Sound consists of waves and is described by frequency, wavelength, amplitude, intensity, and the propagation of speed. An ultrasound is a sound with a frequency higher than the human ear can detect. The frequency of ultrasound used in medicine for fetal imaging is in the range of 2 to 12 million cycles per second (megahertz, MHz). Such high-frequency ultrasound is generated by high mechanical deformation of certain materials (eg, crystals or ceramics) caused by electrical stimulation, which produces a generation of waves at ultrasound frequencies. This is described as the piezoelectric phenomenon, and it also works in reverse.² An ultrasound “receptor” will resonate at certain frequencies, creating electrical impulses when stimulated by reflected ultrasonic waves (echoes).

Materials amenable to the piezoelectric phenomenon make up the core of the ultrasound transducer. These crystals are arrayed at the tip of the ultrasound probe. An ultrasound wave is generated by the electric pulse, transmitted through the tissue, and at some tissue depth is reflected and returned to the transducer. Returned “echoes” are detected and converted by the same transducer into electric impulses of equivalent amplitude that correspond to the depth of the returned ultrasonic wave. The array of the electric impulses is analyzed by the computer software and converted into the image. The arrangement of the crystals and the shape of the transducer alter the image obtained. Depending on the mode of data analysis, we are able to demonstrate tissue structures by: B-mode, two-dimensional real-time sonography; M-mode (eg, used for assessment of heart motion); a pulsed Doppler modality that pictures blood flow in the form of waveforms, which correspond to the systolic and diastolic components of the cardiac cycle; color and power Doppler modes (superimposed blood flow in the form of colored dots/areas over the B-mode picture); and, more recently, three-dimensional sonography that renders analyzed structures in a static three-dimensional image, or
four-dimensional ultrasound, which demonstrates a three-dimensional image in real time (Table 12.1). All of these advances were possible because of the tremendous advancements in computer technology and software systems that enable quick and accurate analysis of the received ultrasound data.²

It is imperative to understand that ultrasound images are generated from an ultrasound beam that is three-dimensional in form. The three dimensions are thickness (azimuthal resolution), width (lateral resolution), and depth (axial resolution).

Any transducer that generates an ultrasound beam is capable of focusing that beam at certain depths via an electromagnetic lens. Generated ultrasound beams are unevenly thick, with the narrowest part at the level of their focus. If the beam is thick, the reflected echoes from the same plane at a certain depth will be unified in one two-dimensional image that may appear blurry. It is especially true for the images that are closest and farthest away from the probe where the ultrasound beam is the thickest. At the same time, the image is most clear at the focus level where the beam is narrowest.

Axial resolution, or parallel to the direction of the sound waves leaving the transducer, is related to the length of the ultrasound pulse. Shorter ultrasound wavelengths or higher frequencies will produce better axial resolution.

In contrast, lateral resolution distinguishes structures that are perpendicular to the acoustic wave. Lateral resolution is equal to the diameter of the ultrasound beam and is significantly affected in curved transducers where the beam diameter increases with depth and becomes equivalent in thickness to the azimuthal resolution. Thus, the image that is away from the probe appears not only blurry but also distorted sometimes. Increasing frequency and a smaller beam diameter may improve lateral resolution.

Although the image quality directly depends on the frequency of the ultrasound probe, resolution has been significantly improved by an increase in the number of transducer crystals (or channels), improvements in transducer crystal technology (creating broadband and high-dynamic-range images), increased array aperture (more crystals firing in a single time frame), faster computational capabilities (faster computer chipsets), improved technical algorithms for focusing on received ultrasound beam (increasing the number of focal zones along the beam), incorporating automatic time-gain controls, and progressively replacing analog portions of the signal path to digital.

The signal path of the beam former (transducer) in the older analog processing data chain ultrasound machines was analyzed based on the axial resolution formed by the use of one focus or multifoci. With the employment of more powerful computers, the whole process became digitized. Super-fast digital beam formers allowed significantly increased numbers of focal points (microfine foci) along the beam to the size of a screen pixel. This technology reduced signal:noise ratio in data processing by several-hundred-fold and created a significantly clearer picture. The most recent advent in use is the so-called harmonic imaging (Figure 12.1). Tissue harmonic imaging utilizes lower frequency echoes
for the ultrasound penetration that receives and processes only the higher frequency echoes generated by the body’s inherent characteristics. The final product is a dramatically cleaner contrast between adjacent tissue structures that is particularly useful in patients with obesity.

The diagnostic ultrasound has widespread acceptance due to its clinical utility, convenience, and noninvasiveness. In the United States, approximately 65% of pregnant women have at least one ultrasound examination.³ We usually reassure any prospective mother that ultrasound is safe and does not have any harmful effects on the baby; therefore, it is of paramount importance to be familiar with ultrasound safety.⁴ Some evidence exists that high-energy ultrasound may produce biological effects in exposed tissues. The most studied effects are the local increase in temperature (thermal changes) and oscillatory and potentially catastrophic motions of bubbles, if present, in the tissues (microcavitation).⁵

The nature of ultrasound is such that, during its propagation through the tissue, portions of its energy are absorbed and converted into heat. Although the heat is dissipated by the adjacent tissues and blood flow through the insonated area, tissue temperature may rise a fraction of a degree Celsius.⁶ Such temperature aberrations normally occur during the human diurnal cycle, and temperature may increase by 3° to 4°C in febrile states. Hyperthermia is a proven teratogenic agent in various animals (mouse, rat, hamster, monkey, sheep, and others) and is considered so in humans. In addition, certain stages of embryonic and fetal development may be more susceptible to thermal effects.⁷ Effects appear to be a threshold phenomenon where temperature increases of 1.5°C or higher are considered necessary for damage to occur. However, the energy output of the diagnostic ultrasound is of such low intensity that it is unlikely to induce temperature changes of such a degree to produce adverse pregnancy effects.⁸ In addition, no recently published study has demonstrated unequivocal adverse effects of diagnostic ultrasound. However, it is a theoretical possibility and should not be completely ignored.

The interaction of sound with microscopic gas bubbles that preexist in tissues may cause a bioeffect termed microcavitation or acoustic cavitation.⁵ Because of the succession of positive and negative pressures that can cause oscillatory motions of bubbles, stable cavitation or implosion of the bubbles, described as transient cavitation, may result. These can result in cell membrane disruption and even in the release of free radicals that are cytotoxic. Another potential effect is radiation stress, caused by acoustic streaming in liquid media secondary to the pressure gradient generated by the moving sound wave. These biological effects have been produced in plants, insects, and some mammalian tissues. Although there is no direct evidence to suggest that in humans, under clinical conditions, ultrasound-induced microcavitation produces biological effects, the U.S. Food and Drug Administration (FDA), together with the American Institute of Ultrasound in Medicine (AIUM), the American College of Obstetricians and Gynecologists (ACOG), and the National Electrical Manufacturers Association, introduced a method of displaying ultrasonic output that would control and minimize possible bioeffects in insonated fetal tissues.^6,8,9,10,11 If an ultrasound machine exceeds predetermined limits for output, either a thermal index or mechanical index must be displayed on the screen. If the thermal index, which is appropriate for Doppler applications, exceeds 1.0, there is a potential for the tissue temperature to rise. If the mechanical index, which is appropriate for scale imaging, exceeds 1.0, there is a potential for cavitational
effects.^6,9 It is important to note that although the more recent epidemiologic studies were published in 1998 through 2002, ultrasound examinations consisted exclusively of B-mode, and all machines used predated 1992, ie, the “new” FDA regulations, allowing output to rise to more than 94 mW/cm², the then-accepted upper limit for fetal application. Those acoustic outputs can be considered “low” by today’s standards. Still, available published evidence showed no difference in the prevalence of delayed speech or motor development; impaired neurological development, growth, vision, or hearing; low birthweight; dyslexia; or childhood cancer among children exposed to ultrasound in utero.^12,13,14 The only well-designed study showing some effect was a 2013 article that presented a small increase in the frequency of non-right-handedness (ambiguity) in male infants of mothers exposed to diagnostic ultrasound.¹⁵ Nevertheless, in general, it is safe to say that when sonography is performed for a valid medical indication by a well-trained individual who respects the basic rules of time and exposure, the information that can be obtained is of such great value that it clearly overshadows the remote risks that may exist.^{16,17,18,19,20}