Doppler Ultrasonography Explained
Doppler ultrasonography is used to quantify the direction and velocity of a moving subject. In veterinary medicine this is most commonly applied to blood flow.
Doppler ultrasonography utilises the Doppler Effect, which is named after Christian Doppler who described the phenomenon in 1842. The effect is the change in frequency of wave (in the case of ultrasound a sound wave) following the interaction of the wave with a moving object. We experience the effect in day-to-day life; a passing ambulance’s siren or car’s horn will change in pitch as the moving object (in this example, a vehicle) emitting the noise travels past us.
When performing ultrasound examinations, the ultrasound wave reflected from a stationary object returns to the transducer at the same frequency it was transmitted at. However, if an ultrasound wave is reflected from an object that is moving, in accordance with the Doppler effect, the frequency of the returning echo will be altered. This change in frequency is known as Doppler Shift and can be detected by the ultrasound machine and displayed as colour pixels or in graphical format.
Using the Doppler effect can provide additional information when performing abdominal ultrasound or echocardiography. As the ultrasonographic appearance of fluid or blood is anechoic or ‘black’, it is difficult to visualise any movement using B-Mode ultrasound. With the use of Doppler, you can effectively visualise blood flow and determine both the direction and velocity.
Doppler ultrasonography is most commonly used in echocardiographic examinations. By mapping the velocity and direction of blood flow, areas of abnormal or turbulent blood flow relating to cardiac changes can be visualised. Additionally, quantifying the velocity of blood within heart chambers gives information about blood pressure changes via the modified Bernoulli equation.
In abdominal ultrasonography, Doppler can be used to document the presence, absence and/or pattern of blood flow to abdominal organs and areas of pathology. This can be useful to avoid damaging local vascular structures when performing tissue sampling via a fine needle aspirate (FNA) or ultrasound guided biopsy. The velocity of blood flow can also provide information regarding pressure changes in organs such as the kidneys or in cases where abnormalities of vascular flow are suspected such as portosystemic shunts.
Types of Doppler System
Pulsed Wave Doppler (PW)
In Pulsed Wave Doppler the ultrasound transducer alternates between emitting a single pulse of ultrasound and waiting to receive the returning echo. Detection of Doppler shift is only performed within a specific area on the ultrasound beam’s path. This is set via the ultrasound operator positioning a “sample volume” or “sample gate” on the B-mode image.
The advantage of PW is that it is depth specific. The main disadvantage is that due to the waiting time for the machine to listen for the returning echoes there is a limit to the velocity of blood flow that can be accurately measured. As the time taken for echoes to return increases with the depth of tissue sampled, the maximal velocity that can be assessed decreases.
Continuous Wave Doppler (CW)
In this Doppler system, a continuous wave of ultrasound is produced and returning echoes are simultaneously interpreted by the probe. Doppler shift is detected for every object along the ultrasound beam’s path.
The advantage of CW is that, unlike PW, higher flow velocities can be accurately assessed. This is especially useful in cases where Doppler is required to assess a high velocity jet of blood flow. E.g. aortic stenosis and left ventricular outflow tract assessment.
The disadvantages of CW are that the depth of returning Doppler signals cannot be determined and that specific crystal arrays are needed to facilitate the continuous emission and simultaneous reception of signals. This means that only specific ultrasound transducer types, such as phased array probes, are capable of CW.
Doppler Display Modes
In this display mode, Doppler Shift data received by the ultrasound probe is displayed as colour pixels within the sample gate shown on the ultrasound B-mode image. This gives information about direction and a semi-quantitative assessment of blood flow velocities.
Conventionally, echoes representing flow away from the ultrasound transducer are displayed as shades of blue and echoes representing flow towards the ultrasound transducer are displayed as shades of red.
Power Doppler ignores the directional information provided by Doppler shift and displays the total Doppler signal strength as shades of one colour. While it does not display any data on flow direction, Power Doppler is a useful tool for examining low velocity blood flow and is more sensitive to flow than colour doppler. This means it can detect flow in smaller blood vessels.
Both Colour Doppler and Power Doppler represent a form of Pulsed Wave Doppler and using one or both forms of display will reduce the frame rate that the ultrasound machine is capable of. As Continuous Wave Doppler is not depth specific it cannot be displayed as colour pixels overlying a B-mode image.
Spectral Doppler displays Doppler shift in a graphical format. Velocity is plotted on the Y - axis and time is plotted on the X - axis. Flow towards the probe is plotted above the baseline and flow away from the probe is plotted below the baseline.
Both Continuous Wave and Pulsed Wave Doppler systems can be displayed in a spectral format.
Ultrasound Transducers and Doppler
Assuming the ultrasound machine has the software capabilities to process Doppler Shift data, all probe types are capable of Pulsed Wave Doppler. However, as Continuous Wave Doppler relies on simultaneous transmission and reception of ultrasound only specific designs of probe can used for CW; a phased array probe, used for echocardiography, is the most common example.
Important Doppler Considerations
Doppler shift is dependent on flow away from or towards the probe making probe positioning extremely important. Ideally, the ultrasonographer should align the direction of the ultrasound in parallel with the direction of blood flow. This maximises the Doppler shift phenomenon and enables accurate velocity assessment. Poor technique in obtaining and optimising B-mode images will result in inaccurate velocity measurements, most commonly resulting in underestimation. When performing echocardiography, perfecting standard B-mode imaging is crucial to maximising the usefulness of Doppler.
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