Doppler Ultrasound
What is Doppler Ultrasound?
Doppler ultrasound is based on the shift in frequency in an ultrasound wave caused by a moving reflector, i.e. blood cells. The change in frequency is termed the Doppler shift. Doppler ultrasound is used to measure the velocity of a moving object. The doppler frequency is given by the product of 2, the incident frequency, the doppler angle and the velocity of blood (m/s) divided by the speed of ultrasound in soft tissue.
By comparing the incident ultrasound frequency with the reflected ultrasound frequency from the blood cells, the velocity of the blood can be calculated. In general, movement towards the transducer causes an increase in frequency, and vice versa.
Doppler frequencies are in the audible range of 15-20kHz. Often a loud speaker or headphones are used by the sonography to aid in positioning and diagnosis.
As the angle of incidence increases with reference to the long axis of blood vessels, the Doppler shift decreases. The preferred Doppler angle is 30-60 degrees. At >60, a small Doppler shift, minor errors in angle accuracy can result in large errors in velocity. At <20, refraction and critical angle interaction can cause problems.
Continuous and Pulsed Doppler
Continuous Doppler is the simplest and least expensive operation mode. It required two transducers, one continuously transmitting ultrasound and one continuously detecting return echoes. However, this is not depth selective, meaning flow from surrounding vessels can be superimposed, making the output signal complex.
Pulsed Doppler uses pulses of ultrasound, enabling range gating. Range gating means, by changing time delays between pulses and echoes, echoes from a particular depth range can be passed for signal processing, excluding other depth ranges. This means information can be obtained from specific points in a specific vessel. For example, the transducer may be set to only receive information from 18-22 microseconds.
Duplex Scanning
Duplex scanning is when we combined 2D B mode imaging and pulsed Doppler data acquisition, e.g. so we can see the blood vessels but also gain information of the blood flow. The 2D B mode creates a real-time image to provide a visual guidance of the vessel of interest. The Doppler gate is positioned over the vessel of interest with a size appropriate for evaluation of blood velocity. The Doppler mode is switched on and blood velocity is calculated.
Colour Doppler
Colour Doppler provides a 2D visual display of moving blood, superimposed upon the conventional grey scale image. Velocities and directions are determined and then colour encoded, e.g. red for blood flowing towards the transducer and blue for blood flowing away from the transducer. The colour intensity varies with flow intensity.
In general, colour Doppler spatial resolution is lower than that of a grey scale image.
Power Doppler
Power Doppler permits detection and interpretation of slow blood flow, but sacrifices the directional and quantitative flow information of Colour Doppler. Power Doppler relies on the strength of the Doppler signal (amplitude) and ignores directional (phase) information. It is more sensitive than standard colour flow imaging and there is enhanced sensitivity in the power Doppler acquisition in areas perpendicular to the beam direction, where signal is normally lost in Colour Doppler. As mentioned, flow directional information is lost in power Doppler. Power Doppler is helpful in imaging internal bleeding or haemorrhage, where blood flow is less than it would be in a blood vessel.
Doppler ultrasound is based on the shift in frequency in an ultrasound wave caused by a moving reflector, i.e. blood cells. The change in frequency is termed the Doppler shift. Doppler ultrasound is used to measure the velocity of a moving object. The doppler frequency is given by the product of 2, the incident frequency, the doppler angle and the velocity of blood (m/s) divided by the speed of ultrasound in soft tissue.
By comparing the incident ultrasound frequency with the reflected ultrasound frequency from the blood cells, the velocity of the blood can be calculated. In general, movement towards the transducer causes an increase in frequency, and vice versa.
Doppler frequencies are in the audible range of 15-20kHz. Often a loud speaker or headphones are used by the sonography to aid in positioning and diagnosis.
As the angle of incidence increases with reference to the long axis of blood vessels, the Doppler shift decreases. The preferred Doppler angle is 30-60 degrees. At >60, a small Doppler shift, minor errors in angle accuracy can result in large errors in velocity. At <20, refraction and critical angle interaction can cause problems.
Continuous and Pulsed Doppler
Continuous Doppler is the simplest and least expensive operation mode. It required two transducers, one continuously transmitting ultrasound and one continuously detecting return echoes. However, this is not depth selective, meaning flow from surrounding vessels can be superimposed, making the output signal complex.
Pulsed Doppler uses pulses of ultrasound, enabling range gating. Range gating means, by changing time delays between pulses and echoes, echoes from a particular depth range can be passed for signal processing, excluding other depth ranges. This means information can be obtained from specific points in a specific vessel. For example, the transducer may be set to only receive information from 18-22 microseconds.
Duplex Scanning
Duplex scanning is when we combined 2D B mode imaging and pulsed Doppler data acquisition, e.g. so we can see the blood vessels but also gain information of the blood flow. The 2D B mode creates a real-time image to provide a visual guidance of the vessel of interest. The Doppler gate is positioned over the vessel of interest with a size appropriate for evaluation of blood velocity. The Doppler mode is switched on and blood velocity is calculated.
Colour Doppler
Colour Doppler provides a 2D visual display of moving blood, superimposed upon the conventional grey scale image. Velocities and directions are determined and then colour encoded, e.g. red for blood flowing towards the transducer and blue for blood flowing away from the transducer. The colour intensity varies with flow intensity.
In general, colour Doppler spatial resolution is lower than that of a grey scale image.
Power Doppler
Power Doppler permits detection and interpretation of slow blood flow, but sacrifices the directional and quantitative flow information of Colour Doppler. Power Doppler relies on the strength of the Doppler signal (amplitude) and ignores directional (phase) information. It is more sensitive than standard colour flow imaging and there is enhanced sensitivity in the power Doppler acquisition in areas perpendicular to the beam direction, where signal is normally lost in Colour Doppler. As mentioned, flow directional information is lost in power Doppler. Power Doppler is helpful in imaging internal bleeding or haemorrhage, where blood flow is less than it would be in a blood vessel.
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