Jacques S. Abramowicz - First-Trimester Ultrasound: A Comprehensive Guide

Department of Obstetrics and Gynecology, Wayne State University School of Medicine, 3990 John R. Street, Detroit, MI 48201, USA

Jacques S. Abramowicz

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Ultrasound is a waveform with a succession of positive and negative pressures [].

Until 1992 acoustic outputs of clinical ultrasound machines had specific limits. For instance, the upper limit of the spatial peak temporal average intensity or I SPTA (the most clinically useful intensity used to determine acoustic power of the ultrasound beam) for adult use was 720 mW/cm2 and for fetal use, 94 mW/cm2, which in fact, already had been increased from a previous maximum value of 46 mW/cm2. It was assumed that higher outputs would generate better images and, thus, improve diagnostic accuracy. Hence, end-users required ultrasound manufacturers to increase their machines output. Some worry, however, was expressed regarding the actual amount of energy absorbed by a human fetus during an ultrasound examination. This amount cannot be measured precisely. Not only the lack of an internal recording device is a major issue but, in addition, elements such as variations in maternal body habitus, fetal position changes, and gestational age progression render such a task impossible. To allow clinical users of ultrasound to use their instruments at higher powers than originally intended and to reflect the two major potential biological consequences of ultrasound (thermal and mechanical), the American Institute of Ultrasound in Medicine (AIUM), the National Electrical Manufacturers Association (NEMA) and the US food and Drug Administration (FDA), with representatives from the Canadian Health Protection Branch, the National Council on Radiation Protection and Measurements (NCRP) and 14 other medical organizations developed a standard related to the potential for ultrasound bioeffects []. The TI calculation is based on the formula: where W is the acoustic power while scanning and W deg is the acoustic power required to achieve an increase in temperature of 1 C under similar conditions []. Furthermore, exposure time is not part of the equation, nor is it in the second index, the MI, which represents the potential for cavitation in tissues, but is not based on actual in situ measurements. The MI is defined as: It is a theoretical formulation of the ratio of the peak rarefaction pressure to the square root of the ultrasound frequency (hence, the higher the frequency, the lesser risk of mechanical effect, which is an advantage in endovaginal scanning). As for the TI, exposure time is not part of the calculation. Both the TI and MI can and should be followed as an indication of change in output during the clinical examination. A major component of the implementation of the ODs was supposed to be education of the end-user. Unfortunately, this aspect of the ODS does not seem to have succeeded as end-users knowledge of bioeffects, safety, and output indices is lacking. Both in Europe [].

Ultrasound has permeated the field of infertility and reproductive endocrinology, from diagnosing uterine anomalies []. They wanted to determine how oocytes obtained under ultrasound guidance affected the pregnancy rate. The results obtained with 3.5-MHz probes suggest that exposure of human oocytes to ultrasonic waves during the different phases of meiosis does not significantly influence the developmental potential of the in vitro fertilized embryos. Unfortunately, no researcher describes any of the relevant exposure parameters discussed earlier, except for ultrasound frequency.

There are many valid medical indications to perform ultrasound in early gestation []. There was a timeeffect relation with activation of an enzyme pathway responsible for apoptosis through a mitochondrial pathway with exposures of 20 and 30 min but not 0 (control group) or 10 min.

The growing fetus is very sensitive to external influences. This is especially true in the first 1012 weeks of gestation []. If one, however, considers together the facts that hyperthermia is potentially harmful to the fetus and that ultrasound may, under certain circumstances elevate tissue temperature, then precaution has to be recommended, particularly in early gestation and especially with modes known to emit higher acoustic energy levels (such as pulsed Doppler).

Ultrasound modalities can result in either scanned or unscanned exposure. Scanned conditions are associated with grey-scale B-mode images (the most commonly used real-time application), and Doppler images of tissue cross sections. Unscanned conditions are used for M-mode and pulsed-Doppler studies of tissue movement (such as cardiac valves) or blood velocity waveforms. This is clinically very important because for unscanned beams the power is limited to the area of the beam cross section, often very narrow (1 mm2) in the focal region. For scanned beams the acoustic power is not limited to a narrow area, but may cover large areas in the lateral direction, hence less risk of high exposure at a specific point. Furthermore, a variety of movements intervene during B-mode imaging, such as fetal body motion, observers hand movements, and maternal breathing. During a Doppler examination, however, it is necessary to have the transducer as steady as possible. This is because, in general, blood vessels or heart valves are small in comparison to the general organ or body size being scanned and even small movements will have more undesired effects on the resulting image. As described below, the most commonly used intensity (spatial peak temporal average intensity, I SPTA) associated with Doppler ultrasound is the highest of all the general-use categories, 1180 mW/cm2 for pulsed Doppler, as opposed to 34 mW/cm2 for B-mode, a 35-fold difference. Dwell time (duration of exposure) is also of major importance: Ziskin [].