Introduction to Echocardiography, Part 3

Introduction

Doppler and 2D echocardiography are established as valuable noninvasive tools in clinical cardiology, providing hemodynamic information within the heart chambers and valves. Ultrasound uses high-frequency sound waves above the audible range of human hearing (typically 2-18MHz). These waves are generated by a transducer and propagate through the body tissues. Doppler allows us to be able to focus on specific information to give us the velocity flow patterns of a particular area within the heart. These can help evaluate many different pathology and structures, including; valvular function, intracardiac shunts, dysfunction of a native or prosthetic valve, or the obstruction within the heart. Understanding cardiac physiology and hemodynamics is critical to the interpretation of Doppler information. In addition, the sonographer must clearly understand Doppler principles, artifacts, and pitfalls in order to provide the physician with the information needed to diagnose and treat the patient.

The Basics

Sound Waves

Ultrasound imaging relies on the properties of sound waves to create the image we see on the screen. The sound waves produced by the transducer propagate through the body tissues. They travel in a straight line until they encounter a boundary between tissues of different densities. At the tissue boundary, some of the sound waves are reflected back to the transducer, while others continue to travel deeper into the body.
The Doppler effect is the change in the frequency of sound or light waves emitted by a source (transducer) as it moves away from or toward an observer. When the source moves toward the listener, the perceived frequency is higher than the emitted frequency, thus creating a higher-pitched sound. If the sound moves away from the listener, the perceived frequency is lower than the transmitted frequency, and the sound will have a lower pitch, as shown in Figure 1 below.

Figure 1

The ultrasound transducer is the key component of the ultrasound system. It contains piezoelectric crystals that convert electrical energy into sound waves and vice versa. When an electrical current is applied to the transducer, the crystals vibrate and emit sound waves. Conversely, when sound waves strike the crystals, they generate electrical signals

Reflection, Refraction, and Attenuation

When sound waves encounter a tissue boundary, several phenomena occur:

Reflection: Some of the sound waves bounce back (reflect) off the tissue boundary. The reflected waves carry information about the internal structures and are detected by the transducer.
Refraction: Sound waves can change direction (refract) as they pass through tissues of varying densities. Refraction can occur when the waves encounter oblique boundaries.
Attenuation: Sound waves gradually lose their energy as they travel through tissues. This attenuation is caused by the absorption and scattering of the sound waves, leading to a decrease in signal strength with increasing depth.

Echo Formation and Detection

The reflected sound waves, known as echoes, return to the transducer and are converted back into electrical signals. The transducer acts as a receiver, detecting these echoes. The time taken for the sound waves to travel to the tissue boundary and back is used to determine the depth of the reflecting structure.

Image Formation

The electrical signals from the transducer are processed by the ultrasound system. Advanced algorithms analyze the time, amplitude, and frequency characteristics of the echoes to create a two-dimensional (2D) image. These images are displayed on a monitor, providing real-time visualization of the internal structures.

Doppler Effect

In addition to imaging, ultrasound can also be used to assess blood flow. The Doppler effect is utilized to detect changes in the frequency of sound waves reflected from moving blood cells. By analyzing these frequency shifts, information about blood flow velocities and patterns can be obtained.
Ultrasound scanning is non-invasive, radiation-free, and widely used in various medical specialties for diagnostic purposes. It provides valuable insights into the body's internal structures, aiding in the detection and evaluation of diseases and abnormalities.

Doppler Angle

Doppler Angle. The angle that the reflector path makes with the ultrasound beam is called the Doppler angle. As the Doppler angle increases from 0 to 90 degrees, the detected Doppler frequency shift decreases, Figure 2.

Figure 2. Schematic demonstrates the importance of the doppler angle to record accurate flow velocity

At 90 degrees, the Doppler shift is zero, regardless of flow velocity. The frequency of the Doppler shift is proportional to the cosine of the Doppler angle. Therefore, the beam should be parallel to the flow to obtain the maximum velocity. The closer the Doppler angle is to zero, the more accurate the flow velocity. If the angle of the beam to the reflector exceeds 60 degrees, velocities will no longer be accurate.

Spectral Analysis

Blood flow through a vessel may be laminar or turbulent, as seen in Figure 3.

Figure 3. Laminar vs Turbulent blood flow

Laminar flow is the normal pattern of vessel flow, which occurs at different velocities, as flow in the center of the vessel is faster than it is at the edges. When the range of velocities increases significantly, the flow pattern becomes turbulent. The audio of the Doppler signal enables the sonographer to distinguish laminar flow from turbulent flow patterns.

Spectral Doppler

When activating pulsed or continuous waves on the ultrasound machine, the flow coming toward the transducer is displayed above the baseline, and the flow away from the transducer is displayed below the baseline.

Pulsed Wave (PW) Doppler:
- This is a technique used to measure blood flow velocities at specific locations within blood vessels, cardiac chambers, or valves. It provides a graphical representation of blood flow velocity over time.
- PW uses a single transducer that alternates between transmitting and receiving ultrasound waves. The transducer emits short pulses of ultrasound and then listens for the returning echoes. PW Doppler allows for depth resolution and is used for measuring velocities in specific locations.
- The protocol involves the following steps:
  - Probe Placement: The ultrasound probe is placed over the area of interest, typically a cardiac valve or chamber. The probe emits ultrasound waves and receives the echoes produced by red blood cells.
  - Doppler Angle Adjustment: The Doppler angle, which is the angle between the direction of blood flow and the ultrasound beam, needs to be optimized. This is crucial for accurate velocity measurements. The angle should be as close to 0 degrees as possible for precise velocity calculations.
  - Sample Volume Placement: A small region within the vessel or chamber is selected for analysis. This region is known as the sample volume or sample gate. The ultrasound machine measures the velocity of blood cells passing through this sample volume.
  - Velocity Measurement: The spectral Doppler waveform is displayed on the ultrasound monitor, representing the velocity of blood flow over time. The waveform shows the direction and speed of blood flow. Common parameters measured include peak systolic velocity, end-diastolic velocity, and resistive index.
- Continuous Wave (CW) Doppler
  - Utilizes two separate transducers, one for transmitting continuous ultrasound waves and the other for receiving the Doppler-shifted frequencies. CW Doppler is capable of measuring high velocities but lacks depth resolution.
  - This is a technique used to measure peak velocities and gradients through cardiac valves or chambers. The protocol involves the following steps:
  - Probe Placement: The ultrasound probe is placed over the area of interest, typically a cardiac valve or chamber. Select CW on the machine, and the probe emits ultrasound waves and receives the echoes produced by red blood cells.
  - Doppler Angle Adjustment: The Doppler angle, which is the angle between the direction of blood flow and the ultrasound beam, needs to be optimized. This is crucial for accurate velocity measurements. The angle should be as close to 0 degrees as possible for precise velocity calculations.
  - Unlike PW, CW doesn’t have a specific sample volume that can be selected. Instead, it takes samples along the line where the cursor is placed.

In both CW and PW Doppler, the received echoes are analyzed to calculate the frequency shift caused by moving blood cells. This frequency shift corresponds to the velocity and direction of blood flow.

Figure 4

Doppler ultrasound has revolutionized the field of diagnostic imaging by providing valuable information about blood flow and vascular conditions. It is non-invasive, safe, and widely available, making it a valuable tool in various medical specialties.

Color Flow

Color flow imaging provides a visual representation of blood flow direction and velocity using color-coded maps. It helps to assess blood flow patterns, detect abnormalities, and estimate flow volumes. The color flow protocol involves the following steps:
- Probe Placement: Similar to spectral Doppler, the ultrasound probe is placed over the area of interest, such as a blood vessel or cardiac chamber. The probe emits ultrasound waves and receives the echoes from moving blood cells.
- Color Box Placement: A rectangular or sector-shaped color box is positioned over the area of interest. This box contains multiple sample volumes or gates for blood flow analysis.
- Color Mapping: The ultrasound machine assigns colors to different velocities of blood flow. Typically, red is used to represent blood flowing toward the transducer, and blue represents blood flowing away from the transducer. Intermediate colors may be used to indicate different velocities or turbulence.
- Real-Time Visualization: The color flow map is displayed on the ultrasound monitor in real time. The colors overlay the B-mode (grayscale) image, showing the location, direction, and speed of blood flow. Abnormalities such as stenosis, regurgitation, or turbulence can be detected.

Optimizing Cardiac 2D Utrasound Examination

Two-Dimensional (2D) Imaging: In cardiac ultrasound, the 2D imaging mode displays a cross-sectional "slice" of the heart. By scanning from different angles and positions, multiple 2D images are acquired, allowing visualization of the heart's chambers, valves, walls, and other structures.

Frame Rate: the frame rate is the number of 2D images displayed per second. In cardiac ultrasound, a higher frame rate is desirable to capture the fast movements of the heart.
Getting started:

Patient Positioning: Proper patient positioning is essential to obtain optimal imaging. Position the patient in a supine or left lateral decubitus position to minimize interference from the ribs and lung tissue. Ensure the patient is relaxed and comfortable during the procedure.
Transducer Selection: Choose the appropriate transducer for cardiac imaging. Higher-frequency transducers (e.g., 5-7 MHz) are generally used for better resolution in adult patients, while lower-frequency transducers (e.g., 3.5-5 MHz) may be suitable for larger patients. Consider phased array or sector transducers for cardiac imaging.
Gain Adjustment: Adjust the gain settings to optimize image brightness and contrast. Proper gain adjustment enhances tissue differentiation and reduces noise. Start with low gain settings and gradually increase until the desired image quality is achieved, being cautious not to over-amplify or saturate the image.
Depth Setting: Adjust the depth setting to visualize the region of interest adequately. Ensure that the depth is set deep enough to capture the entire heart, including the apex and base, while avoiding excessive depth that can compromise image resolution.
Focus Positioning: Utilize the focus function to position the focal zone at the region of interest. Typically, the focal point is placed in the middle of the structure of interest, such as the heart valves or myocardium, to maximize image clarity in that specific area.
Imaging Views: Obtain images from multiple standard views to comprehensively assess cardiac anatomy and function. Common views include the parasternal long-axis, parasternal short-axis, apical 4-chamber, apical 5-chamber, apical 2-chamber, apical 3-chamber, and subcostal views. Adjust the transducer angulation and position to optimize the visualization of specific structures.
Image Optimization Controls: Familiarize yourself with the image optimization controls on the ultrasound machine. These controls include depth, gain, time gain compensation (TGC), compression, and persistence. Understanding how to adjust these settings can significantly impact image quality.
Image Post-Processing: After acquiring the images, utilize post-processing tools available on the ultrasound system to optimize image quality further or to do more precise measurements. These tools may include image filtering, edge enhancement, speckle reduction, and contrast adjustment.

Optimizing Cardiac Flow Examination

Color Flow Doppler

In Color Flow Doppler, a real-time image is displayed with both grayscale and color flow within an adjustable color sector or box. The sector height can be adjusted without decreasing the frame rate much; however, keep the sector width narrow enough to obtain the ideal frame rate (FR) ≥17 Hz.
The color flow map overlays the 2D image, providing information about the location, direction, and speed of blood flow. Abnormalities such as valvular regurgitation (backward flow) or stenosis (narrowing) can be detected and evaluated.
Once the 2D images are obtained and color flow Doppler is activated, the ultrasound machine assigns colors to the moving blood cells within the heart. Typically, red is used to represent blood flowing toward the transducer, and blue represents blood flowing away from the transducer. Intermediate colors may be used to indicate different velocities or turbulence. The color box is positioned over the area of interest, typically the heart valves. This box contains multiple sample volumes or gates for blood flow analysis. (see Figure 5)

Figure 5. An example of a color map that demonstrates flow towards the transducer as red to yellow and flow away from the transducer as dark blue to light blue.

Aliasing occurs in color flow imaging when Doppler frequencies exceed the Nyquist limit, just as in spectral Doppler. This appears as a wrap-around of the displayed color. The velocity scale (pulse repetition frequency) may be adjusted to avoid aliasing. Color arising from sources other than moving blood is referred to as flash artifact or ghosting.

The figure below shows what can be assessed in each echo scanning window with color Doppler.

Figure 6. Example Protocol of color Doppler images to obtain during an echocardiogram.

Parasternal Long-Axis View for Color Flow Mapping.

Parasternal Short-Axis View for Color Flow Mapping.

Apical View for Color Flow Mapping

Figure 7. Color of TR Color.

Subcostal View for Color Flow Mapping

Subcostal Long-Axis View of the Inferior Vena Cava

Figure 8. The subcostal view of the inferior vena cava shows the blue flow of the blood returning to the heart and the yellow flow into the hepatic veins.

Suprasternal View for Color Flow Mapping

Figure 9. Color flow Doppler of the aortic arch demonstrates the red-yellow color in the ascending aorta, and the blue-yellow color in the descending aorta.

Pulsed Wave Doppler

Pulsed wave Doppler (PW) is a technique used in echocardiography to assess blood flow velocities and patterns within the heart and blood vessels. It provides valuable information about the direction, velocity, and timing of blood flow, helping to evaluate various cardiac conditions. Here's an overview of PW Doppler in echocardiography:

Procedure:

Probe Placement: The ultrasound probe is positioned on the patient's chest or other desired locations to obtain the Doppler signal of interest. The location depends on the specific area or vessel being examined, such as the heart valves, great vessels, or structures.
Doppler Gate Placement: The operator selects the region of interest where blood flow velocity will be measured. This is done by placing a sample volume or "Doppler gate" within the vessel or flow area of interest on the ultrasound display. The gate is a defined area within the vessel where blood flow velocity will be analyzed.
Doppler Signal Acquisition: When the pulsed wave Doppler is activated, the ultrasound machine emits short bursts of ultrasound waves within the selected sample volume or Doppler gate. These waves travel into the tissue and encounter moving red blood cells. The reflected waves are received by the transducer and analyzed to extract the Doppler shift information.
Doppler Spectrum: The received Doppler signals are displayed as a Doppler spectrum, which represents the distribution of blood velocities within the selected sample volume. The spectrum is displayed graphically with velocity along the y-axis and time or distance along the x-axis.
Velocity Measurement: The Doppler spectrum provides information about the velocity and direction of blood flow. Positive velocities represent blood flow toward the transducer (e.g., during systole), while negative velocities represent blood flow away from the transducer (e.g., during diastole). The maximum velocity, peak velocity, and time-velocity integrals can be measured and analyzed.

Clinical Applications: Pulsed wave Doppler has numerous clinical applications in echocardiography, including:

Assessing blood flow across heart valves to evaluate stenosis (narrowing) or regurgitation (leakage).
Evaluating diastolic function by assessing the filling patterns and velocities of blood flow across the mitral valve.
Estimating pressure gradients across stenotic valves using the simplified Bernoulli equation.
Assessing blood flow within the great vessels, such as the aorta and pulmonary artery.
Detecting and characterizing intracardiac shunts or abnormal blood flow patterns.
Evaluating the coronary arteries for flow abnormalities and detecting ischemia.

Pulsed wave Doppler provides valuable hemodynamic information and aids in the diagnosis and management of various cardiovascular conditions. However, it is important to interpret the Doppler findings in conjunction with other echocardiographic measurements and clinical information for a comprehensive assessment of cardiac function.

Continuous wave (CW) Doppler

This is another technique used in echocardiography to assess blood flow velocities within the heart and blood vessels. It is particularly useful for detecting high-velocity blood flow and evaluating conditions such as valvular stenosis or regurgitation. Here's an overview of continuous wave Doppler during an echocardiogram:

Principle: Continuous wave Doppler utilizes the Doppler effect, similar to pulsed wave Doppler. It measures blood flow velocities by continuously transmitting and receiving ultrasound waves along a specific line of sight. Unlike pulsed wave Doppler, continuous wave Doppler does not employ a specific sample volume or gate. Instead, it provides a continuous stream of velocity information throughout the cardiac cycle.

Procedure:

Probe Placement: The ultrasound probe is positioned on the patient's chest or other desired locations to obtain the continuous wave Doppler signal of interest. The location depends on the specific area or valve being examined, such as the aortic valve, mitral valve, or other blood vessels.
Doppler Signal Acquisition: When continuous wave Doppler is activated, the ultrasound machine simultaneously transmits and receives continuous ultrasound waves along the chosen line of sight. This allows for continuous measurement of blood flow velocities over a larger region compared to pulsed wave Doppler.
Doppler Spectrum: The received continuous wave Doppler signals are displayed as a Doppler spectrum, which represents the distribution of blood velocities along the line of sight. The spectrum is displayed graphically with velocity along the y-axis and time or distance along the x-axis.
Velocity Measurement: The Doppler spectrum provides information about the velocity and direction of blood flow. Positive velocities represent blood flow toward the transducer, while negative velocities represent blood flow away from the transducer. The maximum velocity, peak velocity, and other velocity parameters can be measured and analyzed.
Continuous Wave vs. Pulsed Wave Doppler: Continuous wave Doppler differs from pulsed wave Doppler in that it does not provide depth or spatial resolution. It measures velocities continuously along a specific line, allowing for detection of high-velocity flows but without the ability to precisely localize the exact origin of the flow.

Clinical Applications - Continuous wave Doppler has several clinical applications in echocardiography, including:

Assessing the severity of valvular stenosis by measuring the peak and mean pressure gradients across the valve.
Evaluating the presence and severity of valvular regurgitation by assessing the direction and magnitude of blood flow reversal.
Detecting and characterizing abnormal flow patterns, such as intracardiac shunts or abnormal vessel flow.
Assessing blood flow velocities within blood vessels, such as the aorta or pulmonary artery.
The Pedof CW (non-imaging) probe is a pencil-like probe also used and has advantages in obtaining a good Doppler study. Most typically used to assess aortic stenosis from multiple windows.

Because of its small diameter, this allows greater flexibility to angle between rib interspaces or obtain signals from the suprasternal notch or right parasternal border. The audio portion of the Doppler examination becomes a critical factor in this study because there is no 2D image available to guide the transducer location.

Figure 10. The non-imaging Pedof probe with continuous wave Doppler.

Continuous wave Doppler provides important hemodynamic information and aids in the diagnosis and management of various cardiovascular conditions. However, it is important to interpret the Doppler findings in conjunction with other echocardiographic measurements and clinical information for a comprehensive assessment of cardiac function. (figure 11)

Figure 11. Pulsed Wave Doppler uses one crystal that pulses to transmit the signal while also receiving the returning signal, while continuous wave Doppler continuously sends and receives a signal.

With pulsed Doppler, for accurate detection of Doppler frequencies to occur, the Doppler signal must be sampled at least twice for each cycle in the wave. This phenomenon is known as the Nyquist limit. When the Nyquist limit is exceeded, an artifact called aliasing occurs. Aliasing presents on the spectral display as an apparent reversal of flow direction and a “wrapping around” of the Doppler spectral waveform, Figure 12.

Figure 12. High-velocity flow with pulsed Doppler may exceed the Nyquist limit, causing aliasing – the flow appears to wrap around and appear along the lower margin of the image (arrow). B, To avoid this, continuous wave Doppler is used to record high velocity flow.

The table below shows some tips on optimizing CW and PW Doppler.

Parasternal Short-Axis Window for Doppler

Figure 13. Spectral Doppler of TR flow.

Apical 4 Chamber Window for Doppler.

Figure 14. PW of normal mitral valve flow.

Figure 15 PW Medial tissue Doppler

Figure 16 CW tricuspid TR flow

Apical 5 Chamber Window for Doppler.

Figure 17 PW of normal aortic flow.

Figure 18 CW of normal aortic flow

Subcostal Window for Doppler.

Figure 19. Hepatic vein PW Doppler showing systolic flow reversal

Suprasternal Window for Doppler.

Figure 20. Spectral Doppler of the normal flow in the descending aorta.