous presence of some other teratogen(s). Given that the rate of autism spectrum disorders in the United States was recently presented as 1 in 88 births, it seems reasonable to suspect that such combinational regimens are much more prevalent than previously thought. Our hypothesis is that synergistic birth defect levels from combinational regimens are consistent with Arrhenius thermodynamics.

Prenatal ultrasound heating impacts on fluctuations in haematological analysis of Oryctolagus cuniculus

Prenatal Ultrasound (US) is commonly used as a routine procedure on pregnant women. It is generally perceived as a safe procedure due to the use of non-ionizing radiation. However, the neurotoxicity of diagnostic prenatal US was detected to have a correlation with high susceptibility to early developing fetus. This research involved in vivo experimental model by using 3(rd) trimester pregnant Oryctolagus cuniculus and exposing them to US exposures for 30, 60, and 90 minutes at their gestational day (GD) 28-29. The output power and intensities, spatial peak temporal average intensity (ISPTA) of US were varied from 0.4 to 0.7 W and 0.13 to 0.19 W/cm(2) respectively were tested initially in free-field, water. Haematological analysis was carried out to detect any changes in blood constituents. Statistically significant differences were detected in red blood cell (RBC) count (P<0.001), haemoglobin (Hb) concentration (P<0.001) and also platelet (PLT) count (P<0.001) in newborn of Oryctolagus cuniculus. These findings indicate the possibility of US heating in causing defects on studied animal.

Trends in diagnostic ultrasound acoustic output from data reported to the US Food and Drug Administration for device indications that include fetal applications

OBJECTIVES

A survey was conducted of acoustic output data received by the US Food and Drug Administration for diagnostic ultrasound devices whose indications for use include fetal applications to assess trends in maximum available acoustic output over time.

METHODS

Data were collected from 124 regulatory submissions received between 1984 and 2010. Data collection excluded transducers not indicated for diagnostic fetal imaging. The output parameters of ultrasonic power, mean center frequency, and bone thermal index (TIB) were extracted or computed from the submissions for 3 periods: 1984-1989, 1992-1997, and 2005-2010. The data were stratified according to the following imaging modes: M-mode, B/M-mode, pulsed wave Doppler, color flow Doppler, and continuous wave Doppler.

RESULTS

Ultrasonic power and maximum TIB values have increased roughly an order of magnitude from pre-1991 to post-1991 periods; the center frequency has decreased somewhat (4.2 to 3.4 MHz). The percentage of Doppler-mode transducers has increased substantially over time, with the majority of the diagnostic fetal imaging transducers currently designed to operate in Doppler modes; this increase is particularly important, since Doppler modes generate much higher TIB levels than B/M-modes. Color flow Doppler ultrasound currently operates at the highest mean ultrasonic power level (with a 14-fold increase over time).

CONCLUSIONS

The observed trends in increased acoustic output for both Doppler and non-Doppler modes underscore the widely recognized importance of adherence to the ALARA (as low as reasonably achievable) principle and prudent use in fetal ultrasound imaging.

Statistics of acoustically induced bubble-nucleation events in in vitro blood: a feasibility study

Bubbles can form in biological tissues through ultrasonic activation of natural gas nuclei. The damaging aftereffects raise safety concerns. However, the population of nuclei is currently unknown, and bubble nucleation is stochastic and thus unpredictable. This study investigates the statistical behavior of bubble nucleation experimentally and introduces a model-based analysis to determine the distribution of nuclei in biological samples-two pig blood samples in vitro. Combined ultra-fast passive and active cavitation detection with a linear array was used to detect nucleation from pulsed ultrasound excitations at 660 kHz. Single nucleation events were detected at peak rarefaction pressures from -3.6 to -24 MPa, and the nucleation probability over the range 0 to 1 was estimated from more than 330 independent acquisitions per sample. Model fitting of the experimental probability revealed that the distribution of nuclei is most likely continuous, and nuclei are rare in comparison to blood cells.

Are prenatal ultrasound scans associated with the autism phenotype? Follow-up of a randomised controlled trial

An existing randomised controlled trial was used to investigate whether multiple ultrasound scans may be associated with the autism phenotype. From 2,834 single pregnancies, 1,415 were selected at random to receive ultrasound imaging and continuous wave Doppler flow studies at five points throughout pregnancy (Intensive) and 1,419 to receive a single imaging scan at 18 weeks (Regular), with further scans only as indicated on clinical grounds. There was no significant difference in the rate of Autism Spectrum Disorder between the Regular (9/1,125, 0.8 %) and Intensive (7/1,167, 0.6 %) groups, nor a difference between groups in the level of autistic-like traits in early adulthood. There is no clear link between the frequency and timing of prenatal ultrasound scans and the autism phenotype.

Improving image quality of diagnostic ultrasound by using the safe use time model with the dynamic safety factor and the effect of the exposure time on the image quality

Resolution and penetration are primary criteria for image quality of diagnostic ultrasound. In theory (and usually in practice), the maximum depth of imaging in a tissue increases as power (pressure) is increased. Alternatively, at a particular effective penetration, an increased power may be used to allow a higher ultrasound frequency for higher resolution and tissue contrast. Recently, Karagoz and Kartal proposed a safety parameter for thermal bioeffects of diagnostic ultrasound; that is, SUT (safe use time). The SUT model is constructed to determine how long one piece of tissue can be insonated safely according to a threshold exposure. Also, Karagoz and Kartal suggested that an increase in acoustic intensity beyond the current US Food and Drug Administration (FDA) limit of intensity can be theoretically possible by using SUT model while staying within the safe limit. The present study was motivated particularly by the goals of higher resolution and/or deeper penetration by using SUT model. The results presented here suggest that the safe use of higher exposure levels than currently allowed by the FDA may be possible for obtaining substantial improvements in penetration depth and/or resolution. Also, the study reveals that image quality can be functionally related to exposure time in addition to acoustic energy and frequency.

In vivo bubble nucleation probability in sheep brain tissue

Gas nuclei exist naturally in living bodies. Their activation initiates cavitation activity, and is possible using short ultrasonic excitations of high amplitude. However, little is known about the nuclei population in vivo, and therefore about the rarefaction pressure required to form bubbles in tissue. A novel method dedicated to in vivo investigations was used here that combines passive and active cavitation detection with a multi-element linear ultrasound probe (4-7 MHz). Experiments were performed in vivo on the brain of trepanated sheep. Bubble nucleation was induced using a focused single-element transducer (central frequency 660 kHz, f-number = 1) driven by a high power (up to 5 kW) electric burst of two cycles. Successive passive recording and ultrafast active imaging were shown to allow detection of a single nucleation event in brain tissue in vivo. Experiments carried out on eight sheep allowed statistical studies of the bubble nucleation process. The nucleation probability was evaluated as a function of the peak negative pressure. No nucleation event could be detected with a peak negative pressure weaker than -12.7 MPa, i.e. one order of magnitude higher than the recommendations based on the mechanical index. Below this threshold, bubble nucleation in vivo in brain tissues is a random phenomenon.

Thermal aspects of exposure to radiofrequency energy: report of a workshop

This special issue contains papers presented at an international workshop entitled 'Thermal Aspects of Radio Frequency Exposure' convened in Gaithersburg, Maryland, USA on 11-12 January 2010, and co-sponsored by the Mobile Manufacturers Forum, the GSM Association, and the US Food and Drug Administration. The goals of the workshop were to (1) identify appropriate health endpoints associated with thermal hazards and their time-dependence thresholds, and (2) outline future directions for research that might lead to an improved understanding of health and safety implications of human exposure to radiofrequency energy and design of improved exposure limits for this energy. This present contribution summarises some of the major conclusions of the speakers, and offers comments by one of the present authors on proposed research priorities and the implications of the material presented at the workshop for setting improved thermally based limits for human exposure to RF energy.

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.

Ultrasonic imaging: safety considerations

Modern ultrasound imaging for diagnostic purposes has a wide range of applications. It is used in obstetrics to monitor the progress of pregnancy, in oncology to visualize tumours and their response to treatment, and, in cardiology, contrast-enhanced studies are used to investigate heart function and physiology. An increasing use of diagnostic ultrasound is to provide the first photograph for baby's album-in the form of a souvenir or keepsake scan that might be taken as part of a routine investigation, or during a visit to an independent high-street 'boutique'. It is therefore important to ensure that any benefit accrued from these applications outweighs any accompanying risk, and to evaluate the existing ultrasound bio-effect and epidemiology literature with this in mind. This review considers the existing laboratory and epidemiological evidence about the safety of diagnostic ultrasound and puts it in the context of current clinical usage.

Ethical Analysis of Non-Medical Fetal Ultrasound

Obstetric ultrasound is the well-recognized prenatal test used to visualize and determine the condition of a pregnant woman and her fetus. Apart from the clinical application, some businesses have started promoting the use of fetal ultrasound machines for nonmedical reasons. Non-medical fetal ultrasound (also known as ‘keepsake’ ultrasound) is defined as using ultrasound to view, take a picture, or determine the sex of a fetus without a medical indication. Notwithstanding the guidelines and warnings regarding ultrasound safety issued by governments and professional bodies, the absence of scientifically proven physical harm to fetuses from this procedure seems to provide these businesses with grounds for rapid expansion. However, this argument is too simplistic because current epidemiological evidence is not synchronous with advancing ultrasound technology. As non-medical fetal ultrasound has aroused very significant public attention, a thorough ethical analysis of this topic is essential. Using a multifaceted approach, we analyse the ethical perspective of non-medical fetal ultrasound in terms of the expectant mother, the fetus and health professionals. After applying four major theories of ethics and principles (the precautionary principle; theories of consequentialism and impartiality; duty-based theory; and rights-based theories), we conclude that obstetric ultrasound practice is ethically justifiable only if the indication for its use is based on medical evidence. Non-medical fetal ultrasound can be considered ethically unjustifiable. Nevertheless, the ethical analysis of this issue is time dependent owing to rapid advancements in ultrasound technology and the safety issue. The role of health professionals in ensuring that obstetric ultrasound is an ethically justifiable practice is also discussed.

The emerging role of functional MRI for evaluating fetal brain activity

Although functional magnetic resonance imaging is a technique that is widely used in adult populations, its use within a fetal environment has been extremely limited. Problems associated with movement and technical scanning issues have limited its effectiveness in providing reliable and spatially accurate details of fetal brain activity. However, initial research has indicated that it is a viable tool for assessing functional maturation in the fetus, and recent advances in echo-planar imaging sequences on the abdomen at 3-T provide the potential for more reliable activation detection and higher resolution spatial information. If the technique can be further developed such that a similar reliability in activity patterns is observed as in conventional functional MRI, then fetal functional MRI could offer a useful contribution at a clinical level as well as at a research one in the assessment of brain development and maturation.

A new safety parameter for diagnostic ultrasound thermal bioeffects: safe use time

It is widely accepted that diagnostic ultrasound has the potential to elevate the temperature of tissue being scanned. Because both the maximum value of the temperature rise and the temporal profile of that rise are necessary to estimate the risk correctly, the temperature rise [DeltaT(t)] at an observation point for an exposure condition is presumed to have two components, that is, DeltaT(t)=DeltaT(max)X(t). The amplitude component DeltaT(max) is the maximum value of DeltaT(t), and the exposure time component X(t) represents the time dependency of that DeltaT(t). Ninety-six cases were investigated to obtain the proposed DeltaT(t) model at six frequencies, four source diameters, and four f-numbers. Then, using the relative change in the rate of induction of a thermal effect due to ultrasound exposure that produces DeltaT(t) different from a threshold exposure, the safe use time (SUT) model was constructed. SUT informs the user of the maximum duration of exposure in a region at a particular output level that would be no more hazardous than scanning at the threshold exposure. Using the SUT model, high power ultrasound can be applied for a short time so that the user can improve imaging performance while staying within safe limits.

Ultrasound biosafety considerations for the practicing sonographer and sonologist

The purpose of this article is to present the practicing sonographer and sonologist with an overview of the biohazards of ultrasound and guidelines for safe use.

In vivo quantification of embryonic and placental growth during gestation in mice using micro-ultrasound

BACKGROUND

Non-invasive micro-ultrasound was evaluated as a method to quantify intrauterine growth phenotypes in mice. Improved methods are required to accelerate research using genetically-altered mice to investigate the interactive roles of genes and environments on embryonic and placental growth. We determined (1) feasible age ranges for measuring specific variables, (2) normative growth curves, (3) accuracy of ultrasound measurements in comparison with light microscopy, and (4) weight prediction equations using regression analysis for CD-1 mice and evaluated their accuracy when applied to other mouse strains.

METHODS

We used 30-40 MHz ultrasound to quantify embryonic and placental morphometry in isoflurane-anesthetized pregnant CD-1 mice from embryonic day 7.5 (E7.5) to E18.5 (full-term), and for C57Bl/6J, B6CBAF1, and hIGFBP1 pregnant transgenic mice at E17.5.

RESULTS

Gestational sac dimension provided the earliest measure of conceptus size. Sac dimension derived using regression analysis increased from 0.84 mm at E7.5 to 6.44 mm at E11.5 when it was discontinued. The earliest measurement of embryo size was crown-rump length (CRL) which increased from 1.88 mm at E8.5 to 16.22 mm at E16.5 after which it exceeded the field of view. From E10.5 to E18.5 (full term), progressive increases were observed in embryonic biparietal diameter (BPD) (0.79 mm to 7.55 mm at E18.5), abdominal circumference (AC) (4.91 mm to 26.56 mm), and eye lens diameter (0.20 mm to 0.93 mm). Ossified femur length was measureable from E15.5 (1.06 mm) and increased linearly to 2.23 mm at E18.5. In contrast, placental diameter (PD) and placental thickness (PT) increased from E10.5 to E14.5 then remained constant to term in accord with placental weight. Ultrasound and light microscopy measurements agreed with no significant bias and a discrepancy of less than 25%. Regression equations predicting gestational age from individual variables, and embryonic weight (BW) from CRL, BPD, and AC were obtained. The prediction equation BW = -0.757 + 0.0453 (CRL) + 0.0334 (AC) derived from CD-1 data predicted embryonic weights at E17.5 in three other strains of mice with a mean discrepancy of less than 16%.

CONCLUSION

Micro-ultrasound provides a feasible tool for in vivo morphometric quantification of embryonic and placental growth parameters in mice and for estimation of embryonic gestational age and/or body weight in utero.

Calibration and measurement issues for therapeutic ultrasound

This review paper examines some of the issues relating to calibration and measurement of therapeutic medical ultrasonic equipment (MUE). This is not intended to be an all-encompassing review of all aspects of characterising therapeutic ultrasound. Instead it concentrates on issues related to the acoustic output of two applications: physiotherapy and high intensity focused ultrasound surgery (HIFUS or HIFU; also referred to as high intensity therapeutic ultrasound, HITU). Physiotherapy has a well-established standards infrastructure for calibration: the requirements are small in number and well-defined. The issue for physiotherapy is not so much 'How to calibrate?' but rather, 'How to ensure that equipment IS calibrated?' The situation in the much newer area of HIFU is very different: the first steps towards writing standards are just starting and even the very basic questions of what to measure and with what type of sensor are open for debate. Readers whose main interest is in other ultrasound therapies will find ideas of relevance to their own specialty.

Safety assurance in obstetrical ultrasound

Safety assurance for diagnostic ultrasound in obstetrics began with a tacit assumption of safety allowed by a federal law enacted in 1976 for then-existing medical ultrasound equipment. The implementation of the 510(k) pre-market-approval process for diagnostic ultrasound resulted in the establishment of guideline upper limits for several examination categories in 1985. The obstetrical category has undergone substantial evolution from initial limits (ie, 46 mW/cm2 spatial peak temporal average [SPTA] intensity) set in 1985. Thermal and mechanical exposure indices, which are displayed onscreen according to an Output Display Standard, were developed for safety assurance with relaxed upper limits. In 1992, with the adoption of the Output Display Standard, the allowable output for obstetrical ultrasound was increased in terms of both the average exposure (eg, to a possible 720 mW/cm2 SPTA intensity) and the peak exposure (via the Mechanical Index). There has been little or no subsequent research with the modern obstetrical ultrasound machines to systematically assess potential risks to the fetus using either relevant animal models of obstetrical exposure or human epidemiology studies. The assurance of safety for obstetrical ultrasound therefore is supported by three ongoing means: (1) review of a substantial but uncoordinated bioeffect research literature; (2) the theoretical evaluation of diagnostic ultrasound exposure in terms of thermal and nonthermal mechanisms for bioeffects; and (3) the skill and knowledge of professional sonographers. At this time, there is no specific reason to suspect that there is any significant health risk to the fetus or mother from exposure to diagnostic ultrasound in obstetrics. This assurance of safety supports the prudent use of diagnostic ultrasound in obstetrics by trained professionals for any medically indicated examination.

A proposal to clarify the relationship between the thermal index and the corresponding risk to the patient

The thermal index (TI) displayed on the screens of most modern diagnostic ultrasound machines is linearly proportional to the absorbed power or, equivalently, to the in-situ intensity or temperature rise. Users are instructed to interpret the TI as a "relative indication of bioeffect risk." The thermal dose is a well-known empirical relationship between the temperature T of a biological system and the time t needed for that temperature to induce a deleterious effect. For any two temperatures, T1 and T2, and the corresponding times t1 and t2, required to produce the same level of effect, this general relation holds: t1/t2=RT2-T1, where R is the thermal normalization constant. Hence, it is experimentally determined that the rate of induction, or risk, of a thermal effect increases exponentially with temperature. Because exponential relationships are not intuitive to many users, there is a significant potential for underestimation of the thermal risk associated with exposure to diagnostic ultrasound. To better quantify this risk and thereby make the displayed information more useful, the current linear display of the calculated value of the thermal index, i.e., of TIcur, should be altered to an exponential form based on the thermal dose and representing the excess risk associated with the exposure: TInew=(RTIcur-1)/(R-1). This expression has the advantage that for the usual choice of R=4 for T<or=43 degrees C, TInew approximately TIcur in the range most often seen onscreen, i.e., TIcur<1.2, minimizing any confusion during a transition from TIcur to TInew. For the relatively rare but potentially much more serious circumstances when TIcur>3.5, the displayed TInew>>TIcur, consistent with empirical observations of the likelihood of harm. Additional advantages also obtain.