Acoustic output as measured by mechanical and thermal indices during routine obstetric ultrasound examinations

Safety of Obstetric Ultrasound: Mechanical and Thermal Indexes-A Systematic Review

Background/Objectives: Obstetric ultrasound is one of the most commonly used imaging modalities during pregnancy to detect any fetal abnormalities. The aim of this systematic review was to appraise all available scientific literature and summarize current evidence regarding the safety of fetal ultrasound by using the thermal index (TI) and mechanical index (MI). Methods: We applied the PRISMA guidelines in order to prepare the review, and a 2-step process was performed in order to evaluate the available literature and decide which studies to be included. A thorough search of the Medline, Scopus, and Google Scholar databases was performed. Randomized and non-randomized studies were considered for review. The MI and TI were available in ultrasound machines after 1993; thus, studies before that year would not provide data on these two indexes. Results: A total of 21 studies were included in this review, including prospective, retrospective, cross-sectional, and survey-type studies. A common theme of the majority of the studies is the increased acoustic output available to the machines with time and the limited awareness of where the MI/TI indexes are from the operators. Conclusions: This review indicates that, while obstetric ultrasound is predominantly safe, there is a need for operators to consistently observe MI/TI indexes and adhere to the ALARA principle to minimize potential risks.

Evaluation of the Scan Duration and Mechanical and Thermal Indices Applied for the Diagnostic Ultrasound Examinations

OBJECTIVE

Although epidemiological studies have failed to show the harmful effects of ultrasound in humans, as a form of energy, ultrasound has the potential to cause bio-effects in tissues. Therefore, clinical guidelines have been established for ultrasound technology related to human safety, which include Thermal (TI) and mechanical (MI) indices. The appropriate TI and MI ranges for embryonic examinations are between 0-1.0 and 0-0.4, respectively. The accepted TI and MI ranges are 0-2.0 and 0-1.9, respectively, for general ultrasound examinations. In addition, the scan duration should be kept as low as possible. Therefore, the present study aimed at evaluating the scan duration, TI, and MI as measures of acoustic output during ultrasound studies.

METHODS

A cross-sectional descriptive study was conducted for patients undergoing pregnancy checkups, routine checkups, and initial diagnosis ultrasound examinations. Samples were selected from imaging departments of 4 educational hospitals based on convenience sampling and 321 checklists completed by direct observation of ultrasound examinations.

RESULTS

For pregnancy scans, the mean TI and MI were obtained as 0.32 ± 0.27 and 1.15 ± 0.13, respectively. For non-pregnancy examinations, the mean value of TI and MI were 0.30 ± 0.29 and 1.07 ± 0.35, respectively. Therefore, mean TI for pregnancy and non-pregnancy examinations and mean MI for non-pregnancy studies obtained lower than the permitted values, while the mean MI was higher than the permitted level for first trimester of pregnancy. Also, relatively suitable scan durations were seen in reviewed studies.

CONCLUSION

From this study, it may be concluded that the reported ultrasound scans were safe.

Obstetric ultrasound: where are we and where are we going?

Diagnostic ultrasound (DUS) is, arguably, the most common technique used in obstetrical practice. From A mode, first described by Ian Donald for gynecology in the late 1950s, to B mode in the 1970s, real-time and gray-scale in the early 1980s, Doppler a little later, sophisticated color Doppler in the 1990s and three dimensional/four-dimensional ultrasound in the 2000s, DUS has not ceased to be closely associated with the practice of obstetrics. The latest innovation is the use of artificial intelligence which will, undoubtedly, take an increasing role in all aspects of our lives, including medicine and, specifically, obstetric ultrasound. In addition, in the future, new visualization methods may be developed, training methods expanded, and workflow and ergonomics improved.

Ultrasound biosafety: Knowledge and opinions of health practitioners who perform obstetric scans in South Africa

BACKGROUND

Diagnostic ultrasound is generally considered as a safe test in pregnancy. To date there is no evidence that ultrasound has caused harm to the developing foetus. However, with the number of obstetric scans on the rise and the steep increase in acoustic output achieved by modern machines, the lack of evidence of absolute safety remains a concern. Acoustic output is under the direct control of the operator and is therefore the operator's responsibility to keep the intensity as low as reasonably achievable. A situation analysis in the South African context was deemed necessary to determine end user knowledge and opinions on safe antenatal ultrasound practice.

AIM

The aim of this quantitative descriptive, cross-sectional study was to evaluate the knowledge and practice of health practitioners who perform antenatal scans regarding safety aspects of diagnostic ultrasound.

SETTING

A self-administered questionnaire was distributed at two national congresses, hosted by the South African Society of Ultrasound and Obstetrics (SASUOG) and South African Society of Obstetricians (SASOG) committees.

METHOD

Quota non-probability sampling allowed for the identification of professional categories capable of providing information relevant to the study objectives. The sample represented a population with experience in obstetric ultrasound.

RESULTS

Compared to international studies, South African end users demonstrated better knowledge of safety indices than their international counterparts. It is, however, discouraging that end users still demonstrate insufficient knowledge regarding factors contributing to adverse biological effects.

CONCLUSION

With room for improvement, an effort should be made to comply with international standards through increased training efforts and raising awareness.

Women's Attitudes Toward Self-Monitoring of Their Pregnancy Using Noninvasive Electronic Devices: Cross-Sectional Multicenter Study

Background

Pregnancy can be distressing, particularly if expectant mothers are worried about the well-being of their fetus. Consequently, the desire for reassurance and frequent fetal monitoring is often pronounced. Smart wearable devices and telemedicine are promising tools that could assist women in self-monitoring their pregnancy at home, hence disburdening emergency departments (EDs). They present the possibility to clarify the need for urgent care remotely and offer tighter pregnancy monitoring. However, patients’ acceptance of such new technologies for fetal monitoring has not yet been explored extensively.

Objective

This survey aimed to elucidate the attitudes of women toward self-monitoring of their pregnancy using noninvasive electronic devices. The technical details of the proposed devices were not specified.

Methods

A cross-sectional multicenter study was conducted at the departments of obstetrics of the University Hospitals of Heidelberg and Leipzig, Germany. All patients seen in the obstetrics clinic who were above 18 years were offered participation. We designed a survey questionnaire including validated instruments covering population characteristics, issues in current and past pregnancies, as well as attitudes toward self-monitoring of pregnancy with smart devices.

Results

A total of 509 pregnant women with no previous experience in telemedicine participated. Only a small minority of 5.9% (29/493) regarded self-monitoring with wearable devices as an alternative to consulting their physicians. Along these lines, only 7.7% (38/496) strongly believed they would visit the ED less often if such devices were readily available. However, if the procedure were combined with a Web-based telemetric physician consult, 13.5% (66/487) would be highly motivated to use the devices. Furthermore, significantly more women regarded it as an alternative prior to seeing a doctor when they perceived a decline in fetal movements (P<.001). Interestingly, women with university degrees had a higher propensity to engage in pregnancy self-monitoring compared with women without one (37% vs 23%; P=.001). Of the participants, 77.9% (381/489) would like smart wearable devices to measure fetal heart sounds, and 62.6% (306/489) wished to use the devices on their own. Feedback from a doctor or midwife was also very important in their choice of such devices (61.8%, 301/487 wished feedback). The intended frequency of use differed vastly among women, ranging from 13.8% (65/471) who would like to use such a device several times per day to 31.6% (149/471) who favored once per week at most.

Conclusions

Our results point to a skeptical attitude toward pregnancy self-monitoring among pregnant women. Nevertheless, many women are open to using devices for pregnancy monitoring in parallel to consulting their physician. The intention to use such devices several times daily or weekly, expressed by more than half of the participants, highlights the potential of such technologies.

Study of the association between the incidences of congenital anomalies and hydrocephalus in Sudanese fetuses

This study was designed with an aim to detect the congenital anomalies appear to be linked to and in conjunction with hydrocephalus fetuses in Sudan, when ultrasound is used to exam fetuses in the second and third trimesters of pregnancy. This prospective cohort study was performed from December 2011 to December 2013, in a group consists of 5000 single gestation pregnant Sudanese women. In all cases, maternal ages were 35 years up to 48 years; mean age of 42.5 years. Pelvic; obstetric ultrasound scanning protocol used should meet the standards established by the American Institute of Ultrasound in Medicine (AIUM) for scanning in the second and third trimesters of pregnancy. Statistical Package for the Social Sciences (SPSS) was used to analyze the results. Diagnosed hydrocephalus cases (0.4%) were found to be associated with other fetal anomalies as aqueduct stenosis (45%), spina bifida (30%), Arnold-Chiari malformation (20%) and Dandy-Walker malformation (5%). The incidence of congenital anomalies and hydrocephalus in Sudanese fetuses showed considerable variation among different regions of Sudan. Hydrocephalus is associated with certain congenital anomalies. In agreement with previous studies, hydrocephalus is predominantly in male rather than female fetuses. The prevalence of fetal anomalies and hydrocephalus are within previously reported ranges.

The effect of ultrasound output level on obstetric biometric measurements

We tested the effect of reducing the ultrasound output energy on obstetric measurements made from two-dimensional (2-D) grayscale images. We measured three fetal structures (femur, umbilical vein and lateral cerebral ventricle) in 113 pregnant women, commencing with an output energy that provided a thermal index for bone (TIB) of ≤1.0, and then repeated the measurements at TIB ≤ 0.5 and then TIB ≤ 0.1. There was a trend toward shorter femur measurements and larger umbilical vein diameter measurements with decreasing output energy. However, since these effects (which did not exceed 0.03 mm) were very small compared with the inter- and intraobserver variations, they were considered to be of no clinical significance. Furthermore, the residuals did not increase when the output energy decreased, indicating that the precision of the measurements was unaltered. We conclude that using a reduced output energy setting corresponding to a TIB of 0.1 does not compromise ultrasound biometry findings.

Handedness in the helsinki ultrasound trial

OBJECTIVES

To determine whether exposure to prenatal ultrasound increases non-right-handedness in boys.

METHODS

The association between exposure to prenatal ultrasound and handedness was tested, using logistic regression analysis, in the Helsinki Ultrasound Trial data. We applied an intention-to-treat approach in this analysis of a subset of 4150 subjects whose parents answered a follow-up questionnaire on handedness when the children were aged 13-15 years.

RESULTS

The odds ratio for non-right-handedness of children who had been exposed to prenatal ultrasound was 1.16 (0.98-1.37) for all subjects, 1.12 (0.89-1.41) for boys and 1.24 (0.97-1.58) for girls.

CONCLUSIONS

We could not confirm the hypothesis that prenatal ultrasound exposure and handedness are associated. Our findings were independent of the particular definition of handedness used, whether it was considered according to the writing hand alone or defined using a laterality quotient.

Prenatal ultrasound exposure and children's school performance at age 15-16: follow-up of a randomized controlled trial

OBJECTIVE

To evaluate the association between prenatal ultrasound exposure and school performance at 15-16 years of age.

METHODS

The study population consisted of children born to women who participated in a randomized controlled trial on the second-trimester ultrasound examination in Sweden from 1985 to 1987. Information about the children's grades when graduating from primary school and information on socioeconomic factors was obtained from Swedish nationwide registers. Comparisons were made using linear and logistic regression analyses according to randomization to ultrasound, ultrasound exposure in the second trimester and ultrasound exposure at any time during pregnancy. Boys and girls were analyzed separately.

RESULTS

Of the 4756 singleton children from the randomized trial, we identified 4458 (94%) in the National School Register. There were no statistically significant differences in school performance for boys or girls according to randomization or exposure to ultrasound in the second trimester. Compared to those who were unexposed, boys exposed to ultrasound at least once at any time during fetal life had a tendency towards lower mean school grades in general (-4.39 points; 95% CI, -9.59 to 0.81 (max possible, 320) points) and in physical education (-0.45 points; 95% CI, -0.91 to 0.01 (max possible, 20) points), but the differences did not reach significance.

CONCLUSION

In general, routine ultrasound examination in the second trimester had no effect on overall school performance in teenagers.

No midterm benefit from low intensity pulsed ultrasound after chevron osteotomy for hallux valgus

Chevron osteotomy is a widely accepted method for correction of symptomatic hallux valgus deformity. Full weight bearing in regular shoes is not recommended before 6 weeks after surgery. Low intensity pulsed ultrasound is known to stimulate bone formation leading to more stable callus and faster bony fusion. We performed a randomized, placebo-controlled, double-blinded study on 44 participants (52 feet) who underwent chevron osteotomy to evaluate the influence of daily transcutaneous low intensity pulsed ultrasound (LIPUS) treatment at the site of osteotomy. Follow-up at 6 weeks and 1 year included plain dorsoplantar radiographs, hallux-metatarsophalangeal-interphalangeal scale and a questionnaire on patient satisfaction. There was no statistical difference in any pre- or postoperative clinical features, patient satisfaction or radiographic measurements (hallux valgus angle, intermetatarsal angle, sesamoid index and metatarsal index) except for the first distal metatarsal articular angle (DMAA). The DMAA showed statistically significant (p = 0.046) relapse in the placebo group upon comparison of intraoperative radiographs after correction and fixation (5.2 degrees) and at the 6-week follow-up (10.6 degrees). Despite potential impact of LIPUS on bone formation, we found no evidence of an influence on outcome 6 weeks and 1 year after chevron osteotomy for correction of hallux valgus deformity.

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.

Fetal thermal effects of diagnostic ultrasound

Processes that can produce a biological effect with some degree of heating (ie, about 1 degrees C above the physiologic temperature) act via a thermal mechanism. Investigations with laboratory animals have documented that pulsed ultrasound can produce elevations of temperature and damage in biological tissues in vivo, particularly in the presence of bone (intracranial temperature elevation). Acoustic outputs used to induce these adverse bioeffects are within the diagnostic range, although exposure times are usually considerably longer than in clinical practice. Conditions present in early pregnancy, such as lack of perfusion, may favor bioeffects. Thermally induced teratogenesis has been shown in many animal studies, as well as several controlled human studies; however, human studies have not shown a causal relationship between diagnostic ultrasound exposure during pregnancy and adverse biological effects to the fetus. All human epidemiologic studies, however, were conducted with commercially available devices predating 1992, that is, with acoustic outputs not exceeding a spatial-peak temporal-average intensity of 94 mW/cm2. Current limits in the United States allow a spatial-peak temporal-average intensity of 720 mW/cm2 for fetal applications. The synergistic effect of a raised body temperature (febrile status) and ultrasound insonation has not been examined in depth. Available evidence, experimental or epidemiologic, is insufficient to conclude that there is a causal relationship between obstetric diagnostic ultrasound exposure and obvious adverse thermal effects to the fetus. However, very subtle effects cannot be ruled out and indicate a need for further research, although research in humans may be extremely difficult to realize.

First-trimester sonography: is the fetus exposed to high levels of acoustic energy?

PURPOSE

As a form of energy, diagnostic ultrasound has bioeffects on living tissues. The thermal index (TI), TIS (TI for soft tissue), TIB (TI for bone), TIC (TI for cranial bone) expresses the potential for rise in temperature at the ultrasound beam's focal point. The mechanical index (MI) indicates the potential for the ultrasound beam to induce inertial cavitation in tissues. The goal of this study was to characterize the acoustic output of clinical ultrasound instruments, as expressed by TI and MI, during routine first-trimester sonographic examinations.

METHODS

A prospective observational study was conducted. First-trimester patients were randomly selected from those scheduled for viability scans. An obstetrician collected data. Sonographers were blinded to the data being sought, which included gestational age, duration of the examination, and every variation in the MI and TI during each sonographic examination.

RESULTS

A total of 52 first-trimester examinations were evaluated. The mean gestational age was 8.9 +/- 1.9 weeks. The mean duration of the sonographic examinations was 8.1+/- 1.4 minutes. During the examinations, there were 178 MI variations (mean +/- SD, 0.9 +/- 0.3) and 167 TI variations (mean +/- SD, 0.2 +/- 0.1).

CONCLUSION

First-trimester sonographic examinations are associated with a negligible rise in TI.

What do clinical users know regarding safety of ultrasound during pregnancy?

OBJECTIVE

The main goal of this study was to determine end users' knowledge regarding safety aspects of diagnostic ultrasound during pregnancy. End users' attitudes toward the use of ultrasound in low-risk pregnancies were also assessed.

METHODS

A questionnaire was distributed to ultrasound end users attending review courses and hospital grand rounds between April and June 2006.

RESULTS

One hundred thirty end users completed the questionnaires (63% response rate). Sixty-three percent were physicians (n=84), most of them obstetricians (81.7%). About 18% of participants routinely performed Doppler ultrasound examinations during the first trimester. Fifty percent of end users thought that the number of ultrasound examinations in low-risk pregnancy should be limited to 1 to 3 (mean +/- SD, 2.6 +/- 0.9). Almost 70% disapprouved of "keepsake/entertainment" ultrasound. Although 32.2% of the participants were familiar with the term thermal index, only 17.7% actually gave the correct answer to the question on the nature of the thermal index. About 22% were familiar with the term mechanical index, but only 3.8% described it properly. Almost 80% of end users did not know where to find the acoustic indices. Only 20.8% were aware that they are displayed on the sonographic monitor during the examinations. End users with higher knowledge of safety issues thought that there should be limitations on the number of ultrasound examinations in low-risk pregnancies (odds ratio, 3.3; 95% confidence interval, 1.1-10.0; P=.028). Likewise, these end users were more likely to respond that ultrasound might have adverse effects during pregnancy (odds ratio, 3.2; 95% confidence interval, 1.1-12.5; P=.045).

CONCLUSIONS

Ultrasound end users are poorly informed regarding safety issues during pregnancy. Further efforts in the realm of education and training are needed to improve end user knowledge about the acoustic output of the machines and safety issues.

A comparison between acoustic output indices in 2D and 3D/4D ultrasound in obstetrics

OBJECTIVE

Three-dimensional (3D) ultrasound is gaining popularity in prenatal diagnosis. While there are no studies regarding the safety of 3D ultrasound, it is now widely performed in non-medical facilities, for non-diagnostic purposes. The present study was aimed at comparing the acoustic output, as expressed by thermal index (TI) and mechanical index (MI), of conventional two-dimensional (2D) and 3D/4D ultrasound during pregnancy.

METHODS

A prospective, observational study was conducted, using three different commercially available machines (iU22, Philips Medical Systems; Prosound Alfa-10, Aloka; and Voluson 730 Expert, General Electric). Patients undergoing additional 3D/4D ultrasound examinations were recruited from those scheduled for fetal anatomy and follow-up exams. Fetuses with anomalies were excluded from the analysis. Data were collected regarding duration of the exam, and each MI and TI during 2D and 3D/4D ultrasound exams.

RESULTS

A total of 40 ultrasound examinations were evaluated. Mean gestational age was 31.1 +/- 5.8 weeks, and mean duration of the exam was 20.1 +/- 9.9 min. Mean TIs during the 3D (0.27 +/- 0.1) and 4D examinations (0.24 +/- 0.1) were comparable with the TI during B-mode scanning (0.28 +/- 0.1, P = 0.343). The MIs during the 3D volume acquisitions were significantly lower than those in the 2D B-mode ultrasound studies (0.89 +/- 0.2 vs. 1.12 +/- 0.1, P = 0.018). The 3D volume acquisitions added 2.0 +/- 1.8 min of actual ultrasound scanning time (i.e. not including data processing and manipulation, or 3D displays, which are all post-processing steps). The 4D added 2.2 +/- 1.2 min.

CONCLUSIONS

Acoustic exposure levels during 3D/4D ultrasound examination, as expressed by TI, are comparable with those of 2D B-mode ultrasound. However, it is very difficult to evaluate the additional scanning time needed to choose an adequate scanning plane and to acquire a diagnostic 3D volume.

An increased thermal index can be achieved when performing Doppler studies in obstetric sonography

OBJECTIVE

The present study was aimed at evaluating acoustic outputs during clinical ultrasound examinations, as expressed by the thermal index (TI) and the mechanical index (MI), during the second half of pregnancy and comparing acoustic outputs between B-mode and Doppler examinations.

METHODS

Patients with suspected fetal growth problems undergoing Doppler studies of the fetal circulation in addition to B-mode sonography were selected. Examinations took place between 21 and 40 weeks' gestation. An obstetrician collected data prospectively. Sonographers were unaware of the data being sought. The analysis of variance test was applied for differences in continuous variables.

RESULTS

A total of 63 examinations were evaluated. The mean gestational age +/- SD was 31.6 +/- 5.1 weeks. The mean duration of the total examinations was 17.6 +/- 8.6 minutes, whereas the Doppler studies lasted 0.9 +/- 0.8 minutes. The TI was significantly higher in the pulsed wave Doppler studies (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) compared with B-mode sonography (mean, 0.3 +/- 0.1; range, 0.1-0.7; P < .01). During the examination, 190 B-mode MI variations were recorded (mean, 1.1 +/- 0.1), which were comparable with those of the 31 color flow Doppler studies (mean, 1.0 +/- 0.1; P = .09) but higher than the 190 pulsed wave Doppler MI variations (mean 0.9 +/- 0.2; P < .001).

CONCLUSIONS

Increased acoustic output levels, as expressed by TI levels, are reached during obstetric Doppler studies. In particular, TI levels may reach 1.5 and higher. Doppler procedures should be performed with caution and be as brief as possible during obstetric sonography.

Medical and non-medical protection standards for ultrasound and infrasound

Protection from inappropriate or hazardous exposure to ultrasound is controlled through international standards and national regulations. IEC standard 60601 part 1 establishes requirements for the mechanical, electrical, chemical and thermal safety for all electro-medical equipment. The associated part 2 standard for diagnostic medical ultrasonic equipment sets no upper limits on ultrasonic exposure. Instead, safety indices are defined that are intended to advise users on the degree of thermal and mechanical hazard. At present the display of these safety indices satisfies regulatory requirements in both the USA and Europe. Nevertheless there are reservations about the effectiveness of this approach to protection management. In the USA, there are national regulatory limits on diagnostic exposure, based on acoustic output from clinical equipment in use over 20 years ago. The IEC 60601 part 2 standard for therapeutic equipment sets 3 W cm(-2) as the limit on acoustic intensity. Transducer surface temperature is controlled for both diagnostic and therapy devices. For airborne ultrasound, interim guidelines on limits of human exposure published by the IRPA are now 2 decades old. A limit on sound pressure level of 100 dB for the general population is recommended. The absence of protection standards for infrasound relates to difficulties in measurement at these low frequencies.