Pulsed-Wave vs. Continuous-Wave Doppler
1. A 25-year-old woman is admitted in septic shock from a suspected urinary source. After a 30 mL/kg intravenous (IV) fluid bolus, her HR is 110 bpm and BP is 100/55 mm Hg. A point-of-care echocardiogram is performed to measure the velocity-time integral (VTI) at the left ventricular outflow tract (LVOT) to predict fluid responsiveness. Which of the following statements is most accurate with regard to this mode of spectral Doppler (Figure 2.1)?
A. Ultrasound waves are transmitted and received continuously.
B. The sampling rate of the ultrasound transducer is at least half the frequency of blood flow.
C. Range specificity is affected by the physical length of the pulse.
D. The duty factor for this mode is 1.
1. Correct Answer: C. Range specificity is affected by the physical length of the pulse
Rationale: Spectral Doppler echocardiography is used to quantify cardiovascular hemodynamics and blood flow characteristics, including velocities and direction. Spectral Doppler includes pulsed-wave Doppler (PWD) and continuous-wave Doppler (CWD) modes, as well as color flow Doppler, which is a type of PWD. Figure 2.1 shows the PWD tracing of the VTI at the LVOT.
In this mode, ultrasound waves are transmitted intermittently in pulses, and the returning waves are “listened to” during the time between the transmitted pulses (Figure 2.5).
The advantage of PWD is range specificity, that is, it allows calculation of blood velocities from a specific location by isolating and measuring short bursts of reflected frequency signals from that particular location. The ability of PWD to detect acoustic signals from a specific location of interest (range specificity) is affected by the physical length of the pulse (ie, spatial pulse length). A shorter SPL produces better axial resolution (the ability to detect two points as separate), and a longer SPL will lead to loss of range specificity. Among the answer choices, choice C represents the principle of PWD; hence, it is the correct choice.
The sampling rate (frequency) of the ultrasound transducer should be at least twice as fast as the frequency of the signal being measured; hence, choice B is incorrect.
The duty factor is the percentage of time the transducer is actively transmitting ultrasound signals. It is calculated as the pulse duration (PD) divided by the pulse repetition period (PRP). PD is the time from the start of a pulse to the end of that pulse, that is, the time that the pulse is “on.” The time between initiation of transmit events is the PRP. For PWD, the duty factor should be <1; hence, choice D is incorrect. Figure 2.6 depicts pulsed-wave parameters as a function of time and distance.
In contrast to PWD, CWD transmits and receives ultrasound waves continuously. Due to simultaneous transmission and receiving of ultrasound signals, CWD cannot distinguish the origin of the echo, hence it has no range specificity. Figure 2.1 used PWD, hence choice A is incorrect. As CWD is continuously sending and receiving pulses, the duty factor for CWD is 1.
2. Otto CM, Schwaegler RG, Freeman RV. Echocardiography Review Guide. 3rd ed. Chapter 1. Elsevier; 2016.
2. Which of the following is an advantage of continuous-wave Doppler (CWD) over pulsed-wave Doppler (PWD)?
A. Ability to distinguish the location of the acoustic signal
B. Ability to measure high velocities across a stenotic valve
C. Ability to measure low-velocity, high-amplitude signals of the myocardium
D. Ability to measure VTI to calculate stroke volume
2. Correct Answer: B. Ability to measure high velocities across a stenotic valve
Rationale: CWD uses two ultrasound crystals to emit and receive ultrasound signals continuously. This principle allows CWD to detect very high-velocity shifts. However, CWD cannot distinguish the location of the acoustic reflection. Every single velocity along the Doppler line of interrogation is recorded, producing the Doppler envelope. As the blood velocities across a stenotic valve are higher, CWD is employed to measure peak velocities and to calculate pressure gradients.
In contrast, the advantage of PWD is its ability to interrogate the acoustic signal from a specific location. This principle allows the user to place the sample volume at a specific point of interest and measure the frequency shifts from only that location. PWD is used in the measurement of myocardial tissue velocities (tissue Doppler imaging [TDI]) for diastolic function assessment and to measure the blood flow velocity at the LVOT. Tracing the outer edge of the Doppler envelope measured at the LVOT via PWD gives the VTI to calculate the left ventricular stroke volume. Although VTI can often be measured with CWD, it does not confer any advantage for this application.
1. Boyd AC, Schiller NB, Thomas L. Principles of transthoracic echocardiographic evaluation. Nat Rev Cardiol. 2015;12:426-440.
3. Decreasing the wall filter cutoff frequency may be necessary for the evaluation of which of the following velocities?
A. Aortic stenosis jet velocity
B. Pulmonary venous flow
C. Mitral regurgitation
D. Velocity-time integral
3. Correct Answer: B. Pulmonary venous flow
Rationale: Wall filters in spectral Doppler are used to suppress low velocities around the baseline. They eliminate the low-velocity, high-amplitude signals arising from the body tissues (eg, myocardium movement, arterial wall motion) and allow higher frequency signals originating from the blood to pass through. This will improve the signal “clutter.” However, certain situations require adjustment of wall filter settings. Venous flows require the ability to detect low-frequency Doppler shifts; hence, wall filters may need to be set to a lower cutoff frequency to register these changes. Low-frequency Doppler shifts, such as those arising from the pulmonary vein into the left atrium, may be eliminated when the wall filters are set too high. Aortic stenosis, mitral regurgitation, and tricuspid regurgitation jet velocities are typically high-frequency velocities, hence lowering of wall filter cutoff frequency is not needed.
4. The axial resolution of PWD is most affected by which of the following?
A. The spatial pulse length (SPL)
B. The velocity of the ultrasound wave in the medium
C. The ultrasound beam width
D. The pulse repetition period (PRP)
4. Correct Answer: A. The spatial pulse length (SPL)
Rationale: Axial resolution is the ability of the ultrasound wave to distinguish two points as separate in the direction parallel to the ultrasound beam. Wavelength is an important parameter that affects axial resolution. Shorter wavelengths can discriminate two points close to each other as separate in space, hence resulting in better axial resolution. Recall from the pulsed-wave properties diagram (Figure 2.6) that SPL is the physical distance the ultrasound pulse occupies in the medium.
It is the product of wavelength (l) and the number of waves in the pulse. A shorter pulse length will be able to differentiate the structures located at neighboring depths more accurately, hence there is better axial resolution. Axial resolution is equal to half of the SPL because of the roundtrip effect (axial resolution = SPL/2). To understand the roundtrip effect, imagine two structures A and B that are separated by a distance “x.” The acoustic signals reflected from structure A and structure B are separated by twice the actual distance (2x) by which these two structures are separated because a wave traveling between them must travel “there and back.” Because of this, the physical length of the pulse (ie, SPL) can be almost twice as long as the distance between two structures, but still results in two distinct echoes. If the SPL becomes longer than twice the separation distance, the reflecting echoes from both structures cannot be distinguished, resulting in being interpreted as one structure instead of two distinct structures (i.e., loss of axial resolution).
PRP is the time between transmitting a pulse and receiving a pulse (Figure 2.6), hence it is related to the ability to detect changes in time, that is, temporal resolution, not axial resolution.
Ultrasound beam width is related to lateral resolution, that is, the ability to resolve structures located perpendicular to the direction of the ultrasound beam. A narrow beam width gives better lateral resolution.
The velocity of the ultrasound wave (the speed of the sound wave in a medium) is constant for a given medium and does not affect the axial resolution. The propagation velocity is determined by the stiffness and density of the medium. The propagation velocity in soft tissue is assumed to be 1540 m/sec.
1. Edelman SK. Ultrasound Physics and Instrumentation. Chapter 1. ESP Inc; 2002.
5. An 86-year-old man with congestive heart failure and aortic stenosis is admitted to the intensive care unit (ICU) for dyspnea. The following Doppler tracing was recorded in the apical five-chamber view (Figure 2.2).
Which of the following statements is the most accurate with regard to this spectral Doppler mode?