The difference between PW and CW in ultrasound

Pulsed-wave (PW) and continuous-wave (CW) Doppler each have their own characteristics, introduced as follows:

Pulsed-wave Doppler:

Its emitted and received ultrasound waves are both intermittent pulsed waves, displaying the blood flow velocity, direction, and nature at a certain depth on the sound beam.

Advantages:It has distance resolution capability and can pinpoint the instantaneous blood flow spectrum of a small area (sampling line) within the cardiovascular system. Therefore, it can locate abnormal blood flow and distinguish between normal and abnormal blood flow.

Disadvantages:It is easily affected by the Nyquist frequency. If the flow velocity exceeds the maximum display frequency, spectral aliasing will occur on the spectrum, thus preventing the quantitative measurement of high-velocity blood flow.

Continuous-wave Doppler:

Its emitted and received ultrasound waves are continuous, representing the sum of all blood flow signals on the entire sound beam channel.

Advantages:It has strong velocity resolution, and its spectrum can reflect the velocity of high-velocity blood flow without being affected by the Nyquist frequency.

Disadvantages:It has no range gating capability and lacks distance resolution. The information from various points along the path of the sound beam overlaps, being simultaneously received by the transducer, resulting in an unlocalizable output signal.

Spectrum analysis includes:

(1) Frequency shift phase: That is, systole, diastole, or the entire cardiac cycle. The value on the horizontal axis (X-axis) of the spectrum represents time, with units of seconds.

(2) Frequency shift amplitude: The value on the vertical axis (Y-axis) of the spectrum represents the magnitude of blood flow velocity, with units of kHz or cm/s and m/s. It measures blood flow velocity, including maximum flow velocity, average flow velocity, acceleration, and deceleration; calculates transvalvular pressure difference; measures acceleration, deceleration time, ejection phase, ejection time, and half-time of pressure drop.

(3) Frequency shift direction: Based on the baseline of the spectrum, upward frequency shift is positive (forward), indicating that the blood flow direction is toward the probe; downward frequency shift is negative (reverse), indicating that the blood flow direction is away from the probe.

(4) Spectrum brightness: It is represented by brightness and reflects the relative number of red blood cells with the same flow velocity in the sampling volume or the probing sound beam. The greater the number of red blood cells with the same velocity, the greater the scattering signal intensity, and the brighter the spectrum brightness; conversely, the brightness is weak (dark).

(5) Spectrum dispersion: It is represented by the width of the spectrum in the vertical direction, referring to the range of red blood cell velocity distribution in the sampling volume or probing sound beam at a certain moment. If the range of red blood cell velocity distribution is large, then the spectrum is wide; conversely, the spectrum is narrow. Spectrum width is an important factor in identifying changes in hemodynamics.

Laminar flow displays a narrow spectrum, dense spots, a relatively smooth spectrum envelope, and often a window between the blood flow spectrum and the baseline; turbulent flow displays a wide spectrum, sparse spots, a rough spectrum envelope, and a filled state between the blood flow spectrum and the baseline; in the case of eddy current, due to the multidirectional movement of red blood cells, its characteristic is a bidirectional turbulent spectrum.

The following four abnormalities can be deduced from the above analysis and have clinical significance:

(1) Abnormal blood flow velocity: The measured flow velocity is higher or lower than the normal range.

(2) Abnormal blood flow phase: The duration of blood flow is longer or shorter than the normal range.

(3) Abnormal blood flow nature: The blood flow changes from normal laminar flow to turbulent flow.

(4) Abnormal blood flow pathway: Blood flow appears in channels that should not appear in a normal human heart, such as septal defects, abnormal channels, and reflux.

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