Safety assurance in obstetrical ultrasound

Abstract

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.

1

A diagnostic ultrasound image of a first trimester fetus obtained with a modern transvaginal probe. The clarity of such images allow the unambiguous verification of pregnancy and fetal viability. (courtesy GE Healthcare).

2

Oscilloscope traces illustrating the exposure to ultrasound at a point in tissue from a transvaginal probe (E8C, GE Healthcare Logiq 9) operating at 6 MHz. The measurements were made with a hydrophone in a water bath, with a tissue mimicking attenuator between the probe and the hydrophone. The minimum unit of exposure is the pulse (left), which oscillates in pressure at approximately 5.8 MHz for a duration of 240 ns. The peak rarefactional pressure amplitude, a parameter used in calculating the Mechanical Index was 1.3 MPa. The pulses are repeated each 144 μs and form an image pulse sequence (center) at a point as the scanned beam passes by. The imaging mode had three focal zones to improve the apparent depth of focus, and the overall image pulse sequence is made up of three groups displaced slightly in time (one for each focal depth). Finally, the image pulse sequence repeats for each image frame at 9 frames per second (right).

3

A plot of the boundary above which bioeffects, including teratological effects, have been detected in animal models of thermal exposure. The data indicate concern for a 6°C temperature elevation for 1 min declining to 4°C for 16 min, which might be within the range of somespectral Doppler exposures based on the Thermal Index display. Extending the boundary for small elevations above normal body temperature results in very long times (e. g., 4 h 16 min for 39°C) relative to any normal ultrasound examination durations at a fixed scan plane.

4

A plot of the region bounded by the FDA upper limit of MI=1.9 (solid line), and a theoretical estimate of the cavitation threshold in blood for optimal nucleation (dashed curve). Within this region, cavitational bioeffects are presumably possible when suitable nuclei are present. Research results suggest that optimal nuclei are not normally present in mammalian blood, except for contrast enhanced diagnostic ultrasound with stabilized microbubbles. Therefore, the apparent zone of risk between the two curves, which otherwise might be a source of concern, is considered to be immaterial for obstetrical ultrasound.