The effect of ultrasound output level on obstetric biometric measurements

Function and Safety of SlowflowHD Ultrasound Doppler in Obstetrics

SlowflowHD is a new ultrasound Doppler imaging technology that allows visualization of flow within small blood vessels. In this mode, a proprietary algorithm differentiates between low-speed flow and signals attributed to tissue motion so that microvessel vasculature can be examined. Our objectives were to describe the low-velocity Doppler mode principles, to assess the bone thermal index (TIb) safety parameter in obstetric ultrasound scans and to evaluate adherence to professional guidelines. To achieve the latter goals, we retrospectively reviewed prospectively collected ultrasound images and video clips from pregnancy ultrasound scans at >10 wk of gestation over 4 mo. We used a custom-built optical character recognition-based software to automatically identify all images and video clips using this technology and extract the TIb. Overall, a total of 185 ultrasound scans performed by three fetal medicine physicians were included, of which 60, 54 and 71 scans were first-, second- and third-trimester scans, respectively. The mean (highest recorded) TIb values were 0.32 (0.70), 0.23 (0.70) and 0.32 (0.60) in the first, second, and third trimesters, respectively. Thermal index values were within recommended values set by the World Federation for Ultrasound in Medicine and Biology American Institute of Ultrasound in Medicine and British Medical Ultrasound Society in all scans.

Safety Aspects of Perinatal Ultrasound

Ultrasound safety is of particular importance in fetal and neonatal scanning. Fetal tissues are vulnerable and often still developing, the scanning depth may be low, and potential biological effects have been insufficiently investigated. On the other hand, the clinical benefit may be considerable. The perinatal period is probably less vulnerable than the first and second trimesters of pregnancy, and ultrasound is often a safer alternative to other diagnostic imaging modalities. Here we present step-by-step procedures for obtaining clinically relevant images while maintaining ultrasound safety. We briefly discuss the current status of the field of ultrasound safety, with special attention to the safety of novel modalities, safety considerations when ultrasound is employed for research and education, and ultrasound of particularly vulnerable tissues, such as the neonatal lung. This CME is prepared by ECMUS, the safety committee of EFSUMB, with contributions from OB/GYN clinicians with a special interest in ultrasound safety.

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.

Measured acoustic intensities for clinical diagnostic ultrasound transducers and correlation with thermal index

OBJECTIVES

To investigate if the thermal index for bone (TIB) displayed on screen is an adequate predictor for the derated spatial-peak temporal-average (I_SPTA.3 ) and spatial-peak pulse-average (I_SPPA.3 ) acoustic intensities in a selection of clinical diagnostic ultrasound machines and transducers.

METHODS

We calibrated five clinical diagnostic ultrasound scanners and 10 transducers, using two-dimensional grayscale, color Doppler and pulsed-wave Doppler, both close to and far from the transducer, with a TIB between 0.1 and 4.0, recording 103 unique measurements. Acoustic measurements were performed in a bespoke three-axis computer-controlled scanning tank, using a 200-μm-diameter calibrated needle hydrophone.

RESULTS

There was significant but poor correlation between the acoustic intensities and the on-screen TIB. At a TIB of 0.1, the I_SPTA.3 range was 0.51-50.49 mW/cm² and the I_SPPA.3 range was 0.01-207.29 W/cm² . At a TIB of 1.1, the I_SPTA.3 range was 19.02-309.44 mW/cm² and the I_SPPA.3 range was 3.87-51.89 W/cm² .

CONCLUSIONS

TIB is a poor predictor for I_SPTA.3 and I_SPPA.3 and for the potential bioeffects of clinical diagnostic ultrasound scanners. Copyright © 2016 ISUOG. Published by John Wiley & Sons Ltd.