Bioeffects and Safety of Ultrasound in Obstetrics

Instrument Outputs

Over the years, output of ultrasound instruments has increased. Furthermore, many machine controls can alter the output, and various machines can behave differently when manipulating similar controls. For example, keeping in mind that the degree of temperature elevation is proportional to the product of the amplitude of the sound wave times the pulse length and the pulse repetition frequency, it becomes immediately evident why any change (augmentation) in these characteristics can add to the risk of elevating the temperature, a potential mechanism for bioeffects. Three important parameters under end-user control are the (1) scanning/operating mode (including transducer choice), (2) system setup and output control, and (3) dwell time.

System Setup

Starting or default output power is another important ultrasound parameter. Some manufacturers “boot” their machines with high power, which supposedly produces a better image, and the sonographer must act to decrease that power. Other systems boot up with low power and, only if judged necessary, the sonographer will increase that power. In, For example, the Doppler signal in Fig. 29.1A was obtained with a high power, as reflected by an elevated TI of 2.9, whereas in Fig. 29.1B the power was greatly reduced (TI = 0.6), and the image is still diagnostic. Also, the examiner fine-tunes to optimize the image, influencing output but with no visible effect, except to change thermal index (TI) and mechanical index (MI), as discussed in Chapter 2 .

Controls that regulate output include focal depth, usually with greatest power at deeper focus ( Fig. 29.2 ) but occasionally with highest power in the near field; increasing frame rate; and limiting the field of view, as by high-resolution magnification or certain zooms. In Doppler mode, changing sample volume and velocity range (to optimize received signals) will change output. In Fig. 29.3 , only the size of the color box is smaller, which caused increased TI on the output. It should be remembered that receiver gain often has similar effects as these controls on the recorded image (postprocessing), but no effect on the output of the outgoing beam (preprocessing), and therefore it is completely safe to manipulate. In Fig. 29.4A , the output power is high. In Fig. 29.4B , the output power has been greatly reduced (as reflected by a very low TI) and the image is much less diagnostic. In Fig. 29.4C , the receiver gain has been greatly increased, resulting in an image that looks identical to that in Fig. 29.4A but there were no changes in acoustic power, as reflected by unchanged TI and MI or power percentage. In other words, the receiver gain should be maximized before output is increased, which will rarely, if ever, be necessary.

Thermal Effects

That diagnostic ultrasound can induce thermal changes in various animals has been demonstrated. In addition, hyperthermia has clearly been shown to be teratologic to many species. Elevated maternal temperature, whether from illness or exposure to heat, can produce teratogenic effects. A rise less than 2°C is thought to be safe, although any temperature increase for any amount of time may have some effect, and a rise of 2.5°C may be considered significant. A major question is whether diagnostic ultrasound can induce a rise in temperature in the fetus that could reach dangerous levels. Temperature elevation in the human fetus cannot be exactly measured but can be estimated fairly accurately. For prolonged ultrasound exposures, temperature elevations of up to 5°C have been obtained. Thus any temperature increment for any period of time has some effect; the higher the temperature differential or the longer the temperature increment, the greater is the likelihood of producing an effect. Although these assumptions cannot be demonstrated in diagnostic ultrasound and no human data exist, clinicians should keep these facts in mind when performing obstetric ultrasound. This also forms part of the argument against nonmedical or nonindicated ultrasound examinations.

As with any external influence on the pregnancy, gestational age is a vital factor. Milder (in time or intensity) exposures during the preimplantation period (very early gestational age) could have similar or worse consequences than more severe exposures during embryonic and fetal development and could result in fetal demise and abortion or structural and functional defects. Such a dose analysis is not available. As for many other teratogens, the central nervous system (CNS) is most at risk because of a lack of compensatory growth by undamaged neuroblasts. In experimental animals the most common defects associated with temperature increase are of the neural tube (anencephaly, microencephaly) and the eyes (microphthalmia, cataract). Associated with CNS defects are functional and behavioral problems. Other organ defects secondary to hyperthermia include defects of craniofacial development (e.g., clefts) and anomalies of the axial and appendicular skeleton, the body wall, teeth, and heart.

Gestational age is critical when considering heat dispersion. In midterm, there was no significant difference when guinea pig fetal brains were exposed, alive (perfused) or postmortem (nonperfused), in the focal region of the ultrasound beam. However, a significant cooling effect of vascular perfusion was observed when the fetuses reached the stage of late gestation near term, when the cerebral vessels were well developed. In early human pregnancy, less than 6 weeks, the minimal fetal perfusion may reduce heat dispersion. The increased sensitivity of Doppler devices suggests evidence of blood flow within embryonic vesicles after heart formation, with the simultaneous development of a uterine circulatory pathway in the developing placenta. The flow is often referred to as “nonpulsatile” or “percolating” with near-minimal Doppler-measured velocities, as opposed to later in pregnancy. At about week 12 of gestation, the plugs of the spiral arteries are “loosened” and allow for freer blood circulation.

Thus perfusion status is far from approaching that for normal tissue levels (as assumed in the TI algorithm) for much of the first trimester. Only later, when “free circulation” is established (about week 11 or 12 of gestation), does the tissue become normally perfused, when the embryonic circulation actually links up with the maternal circulation. This absence of perfusion may result in underestimation of the actual ultrasound-induced temperature in early gestation. This warrants extreme caution in first-trimester scanning, particularly with the recent increase in utilization of Doppler in the first trimester.

Also, the issue of transducer heating may be particularly relevant in the first trimester, if performing endovaginal scanning. A mitigating factor is motion (even very small) of the examiner's hand, as well as the patient's breathing and body movements (in obstetric ultrasound, both mother and fetus), which tend to spread the region being heated. However, for spectral (pulsed) Doppler studies, it is necessary to have the transducer as steady as possible. Because the intensity and acoustic power associated with Doppler ultrasound are the highest of all the general-use categories, time spent scanning with Doppler ultrasound mode is crucial. Ziskin reported that average duration of 15,973 Doppler ultrasound examinations was 27 minutes (longest, 4 hours!). It is clear that temperature increases of 1°C are easily reached in routine scanning. Elevation of up to 1.5°C were obtained in the first trimester and up to 4°C in the second and third trimesters, particularly with the use of pulsed Doppler. In many clinical machines, TI values of 5 or 6 can be obtained in Doppler mode.