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

Acoustic Power Measurement and Thermal Bioeffect Evaluation of Therapeutic Langevin Transducers

We recently proposed an analytical design method of Langevin transducers for therapeutic ultrasound treatment by conducting parametric study to estimate the effect of compression force on resonance characteristics. In this study, experimental investigations were further performed under various electrical conditions to observe the acoustic power of the fully equipped transducer and to assess its heat-related bioeffect. Thermal index (TI) tests were carried out to examine temperature rise and thermal damage induced by the acoustic energy in fatty porcine tissue. Acoustic power emission, TI values, temperature characteristics, and depth/size of thermal ablation were measured as a function of transducer’s driving voltage. By exciting the transducer with 300 V_pp sinusoidal continuous waveform, for instance, the average power was 23.1 W and its corresponding TI was 4.1, less than the 6 specified by the Food and Drug Administration (FDA) guideline. The maximum temperature and the depth of the affected site were 74.5 °C and 19 mm, respectively. It is shown that thermal ablation is likely to be more affected by steep heat surge for a short duration rather than by slow temperature rise over time. Hence, the results demonstrate the capability of our ultrasonic transducer intended for therapeutic procedures by safely interrogating soft tissue and yet delivering enough energy to thermally stimulate the tissue in depth.

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.

Do current Philips ultrasound systems exceed the recommended safety limits during routine prenatal ultrasounds?

Background

Biological safety of ultrasound is a complex and nuanced subject that is poorly understood by ultrasound users. Little is known about the acoustic output and thermal index levels during the routine use of modern ultrasound machines in prenatal scanning.

Methodology

This study was a retrospective review of thermal index (TI) values encountered during 300 prenatal ultrasound examinations (100 in each trimester) performed on any one of 13 Philips Epiq 7 or Epiq 5 systems, representing approximately 106.5 h of real-time scanning. The TI levels were compared to three international guidelines on the biological safety of ultrasound.

Results

The routine use of current Philips systems was associated with low TI levels. Of the 300 examinations reviewed, virtually all were compliant with the BMUS and Nelson safety guideline. Whether the examination was compliant with the WFUMB guideline is open to interpretation. The highest level of TI encountered was 1.1. In no instance did the TI level incur into the 'not recommended' range or into a range where specific user action was required to reduce the TI within 1 min. The most frequent action associated with TI > 0.7 was the use of M-mode to document the fetal heart rate. In the four instances where TI peaked at 1.1, 3 were associated with the use of M-mode and one with B-mode. Spectral Doppler was not implicated in high TI levels. These results are surprising and open up a range of opportunities for future study.

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.

Acoustic output measured by thermal and mechanical indices during fetal echocardiography at the time of the first trimester scan

We measured acoustic output, expressed as the thermal index (TI) and mechanical index (MI), during fetal echocardiography at the time of the first trimester scan. TI and MI were retrieved from the saved displays during gray-mode, high-definition color flow Doppler and pulsed-wave Doppler (tricuspid flow) ultrasound examinations of the fetal heart and from the ductus venosus assessment. A total of 399 fetal cardiac examinations were evaluated. There was a significant increase in TI values from B-mode studies (0.07 ± 0.04 [mean ± SD]) to color flow mapping (0.2 ± 0.0) and pulsed-wave Doppler studies (0.36 ± 0.05). The TI from ductus venosus assessment (0.1 ± 0.01) was significantly lower than those from Doppler examinations of the heart. MI values from B-mode scans (0.65 ± 0.12) and color flow mapping (0.71 ± 0.11) were comparable, although different, and both values were higher than those from pulsed-wave Doppler tricuspid evaluation (0.39 ± 0.03). There were no differences in MI values from power Doppler assessment between the tricuspid flow and ductus venosus. Safety indices were remarkably stable and were largely constant, especially for color Doppler (TI), tricuspid flow (MI) and ductus venosus assessment (TI, MI). We acquired satisfactory Doppler images and/or signals at acoustic levels that were lower than the actual recommendations and never reached a TI of 0.5.

Do clinical practitioners seeking credentialing for nuchal translucency measurement demonstrate compliance with biosafety recommendations? Experience of the Nuchal Translucency Quality Review Program

OBJECTIVES

The purpose of this study was to evaluate compliance with the ALARA (as low as reasonably achievable) principle by practitioners seeking credentialing for nuchal translucency (NT) measurement between 11 and 14 weeks' gestation.

METHODS

Nuchal Translucency Quality Review Program credentialing requires quantitative scoring of 5 NT measurements from 5 different fetuses. Images submitted by 100 consecutive practitioners were retrospectively evaluated for the output display standard (ODS). The thermal index (TI) type (bone [TIb] or soft tissue [TIs]) and numeric value of the index were recorded. The TIb was considered the correct index for this study. Compliance with the numeric value was evaluated in several ways. Collectively, a TIb lower than 0.5 was considered optimal, lower than 0.7 compliant, and 1.0 or lower satisfactory.

RESULTS

An ODS was present in at least 1 image submitted by 77 practitioners. The TIb was used exclusively by 15 (19.5%), the TIs by 37 (48.1%), and 25 used a combination of the TIb and TIs. Only 4 of 77 providers (5%) used the correct TI type (TIb) at lower than 0.5 for all submitted images, 5 of 77 (6%) at lower than 0.7, and 9 of 77 (12%) at 1.0 or lower. A TI (TIb or TIs) higher than 1.0 was used by 15 of 77 providers (19.5%). Proficiency in NT measurement and educational background (physician or sonographer) did not influence compliance with ALARA.

CONCLUSIONS

Clinicians seeking credentialing in NT do not demonstrate compliance with the recommended use of the TIb in monitoring acoustic output.

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.

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 is safe... right?: resident and maternal-fetal medicine fellow knowledge regarding obstetric ultrasound safety

OBJECTIVES

This study was created to assess the knowledge levels of postgraduate year 4 obstetrics and gynecology residents and maternal-fetal medicine fellows in the United States regarding the safety of obstetric ultrasound and the use of the output display standard.

METHODS

An electronic survey was submitted to each Accreditation Council for Graduate Medical Education-accredited obstetrics and gynecology residency and each maternal-fetal medicine fellowship program in the United States over 2 academic years from 2008 to 2010.

RESULTS

A total of 165 surveys were completed (67 by postgraduate year 4 obstetrics and gynecology residents and 92 by maternal-fetal medicine fellows). In total, 13.4% of residents and 20.9% of maternal-fetal medicine fellows knew how to find or use the output display standard, and 10.9% of residents and 22.7% of fellows reported use of the output display standard during their ultrasound examinations. Overall, 37% to 46% of residents and fellows reported no limitations to the use of obstetric ultrasound and 22% to 39% reported no limitations to the use of Doppler ultrasound in the first, second, and third trimesters. Maternal-fetal medicine fellow knowledge of ultrasound safety generally improved with each year of training; however, only 34.8% of third-year fellows reported use of the output display standard.

CONCLUSIONS

Currently, obstetrics and gynecology resident knowledge of obstetric ultrasound safety is low. Maternal-fetal medicine fellow knowledge is stronger overall; however, few are using the output display standard routinely in their last year of fellowship training. This study provides evidence of the need for improved education on the subject of obstetric ultrasound safety.

In-utero evaluation of the fetal umbilical-portal venous system: two- and three-dimensional ultrasonic study

OBJECTIVES

To describe the normal anatomy of the fetal umbilical-portal venous system (UPVS) and to assess possible anatomical variants of the main portal vein (MPV) insertion into the portal sinus (PS).

METHODS

This was a prospective cross-sectional study of low-risk patients between 14 and 36 weeks of gestation. Two- (2D) and three-dimensional (3D) ultrasound techniques combined with color and high-definition flow Doppler were used to evaluate the fetal UPVS. The standard transverse plane of the fetal upper abdomen, used for measuring the abdominal circumference, was taken in all cases as the point of reference. A longitudinal section was taken to identify the normal course of the umbilical vein and ductus venosus (DV). We performed offline analysis of all gray-scale and color Doppler 2D and 3D volume datasets.

RESULTS

Two hundred and eight fetuses were included in the study. The umbilical vein was observed to course in a cephalad direction from its entry point into the fetal abdomen, joining the L-shaped PS, a confluence of vessels that is the main segment of the left portal vein (LPV). Three branches emerge from the LPV: two to the left, the inferior and superior branches, and one to the right, the medial branch. The main LPV then courses abruptly to the right. Following the emergence of the DV, the communication of the MPV with the LPV marks the point at which the vessel becomes the right portal vein (RPV), giving rise to its anterior and posterior branches. We were able to define three main variants of connection between the MPV and the PS. In 140 (67.3%) fetuses the MPV was connected to the LPV in an end-to-side T-shaped anastomosis, in 26 (12.5%) fetuses the MPV connected with a side-to-side X-shaped anastomosis and in 30 (14.4%) fetuses the two vessels ran in parallel with a short communicating segment, in an H-shaped anastomosis. In the remaining 12 (5.7%) cases classification into one of these three groups was not possible due to intermediate morphology.

CONCLUSIONS

Knowing the normal anatomy of the UPVS and being aware of the possible variants of the connection between the MPV and the PS is a fundamental requirement for accurate prenatal diagnosis of the anomalies of the fetal UPVS.

Temperature measurement by thermal strain imaging with diagnostic power ultrasound, with potential for thermal index determination

Over the years, there has been a substantial increase in acoustic exposure in diagnostic ultrasound as new imaging modalities with higher intensities and frame rates have been introduced; and more electronic components have been packed into the probe head, so that there is a tendency for it to become hotter. With respect to potential thermal effects, including those which may be hazardous occurring during ultrasound scanning, there is a correspondingly growing need for in vivo techniques to guide the operator as to the actual temperature rise occurring in the examined tissues. Therefore, an in vivo temperature estimator would be of considerable practical value. The commonly-used method of tissue thermal index (TI) measurement with a hydrophone in water could underestimate the actual value of TI (in one report by as much as 2.9 times). To obtain meaningful results, it is necessary to map the temperature elevation in 2-D (or 3-D) space. We present methodology, results and validation of a 2-D spatial and temporal thermal strain ultrasound temperature estimation technique in phantoms, and its apparently novel application in tracking the evolution of heat deposition at diagnostic exposure levels. The same ultrasound probe is used for both transmission and reception. The displacement and thermal strain estimation methods are similar to those used in high-intensity focused ultrasound thermal monitoring. The use of radiofrequency signals permits the application of cross correlation as a similarity measurement for tracking feature displacement. The displacement is used to calculate the thermal strain directly related to the temperature rise. Good agreement was observed between the temperature rise and the ultrasound power and scan duration. Thermal strain up to 1.4% was observed during 4000-s scan. Based on the results obtained for the temperature range studied in this work, the technique demonstrates potential for applicability in phantom (and possibly in vivo tissue) temperature measurement for the determination of TI.

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.