To locate the spatial peak within the acoustic field, the ultrasound probes were covered with a Civ-flex latex ultrasound probe cover (Civco, Kalona, Iowa, USA) with a thin layer of acoustic transmission gel (Ecogel, Eco-Med Pharmaceuticals Inc., Ontario, Canada) on the acoustic propagation face, and immersed in an established and validated bespoke three-axis ultrasound calibration tank 24. The scanners and transducers were assigned identification numbers randomly: numbers 1–5 for scanners and 1–10 for transducers. Ten different ultrasound transducers were evaluated, each on their corresponding scanner: four transvaginal transducers: 3D9-3 (Epiq7G), C10-3V (Epiq7G), E8C (Vivid 7) and E8C RS (E6) four curvilinear transducers: C5-1 (iU22), 3.5C (Vivid 7), C1-5 (E6) and 4C (S6) a matrix transducer: X6-1 (iU22) and a sector transducer: M4S sector (Vivid 7). METHODSįive different clinical diagnostic ultrasound scanners from two manufactures were evaluated: a Philips iU22 (Philips Healthcare), a Philips Epiq7G (Philips Healthcare, Cleveland, OH, USA), a GE Vivid 7 (GE Vingmed, Horten, Norway), a GE Voluson E6 (GE Medical Systems, Zipf, Austria) and a GE Voluson S6 (GE Medical Systems, Gyeonggi-do, South Korea). In this study, we investigated whether TIB can be considered indicative of the acoustic intensities, thereby allowing the examiner to evaluate the risk of ultrasound bioeffects. Our previous study revealed a considerable range in acoustic intensities for a given value of TIB for one transducer 23. The benefit of TI has been questioned recently 22, prompting speculation as to whether TIB might be a sufficient predictor of ultrasound intensity. The British Medical Ultrasound Society recommends monitoring the thermal index for bone (TIB) when scanning a fetus > 10 weeks' gestational age 21. There are three variations of the TI, allowing for differences between tissues being scanned. Guidelines recommend that TI ‘be kept to the lowest levels consistent with obtaining diagnostic information’ 20. With intensities not available during scanning, the examiner must rely on just MI and TI to evaluate the risk. These indices were to be displayed on the screen during examinations, with the operator required to monitor them to maintain safety 18, 19. Consequently, the on-screen display standard was introduced: the thermal index (TI), designed to indicate the risk of tissue heating, and the mechanical index (MI), designed to indicate the risk of inducing cavitation. However, the safety of exposing human fetuses to ultrasound at I SPTA.3 720 mW/cm 2 remains unknown. In 1991, the United States Food and Drug Administration (FDA) increased the maximum permitted I SPTA.3 from ≤ 94 to ≤ 720 mW/cm 2, while the I SPPA.3 remained at ≤ 190 W/cm 3, 16, 17. The resulting derated spatial-peak temporal-average intensity (I SPTA.3) is the intensity parameter most commonly used, although some studies on bioeffects use derated spatial-peak pulse-average intensity (I SPPA.3) 15. Ultrasound intensity is measured in water, at the point of maximum intensity (spatial peak), averaged over time (temporal average) and derated by 0.3 dB/MHz/cm to estimate the ‘ in-situ’ intensity in tissues. More pronounced bioeffects in animals have been demonstrated at higher intensities 3, 12- 14. Low-intensity ultrasound can affect human tissue in vivo 6, and recent studies have found bioeffects from clinical scanners used on chicken and rat fetuses 7- 11. Although several studies indicate no harmful effects 1- 3, some studies report potential bioeffects on human fetuses 4, 5. Ultrasound has never been, and will never be, an unconditionally safe modality for use in modern medicine. 3 and for the potential bioeffects of clinical diagnostic ultrasound scanners. 3 range was 0.51–50.49 mW/cm 2 and the I SPPA. There was significant but poor correlation between the acoustic intensities and the on-screen TIB. Acoustic measurements were performed in a bespoke three-axis computer-controlled scanning tank, using a 200- μm-diameter calibrated needle hydrophone. 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. 3) acoustic intensities in a selection of clinical diagnostic ultrasound machines and transducers. 3) and spatial-peak pulse-average (I SPPA. 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.
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