The Doppler Effect in Ultrasound

The Doppler effect describes a phenomenon where there is a change in the frequency of a wave due to the relative motion between the source of the wave and the observer. This effect has been can be harnessed to measure blood flow velocity at various locations in the heart.

The Doppler Effect

When a sound wave is reflected off a moving object, its frequency changes. The frequency increases when the object is moving towards the source of the sound wave, and it decreases when the object is moving away from the source of the sound wave. This change in frequency is known as the Doppler shift. In ultrasound, the Doppler shift in frequency can be used to estimate blood flow velocity.

The signal source is the ultrasound probe (on the TOE) which remains in a fixed position and shifts of moving objects; red blood cells or myocardial tissue (either increased or decreased) are then detected by the probe and translated into blood flow velocity on the screen.

Understanding the doppler effect beyond that which has been outlined above likely will yield little in terms of improved clinical relevance. There is an equation which describes how the amount of doppler shift can be calculated using the following variables:

Transmit frequency (set by the probe)
Flow velocity of the scatterer i.e. the red blood cell or section of myocardial tissue
Speed of sound in tissues - this remains fixed at 1540m/sec
Angle of insonation - this creates error if beam alignment is poor. The user should try to align the doppler beam to be parallel to the direction of blood flow as much as possible. In practical terms a 6% error is seen at angles less than 20 degrees, beyond this the error increases significantly.

The TOE machine uses lower transmit frequency for doppler measurement than those used for imaging (towards the lower 25% of the frequency range) which provides the advantage of being able to “listen” to deeper and fainter echoes from red blood cells.

Continuous Wave Doppler

How Continuous Wave Doppler Works

Continuous wave doppler uses two crystals in the transducer to send and receive sound waves, the crystals must be separate as there is the beam is transmitted and received “continuously”. One crystal emits a continuous wave of sound, while the other crystal receives the sound waves that bounce back from the moving blood cells. The difference in frequency between the emitted and received sound waves is used to calculate the velocity of blood flow (using the doppler effect).

The CWD beam measures velocities along the entire beam length resulting in a large variation in velocities received and this creates the characteristic “filled in” appearance of the CWD waveform.

This TTE image shows a continuous wave doppler beam directed through the LVOT and out into the aorta, through the aortic valve. It is not possible to say where the fastest measured velocity of 4.22m/sec is located along the CWD beam length but is most likely to be at the aortic valve. The “filled in” appearance of the CWD trace differentiates it from PWD which has a dark centre.

Main indications

CWD can be used for measuring velocity of blood flow anywhere in the heart. It is particularly suited to measuring high velocities in excess of >1m/sec making it ideal for quantifying the peak and mean gradients for aortic stenosis. CWD can also be used to assess for mitral stenosis, quantifying the EROA in mitral regurgitation and to quantify the severity of tricuspid regurgitation when estimating PA systolic pressure.

Limitations

Continuous wave doppler cannot provide information about the location along the beam’s length where the measured velocity is originating; this makes CWD “location non-specific”. Additionally, CWD is unable to provide information about the direction of blood flow aside from broadly identifying whether flow is away or towards the probe (above or below the baseline).

Pulsed Wave Doppler Ultrasound

How Pulsed Wave Doppler Works

Pulsed wave doppler uses the same crystal to both transmit and receive information which requires an “off” or “listening” period followed by a burst of transmitted ultrasound waves. The length of the listening period relates to the depth of the “sample volume” being interrogated. Return signal velocities are used to create a waveform that represents the velocity of blood flow in a specific location making PWD “location specific”. T

Main Indications

PWD is commonly used to assess blood flow velocity at a specific location. The PWD trace is shown as a thin line of velocities within a narrow band when flow is laminar (this is less the case if flow is turbulent through a restriction, in this instance”spectral widening” of the PWD trace is seen).

Utility with TOE includes measuring mitral inflow velocities to quantify the extent of diastolic dysfunction and flow velocity in the LVOT to quantify the “LVOT VTI”. LVOT VTI can then be used to quantify left-sided cardiac output and the aortic valve area (AVA) using the continuity equation.

Limitations

PWD is unable to measure high velocities accurately due to the Nyquist limit. This limit is dependent on the pulse repetition frequency (PRF) of the ultrasound waves, which limits the maximum velocity that can be measured. This PRF in turn relates to the depth being interrogated by the PWD “volume box” but overall only velocities at the lower end can be interrogated before aliasing occurs. In reality this usually translates to 1-2m/sec being the maximum that can be accurately measured at depths of 10cm or so.

This TTE image shows PWD with the “gate” (two lines at right angles to the doppler beam) centred just below the leaflet tips of the mitral valve. The PWD above the baseline shows the velocity profile of blood entering the LV from the LA with both the E and A waves clearly visible. The characteristic “black centre” of the PWD trace can also be appreciated (in contrast to the CWD trace above)

Colour Flow Doppler

Colour flow doppler (CFD) uses the Doppler effect to produce a colour-coded image of blood flow within the heart. This technique allows for the visualization of both the direction and velocity of blood flow, making it a valuable tool for diagnosis.

How Colour Flow Doppler Works

Frequency shift data which is returned to the ultrasound probe is used to calculate the velocity and direction of blood flow. The resulting data is overlaid onto a grey-scale image of the heart, creating a colour-coded image within a “colour box” which can be altered in size and shape by the user. Red colour indicates blood flowing towards the transducer, while the blue colour indicates blood flowing away from the transducer. The acronym “BART” is often used for reference: “blue away, red towards”.

This TOE image shows a 2-chamber view of the LA and LV with a large colour box placed across the mitral annulus. Colour flow can be seen “regurgitating” back into the LA with evidence of “aliasing” of the colour flow suggesting turbulent flow at velocities above that which are able to be reliably measured according to the set Niquist limit (set at +40 - 40)

Main Indications

CFD is used frequently during TOE examination to visually quantify blood movement within the heart, especially when the movement is pathological. Conduits or “connections” such as VSD’s, ASD’s and other vascular anomalies can often not be seen in the 2D greyscale image but “applying colour” by targeting the colour box over the area of interest may well show movement of blood.

If it proves challenging to align the CWD beam to a given blood flow direction then CFD can be used simultaneously (at the expense of image quality and frame rate) to try and visually show where exactly the blood flow is going.

Limitations

One limitation is that it is highly dependent on the angle of the ultrasound beam. If the beam is not aligned properly with the direction of blood flow, the resulting image may not accurately reflect the true velocity or direction of blood flow. Additionally, colour flow doppler does not provide information about the actual volume of blood flow, making it difficult to quantify the severity of certain conditions.

Another limitation of colour flow doppler is that it can be affected by interference from other structures within the heart. For example, if there is a large amount of calcification or scar tissue within the heart, it may be difficult to obtain a clear image of blood flow.

Tissue Doppler Imaging

How Tissue Doppler Imaging Works

The TDI technique uses the Doppler effect to measure the velocity of tissue movement in the heart, which allows for the assessment of both the systolic and diastolic function of the heart muscle.

TDI is unique in that it can measure the velocity of tissue movement in any direction, including towards and away from the transducer. This makes it possible to assess the longitudinal and radial motion of the heart muscle.

Main Indications

TDI is commonly used to diagnose and monitor myocardial dysfunction and forms the cornerstone of quantifying whether diastolic dysfunction is present or not.

TDI is also useful in assessing the function of the right ventricle (RV), which is often overlooked in routine echocardiography. The RV is responsible for pumping blood to the lungs, and its dysfunction can lead to various pulmonary diseases.

Limitations

Like other imaging techniques, TDI has its limitations. One limitation of TDI is that it is operator dependent, meaning that the quality of the image and the accuracy of the measurement depend on the skill and experience of the operator.

Another limitation of TDI is that it is affected by the angle of the ultrasound beam. When the beam is perpendicular to the tissue being imaged, the velocity measurement is accurate. However, when the beam is at an angle measurement accuracy decreases.