CHAPTER 2 • Physical Principles and Bioeffects of First Trimester Ultrasound. First Trimester Ultr

 CHAPTER 2 • Physical Principles and Bioeffects of First Trimester Ultrasound

INTRODUCTION

Recent advances in ultrasound technology along with a growing body of literature

expanded the role of obstetric ultrasound in the first trimester. Currently, first trimester

ultrasound is considered an important element of pregnancy care and is clinically used

to accurately date a pregnancy, assess for risk of aneuploidy, and screen for major fetal

malformations. Understanding the basic physical principles of ultrasound is essential for

knowledge of instrument control and also for the safety and bioeffects of this

technology. In this chapter, we present the basic concepts of the physical principles of

ultrasound, define important terminology, and review its safety and bioeffects,

especially with regard to its use in the first trimester of pregnancy. Following chapters

will present the role of first trimester ultrasound in pregnancy dating and in screening

for fetal malformations.

PHYSICAL CHARACTERISTICS OF SOUND

Sound is a mechanical wave that travels in a medium in a longitudinal and straight-line

fashion by transmitting its energy from one molecule to another. Sound therefore cannot

travel in vacuum as it requires a medium for energy transfer. When sound travels

through a medium, the molecules of that medium are alternately compressed (squeezed)

and rarefied (stretched). It is important to note that the molecules oscillate but do not

move as the sound wave passes through them. Seven acoustic parameters describe the

characteristics of a sound wave and are listed in Table 2.1. In this chapter, we will

briefly discuss the frequency, power, and intensity of sound given their importance to

safety of ultrasound. For more details and a broader discussion on ultrasound physics,

the readers are directed to references on this subject.1–3

The frequency of a sound wave is the number of cycles that occurs in 1 second. The

unit Hertz is 1 cycle per second. Frequency is an important characteristic of sound in

ultrasound imaging as it affects penetration of sound and image quality. In general,

higher ultrasound frequencies provide better image quality at the expense of tissuepenetration. Power and intensity of the ultrasound beam relate to the strength of a sound

wave. Power is the rate of energy transferred through the sound wave and is expressed

in Watts. Power can be altered up or down by a control on the ultrasound machine.

Intensity is the concentration of energy in a sound wave and thus is dependent on the

power and the cross-sectional area of the sound beam. The intensity of a sound beam is

thus calculated by dividing the power of a sound beam (Watts) by its cross-sectional

area (cm2), expressed in units of W per cm2.

The sound source, which is the ultrasound machine and/or the transducer, determines

the frequency, power, and intensity of the sound. The propagation speed of sound in soft

tissue is constant at 1,540 m per second. The propagation speed of sound is fastest in

bone and is slowest in air. This is why the use of medical ultrasound is limited in

anatomic regions involving air, such as the lungs or large bowels.

Sound is classified based upon the ability of the human ear to hear it. Sounds sensed

by young healthy adult human ears are in the range of 20 to 20,000 cycles per second or

Hertz, abbreviated as Hz, and this range is termed the audible sound (range of 20 to

20,000 Hz). If the frequency of a sound is less than 20 Hz, it cannot be heard by humans

and is defined as infrasonic or infrasound. If the frequency of sound is higher than

20,000 Hz or 20 kHz, it cannot be heard by humans and is called ultrasonic or

ultrasound. Typical frequencies used in medical ultrasound are 2 to 10 MHz (mega

[million] Hertz). Ultrasound frequencies that are commonly used in obstetrics and

gynecology are between 3 and 10 MHz.

Table 2.1 • Characteristics of a Sound Wave

Frequency

Period

Amplitude

Power

Intensity

Wavelength

Propagation speedFigure 2.1: Ultrasound image of a fetal head (A) and abdomen (B) at 13

weeks of gestation. Note the hyperechoic bones of the skull and anechoic fluid

(asterisk) within the lateral ventricles (LV). Note that the echogenicity of the

choroid plexuses (CP) is less than bone. In B, the hyperechoic rib and anechoic

fluid within the fetal stomach are seen.

ULTRASOUND WAVES

Ultrasound waves are generated from tiny piezoelectric crystals packed within

ultrasound transducers. When an alternate current is applied to these crystals, they

contract and expand at the same frequency at which the current changes polarity and

generate an ultrasound beam. The ultrasound beam traverses into the body at the same

frequency generated. Conversely, when the ultrasound beam returns to the transducer,

these crystals change in shape and this minor change in shape generates a tiny electric

current that is amplified by the ultrasound machine to generate an ultrasound image on

the monitor. The piezoelectric crystals within the transducer therefore transform electric

energy into mechanical energy (ultrasound) and vice versa. A rubber covering on the

ultrasound transducer protects the crystal and helps to decrease the resistance to sound

transmission (impedance) from the crystals to the body and vice versa. In order to

minimize the impact of air, a watery gel is applied on the skin of the patient to facilitate

transfer of sound to and from the transducer.

THE ULTRASOUND IMAGE

Modern ultrasound equipment create a graded gray scale ultrasound image by sending

multiple sound pulses from the transducer at slightly different directions and analyzing

returning echoes received by the crystals. The details of this process are beyond the

scope of this chapter, but it is important to note that tissues that are strong reflectors of

the ultrasound beam, such as bone or air, will result in a strong electric current

generated by the piezoelectric crystals, which will appear as a hyperechoic image(bright) on the monitor (Fig. 2.1). On the other hand, weak reflectors of ultrasound

beam, such as fluid or soft tissue, will result in a weak current, which will appear as a

hypoechoic or anechoic image (dark) on the monitor (Fig. 2.1). The ultrasound image is

thus created from a sophisticated analysis of returning echoes in a gray scale format.

Given that the ultrasound beam travels in a longitudinal format, in order to get the best

possible image, keep the angle of incidence of the ultrasound beam perpendicular to the

object of interest, as the angle of incidence is equal to the angle of reflection.

ULTRASOUND MODES

B-Mode Ultrasound

B-mode ultrasound, which stands for “Brightness mode,” is also known as twodimensional (2D) imaging and is commonly used to describe any form of gray scale

display of an ultrasound image. The image is created based upon the intensity of the

returning ultrasound beam, which is reflected in a variation of shades of gray that form

the ultrasound image (Fig. 2.2). It is important to note that B-mode is obtained in real

time, an important and fundamental characteristic of ultrasound imaging. B-mode, or

gray scale imaging, is the fundamental imaging modality for ultrasound in the first

trimester and as discussed later in this chapter, it carries the least amount of energy.

Figure 2.2: Variations in gray scale in a 2D transvaginal ultrasound image in

the first trimester. Note the hyperechoic bones of the fetal skull, hypoechoic

tissue of the uterus, and anechoic amniotic fluid (AF). The placenta is seen on

the posterior uterine wall and is slightly more echogenic than the uterine wall.

The intensity of the returning beam determines echogenicity.

M-Mode Ultrasound

M-mode ultrasound, which stands for “Motion mode,” is a display that is frequently

used in early gestation to assess the motion of the fetal cardiac chambers and valves inorder to document cardiac activity. The M-mode originates from a single beam

penetrating the body with a high pulse repetition frequency. The display on the monitor

shows the time of the M-mode display on the x-axis and the depth on the y-axis (Fig.

2.3).

Spectral (Pulsed) Doppler

Spectral (pulsed) Doppler modes are ultrasound displays that are dependent on the

Doppler principle (effect). The Doppler principle describes the apparent variation in

frequency of a sound wave as the source of the wave approaches or moves away,

relative to an observer. This apparent change in frequency, or what is termed the

frequency shift, is proportional to the speed of movement of the sound emitting or

reflecting object(s), such as red blood cells within a vessel. This frequency shift is

displayed in a graphic form as a time-dependent plot. In this display, the vertical axis

represents the frequency shift and the horizontal axis represents the temporal change of

this frequency shift as it relays to the events of the cardiac cycle (Fig. 2.4). This

frequency shift is highest during systole, when the blood flow is fastest and lowest

during end diastole, when the blood flow is slowest in the peripheral circulation (Fig.

2.4). Given that the velocity of flow in a particular vascular bed is inversely

proportional to the downstream impedance to flow, the frequency shift therefore derives

information on the downstream impedance to flow of the vascular bed under study. The

frequency shift is also dependent on the cosine of the angle that the ultrasound beam

makes with the targeted blood vessel (see formula in Fig. 2.4). Given that the insonation

angle (angle of incidence) is difficult to measure in clinical practice, indices that rely on

ratios of frequency shifts were developed to quantitate Doppler waveforms.

In spectral Doppler mode, quantitative assessment of vascular flow can be obtained

at any point within a blood vessel by placing a sample volume or the gate within the

vessel (Fig. 2.4). The operator controls the velocity scale, wall filter, and the angle of

incidence. Flow toward the transducer is displayed above the baseline and flow away

from the transducer is displayed below the baseline. In spectral Doppler mode, only one

crystal is typically necessary and it alternates between sending and receiving ultrasound

pulses.Figure 2.3: M-mode ultrasound of the fetal heart in a fetus at 12 weeks of

gestation. Note that the M-mode line intersects the heart and the cardiac

activity is displayed on the M-mode spectrum. This represents the preferred

method (along with saving a movie clip in B-mode) for documentation of cardiac

activity in the first trimester, as it is associated with less energy than spectral

Doppler. Note that the fetal heart rate is measured at 157 beats per minute.Figure 2.4: Spectral Doppler velocimetry of the maternal uterine artery in early

gestation. “S” corresponds to the frequency shift during peak systole and “D”

corresponds to the frequency shift during end diastole. The Doppler effect

formula is also shown in white background with fc corresponding to the

ultrasound frequency, fd corresponding to the frequency shift, V is the velocity

of flow, cosθ represents cosine of the angle of incidence, and c is a constant

related to the milieu that the ultrasound beam is traversing. Spectral Doppler of

the uterine arteries are not associated with added risk to the embryo/fetus as

the sample gate is placed on the uterine vessels outside of the gestational sac.

Color Doppler

Color Doppler mode or Color flow mode is a mode that is superimposed on the realtime B-mode image. This mode is used to detect the presence of vascular flow within

the tissue being insonated (Fig. 2.5). By convention, if the flow is toward the transducer

it is colored red and if the flow is away from the transducer it is colored blue. Low

velocity scales and filters are reserved for low impedance vascular beds such as

placental flow (Fig. 2.5) and high velocity scales and filters are reserved for high

impedance circulation such as intracardiac flow (Fig. 2.6). In order to optimize the

display of color Doppler, the angle of insonation should be as parallel to the direction

of blood flow as possible. If the angle of insonation approaches 90 degrees, no color

flow will be displayed given that the “Doppler effect” is dependent on the cosine of theangle of insonation, and cosine of 90 degrees is equal to zero (Fig. 2.7). Characteristics

and optimization of color Doppler in the first trimester are discussed in detail in

Chapter 3.

Figure 2.5: Color Doppler mode of the cord insertion into an anterior placenta

in a pregnancy at 12 weeks of gestation. Note that blood in the umbilical vein

(UV) is colored blue (away from the placenta) and blood in the umbilical

arteries (UA) is colored red (toward the placenta).Figure 2.6: Color Doppler mode of the four-chamber view of the fetal heart at

14 weeks of gestation. Blood flow in the fetal heart has high velocity and thus is

detected on a high velocity scale (here at 33 cm per second). LV, left ventricle;

RV, right ventricle.

Power or High Definition Doppler Mode

Power or high definition Doppler mode is a sensitive mode of Doppler that is available

on some high-end ultrasound equipment and is helpful in cardiac imaging in the first

trimester (Fig. 2.8). The strength (amplitude) of the reflected signal is primarily

processed. Power Doppler mode is less affected by the angle of insonation than the

traditional color or spectral Doppler.Figure 2.7: Blood flow in an umbilical cord at 13 weeks of gestation showing

the Doppler effect. Yellow arrows show the direction of blood flow in the

umbilical arteries. Note the absence of blood flow on color Doppler (asterisk)

where the ultrasound beam (white arrow) images the cord with an angle of

insonation equal to 90 degrees (cosine of 90 degrees = 0). The circle shows

area of blood flow with an angle of insonation almost parallel to the ultrasound

beam and thus displays the brightest color corresponding to the highest

velocities.Figure 2.8: High definition color Doppler ultrasound of a parasagittal view of

the fetal chest and abdomen at 13 weeks of gestation demonstrating the

inferior vena cava (IVC) and superior vena cava (SVC) entering the right atrium

(RA). High definition color Doppler or power color Doppler allows for a clear

display of fetal vasculature in the first trimester. See text for details.

BIOEFFECTS AND SAFETY OF ULTRASOUND

Ultrasound is a form of energy and its output varies based upon the mode applied. As

the ultrasound wave traverses through tissue, the absorption of energy results in heat

dissipation, referred to as the thermal effect of ultrasound. The passage of the ultrasound

waveform through tissue also produces a direct mechanical effect from the succession

of positive and negative pressures. The thermal and mechanical effects of ultrasound are

reflected in two important indices for measurement of bioeffects of ultrasound: the

thermal index (TI) and the mechanical index (MI). The MI gives an estimation of the

cavitation effect of ultrasound, which results from the interaction of sound waves with

microscopic, stabilized gas bubbles in the tissues. The TI is a predictor of maximum

temperature increase under clinically relevant conditions and is defined as the ratio of

the power used over the power required to produce a temperature rise of 1°C. The TI is

reported in three forms—thermal index soft (TIS) tissue assumes that sound is traveling

in soft tissue and is primarily useful in the first trimester; thermal index bone (TIB)

assumes that sound is at or near bone, useful in late second and third trimester; thermal

index cranial (TIC) assumes that the cranial bone is in the sound beam’s near field, used

for examination in adult patients. Other energy effects of ultrasound include physical

(shock wave) and chemical (release of free radicals) effects on tissue.

In obstetrical scanning, the thermal effect (TI) of ultrasound is of more concern than

the mechanical effect (MI). Hyperthermia has been shown to have a teratogenic effect on

the developing embryo in various species.4,5 As the thermal effect results in an increase

in temperature in the insonated tissue, caution should be undertaken to limit embryo and

fetal exposure to the minimal time that is needed for diagnostic purposes and the benefit

to the patient must always outweigh the risk. A general threshold of 1.5°C above normal

physiologic levels is suggested as a safe threshold for diagnostic imaging.6

In 1992, the output display standard (ODS) was mandated for all diagnostic

ultrasound devices. In this ODS, the manufacturers are required to display in real time

the TI and the MI on the ultrasound screen with the intent of making the user aware of

the bioeffects of ultrasound examination (Fig. 2.9). The user has to be aware of the

power output and make sure that reasonable levels are maintained. Despite the lack of

epidemiologic studies of confirmed harmful bioeffects from exposure to diagnostic

ultrasound, the potential benefit and risk of the ultrasound examination should be

assessed and the principle of ALARA (as low as reasonably achievable) should be

always followed, particularly when adjusting controls of the ultrasound equipment inorder to minimize the risk. This implies that the ultrasound power should be kept as low

as possible and the time of ultrasound exposure as short as possible within the scope of

the clinical ultrasound examination. Always keep track of the TI and MI values on the

ultrasound screen, and keep the TI below 1 and MI below 1 for obstetrical ultrasound

imaging.

Bioeffects and safety of ultrasound is an important topic, especially as it relates to

the developing embryo and fetus in early gestation. Guiding principles on this topic

suggest that the benefit of ultrasound should always be weighed against its risk when

ultrasound is performed in early gestation. Acoustic outputs of B-mode and M-mode are

generally not high enough to produce deleterious effects. Their use therefore appears to

be safe, for all stages of pregnancy.7 Pulsed Doppler however focuses the ultrasound

beam’s energy on a small anatomic target and thus it should not be used routinely in the

first trimester.8 Its use in the first trimester should be limited to clinical situations with

clear pregnancy benefit. When performing Doppler ultrasound, the displayed TI should

be ≤1.0 and exposure time should be kept as short as possible (usually no longer than 5

to 10 minutes) and should not exceed 60 minutes.8 The use of 3D and 4D ultrasound is

associated in general with low TI, comparable to that of B-mode, and is considered as

safe as B-mode for obstetrical scanning.9

Figure 2.9: An ultrasound examination of the four-chamber view at 13 weeks of

gestation in color Doppler. Note the display of MI and TIs in the red circle.

Mechanical index (MI) and thermal index soft (TIS) tissue. TIS is useful in the1.

2.

3.

4.

5.

6.

first trimester given the absence of ossified bony structures. See text for

details.

When attempting to obtain fetal heart rate with a diagnostic ultrasound system, the

American Institute of Ultrasound in Medicine (AIUM) recommends using M-mode at

first, because the time-averaged acoustic intensity delivered to the fetus is lower with

M-mode than with spectral Doppler.10 If this is unsuccessful, spectral Doppler

ultrasound may be used with the following guidelines: use spectral Doppler only briefly

(e.g., 4 to 5 heart beats) and keep the TI (TIS for soft tissues in the first trimester) as

low as possible, preferably below 1 in accordance with the ALARA principle. It is

important to note however that documentation of cardiac activity in early gestation can

also be achieved by saving a movie clip in B-mode.

No independently confirmed adverse effects caused by exposure from present

diagnostic ultrasound instruments have been reported in human patients in the absence of

contrast agents.11 Biological effects (such as localized pulmonary bleeding) have been

reported in mammalian systems at diagnostically relevant exposures,12 but the clinical

significance of such effects is not yet known.

National and international ultrasound societies have developed official statements

that relate to the use of medical ultrasound in obstetrics.7,8,10–16 It is important to note

that official societal statements tend to be updated from time to time and the reader

should consult with the society’s website for the most recent versions. Ultrasound

examinations should be used by qualified health professionals to provide medical

benefit to the patient. Ultrasound exposures during examinations should always be as

low as reasonably achievable (ALARA).16 Knowledge of the bioeffects of ultrasound,

the ALARA principle, and the output display standard is required learning for

healthcare workers involved in ultrasound imaging