INTRODUCTION

Doppler velocimetry was introduced in obstetrics in 1977, and since then it has been used as a safe non-invasive technique that allows the assessment of maternal, placental and fetal vascular districts and circulation.^1,2 It is used for diagnosis, monitoring and delivery timing of several pathological or high-risk obstetrical conditions such as fetal growth restriction,³ pre-eclampsia,⁴ twin pregnancies,⁵ fetal anemia⁶ and others. Its application has been shown to improve pregnancy outcome.⁷ Therefore, Doppler velocimetry plays a crucial role in everyday obstetrical practice for fetal well-being assessment, and diagnosis and management of obstetrics’ complications.

Considering its widespread availability and applications, it is important to understand the principles of Doppler velocimetry and to perform the examination in coherence with the correct methodology.

Herein, we describe the main Doppler velocimetry principles and the methodology for its execution.

PRINCIPLES OF DOPPLER VELOCIMETRY

In 1842, Christian Doppler first described the Doppler velocimetry and its operating principle.⁸ Doppler observed that there is a wave frequency change when there is a motion between the wave source and the observer. In case of Doppler velocimetry, a series of ultrasound pulses, produced by piezoelectric crystals, are transmitted from the ultrasound probe to detect the blood cells (erythrocytes) within the vessels. In case of a still stationary tissue, the time needed for the ultrasound beam to travel to the tissue and to return to the transducer will be the same (Figure 1a).

In the case of a moving object toward the transducer, the time to return is shorter and the frequency higher (Figure 1b and 2a), vice versa in case of a moving object away from the transducer, the return time is longer and the frequency lower (Figure 1c and 2b). These time differences, or wave frequency differences, can be measured and represent the so-called Doppler shift.⁹

The Doppler shift (Fs) depends on: the blood flow velocity (V); the emitted ultrasound frequency (Fe); the ultrasound velocity in the tissue (c) and the cosine of the angle of insonation (cos q, that is the angle between the direction of the ultrasound beam and the direction of the blood flow which is discussed further in this chapter). The ultrasound velocity in the human body is constant (c = 1540 m/s).

Fs = 2Fe (V*cosq/c)

There is a direct proportional correlation between the Doppler shift and the blood flow velocity. The Doppler shift can be processed by ultrasound machine software and displayed in several ways (color Doppler, spectral Doppler, and others).

CONTINUOUS AND PULSED WAVE

There are different Doppler modalities: continuous and pulsed wave Doppler.

The continuous wave (CW) Doppler continuously transmits and receives sound waves. This allows the recording of much higher velocities that might be useful, for example, in cardiology for cardiac valve stenosis evaluation.⁴ This is possible due to the presence of distinct sets of piezoelectric crystals inside the ultrasound probe dedicated to a separate function, one for the transmission and one for the registration of the reflected ultrasound waves (Figure 3). It is not usually employed in obstetrics, because it is unable to determine the velocity location within the beam and it cannot be used to produce color flow images.

In pulsed wave (PW) Doppler the ultrasound beam is released in pulses (Figure 3); thus, the transducer analyzes the reflected waves between the pulses making it possible to determine the distance between the reflector and the transducer. Contrary to continuous wave Doppler, the same piezoelectric crystal alternates rapidly between sending and analyzing the ultrasound waves. Since the same piezoelectric elements generate and receive sound waves, this limits the maximum velocity that can be measured. Pulsed wave Doppler is largely used in obstetrics, as it can determine the location of the measured velocities by placing the sample volume along the Doppler line. The sample volume can be modified in its size and position.

PULSE REPETITION FREQUENCY

Pulse repetition frequency (PRF) indicates the number of ultrasound pulses emitted per second (frequency). It depends on the speed of sound (constant in the human body) and the distance that it must travel. Thus, a lower PRF (fewer ultrasound pulses per second) is required when the target is more distant (deeper), while the time required for the ultrasound beam to travel back is longer. Thus, lower PRF is needed for the visualization of deeper structures and will provide a lower resolution. However, high PRF (more ultrasound pulses per second) is required when the target is closer to the ultrasound probe, the time required for the ultrasound beam to travel back is shorter and it will provide a higher resolution.

In order to assess accurately the blood flow and the velocity, PRF should be sufficiently high.¹⁰ According to Nyquist-Shannon’s theorem,¹⁷ the ultrasound wave should be sampled at least twice per cycle, implying that the PRF for pulsed wave Doppler should be at least twice the Doppler shift. On the other hand, the Doppler shift depends on the flow velocity, thus, the maximum velocity that can be determined is half the PRF, value that represents the so-called Nyquist limit.

ALIASING

Aliasing is the phenomenon that occurs when the blood velocity exceeds the Nyquist limit, that means that the blood velocity is more than half of the PRF and the system cannot correctly determine the velocity and the direction of the flow (Figure 4). As a result, the blood flow with velocities above the Nyquist limit will be displayed on the opposite side of the baseline compared to the blood flow velocities below the Nyquist limit (Figure 4).

When examining low velocities (i.e., venous flow), low PRF should be used: the longer interval between pulses allows better identification of slow flow. Aliasing will occur when low PRF is used to determine high velocity blood flow: in this case PRF should be increased.¹¹ In some cases, it is sufficient to adjust the baseline in order to resolve the aliasing phenomenon (Figure 5) or to apply the automatic setting button present on many ultrasound machines.

COLOR AND POWER DOPPLER

The component analysis of the reflected velocities can be represented with different color modalities, color and power Doppler.

The color Doppler (CD) is usually applied to identify vessels and the flow direction, and it is superimposed to B-mode gray scale image for easier interpretation. It allows an easy identification of vascular districts or vessels (or their anomalies), identification of regurgitation or stenosis (i.e., in case of cardiac valvular insufficiency or stenosis), identification of the correct angle in case of velocity measurement by pulsed wave Doppler, etc. For a determined color box sample, it displays the average of the velocities and not the maximum velocity. It represents an overall view of the flow within a color box sample, thus characterized by poor temporal resolution, particularly when the region of interest is deep. Conventionally, red color indicates a velocity flow toward the transducer, while the blue color indicates the velocity flow away from the transducer (Figure 6). The brighter the color, the higher the velocity. Turbulent flow indicates the presence of a large variation of velocities inside the color box sample.

Color Doppler is also limited by Nyquist limit and aliasing. Thus, in case of velocities that exceed the Nyquist limit, the blue color will turn into red and vice versa.

Color Doppler increases the total power emitted. The color box size should be reduced as much as possible for a better resolution. Next, PRF should be adjusted to study the blood flow velocity that will be correctly displayed on the monitor. As mentioned above, the gain balancing improves the image setting and prevents artifacts, and the insonation angle should be as close as possible to the longest axis of the vessel, that is as close as possible to 0, as discussed below.

Power Doppler (PD) ultrasound uses the amplitude of the reflected signal, rather than the frequencies, to detect the moving elements. It is determined by the amount of blood cells that are present in the sample volume. The advantage of the power Doppler is that it is more sensitive to lower velocity flow and less dependent on the insonation angle, but, contrary to color Doppler, it cannot display the flow direction or different velocities. It is characterized by a poor temporal resolution. The aliasing phenomenon is not an issue in case of power Doppler ultrasound; gain balancing prevents artifacts.⁴

In addition, there are new hybrid modalities of Doppler imaging display provided by different ultrasound machine manufacturers, that provide a “3D like” image of the examined vessel/circulation, allowing for a more dynamic perception of the laminar flow of the blood and a better definition of the vessel borders (Figure 7).

SPECTRAL PULSED DOPPLER

The blood flow presents a laminar parabolic distribution with the highest velocity at the center of the vessel and the lowest velocities in the correspondence of the vessel walls (Figure 8).

Thus, the velocities within a cross-sectional area of a vessel are different. This is the reason why it is important to cover the whole vessel by the sample volume in order to sample all velocities. Another important characteristic of the human circulation is its pulsatile pattern: peaking during systole and reaching its nadir during diastole. These characteristics will result in different Doppler shifts that can be transformed and represented as spectral Doppler. Thus, the spectral Doppler is a graphical representation of different Doppler shifts in the sample volume over time (cardiac cycles) and it allows for velocity and indices calculations.

Different materno-placental-fetal districts present different patterns of spectral Doppler that can change over gestation and with pathological state.

INDICES OF BLOOD FLOW MEASUREMENT

Several indices of quantitative waveform measurement have been proposed. The most known are systolic/diastolic ratio (S/D), resistance index (RI) and pulsatility index (PI) (Figure 9). Low pulsatility or resistance index indicate low resistance blood flow and vice versa. Pulsatility index is preferable while it includes in the formula the mean velocity of the blood flow.

The waveform indices and the velocities depend on several factors such as the heart rate, the pressure and contraction gradient, the viscosity of the blood, the elasticity of the vessel and the peripheral resistance.

The spectral Doppler can be also evaluated visually and provide qualitatively information such as an absence or inversion of the end-diastolic flow (i.e., important for umbilical artery and ductus venosus evaluation).

High amplitude low frequencies, deriving from tissue and/or vessel movements, can cause background “noise”. To eliminate this artifact, it is possible to use “filter” setting. The setting of the filter frequency can be regulated on the ultrasound machine and will determine the minimum velocities that are displayed. However, in some cases an excessive filter preset can hide important clinical information, such as inversion of the end diastolic flow, or can mimic an absence of end diastolic flow (Figure 10).

ANGLE OF INSONATION

All Doppler evaluations are dependent on the angle of insonation, particularly when assessing absolute velocities, as for example in case of the measurement of peak systolic velocity in middle cerebral artery in the assessment of fetal anemia.^4,6 Ideally the ultrasound beam should be aligned to the blood flow or tissue motion, with the angle of insonation (cos q) equal to 0 (Figure 11). In this case the cos 0° will be 1. For increasing values of angle of insonation, the cos q will be less than 1 and will result in velocity underestimation. For example, an insonation angle of 10° is equal to a 2% error in the velocity estimation, while an angle of 20° corresponds to an error of 6%.⁴ An angle of insonation close to 90° (cos 90° = 0) might result in the absence of color image if color Doppler is performed or unreliable velocity values if spectral Doppler is performed (Figure 11b). For small deviation of the angle of insonation, a post-processing adjustment can be performed. Measurements above 30° should be taken with caution, particularly when used for clinical management based on absolute velocities.

SAFETY OF DOPPLER IN MATERNAL-FETAL MEDICINE

The application of diagnostic ultrasound and Doppler seems to have no adverse effect on the embryo/fetus. However, safe practice should be maintained regarding the power output and use of Doppler ultrasound.

In 1998 the World Federation for Ultrasound in Medicine and Biology (WFUMB) statement on the safety of diagnostic ultrasound was published. In these recommendations, the default power of a thermal index should not exceed 0.7. Indeed, the thermal index represents the potential for tissue heating and it is set by default (although it can be adjusted by the operator) and it can be visualized on the screen.¹² The WFUMB statements are not mandatory, but they are intended to educate and advise, especially during the embryogenesis since routine examination during the organogenesis period is considered not thoroughly studied at present.

The “ALARA” (As Low As Reasonably Achievable) principle assumes that power output should always be used adequately to perform the ultrasound. The sonographer should start with a low power output and increase it only if the examination requires it.¹³

B-mode has the lowest power intensity and output. Spectral Doppler and color Doppler have higher power outputs. Pulsed Doppler techniques usually implicate greater temporal average power and intensities than do B- or M-mode and have a greater heating potential. Thus, an attention should be given especially early in pregnancy.¹⁴

HOW TO OPTIMIZE THE DOPPLER ACQUISITION

The ultrasound and Doppler velocimetry interrogation depends on operators’ appropriate training and experience.¹⁵ Inadequate machine settings may represent a source of pitfalls. Some principles are important for an adequate Doppler ultrasound performance as illustrated in Table 1.

DOPPLER VELOCIMETRY APPLIED TO MATERNAL-FETAL MEDICINE

Doppler velocimetry allows the investigation of many maternal, placental and fetal districts. However, the investigation of clinical relevance is limited to some districts: uterine arteries on the maternal side, umbilical arteries on the placental side and middle cerebral artery and ductus venosus on the fetal side. Other districts have been proposed, but are still used mainly in research setting, such as umbilical vein and other districts.¹⁸ Doppler velocimetry is also used to perform fetal echocardiography. The significance of Doppler velocimetry in obstetrical pathological conditions is discussed in subsequent chapters.

PRACTICE RECOMMENDATIONS