Doppler Flashcards
What is the Doppler principle?
The physical principle used to measure the velocity of blood in circulation
Define demodulation in the context of Doppler.
The process of extracting the low Doppler frequency from the transducer’s carrier frequency
What are positive Doppler shifts?
When blood cells move toward the transducer, causing the reflected frequency to be higher than the transmitted frequency
What are negative Doppler shifts?
When blood cells move away from the transducer, causing the reflected frequency to be lower than the transmitted frequency
What does the Doppler shift represent?
The difference between the reflected frequency and the transmitted frequency
Fill in the blank: Doppler shift (Hz) = _______.
reflected freq. - transmitted freq.
What is measured velocity in Doppler?
Defined by a magnitude and a direction
What is the formula for Doppler shift?
Doppler shift = 2 x velocity of blood x transducer freq. x cosine
What does the cosine factor represent in the Doppler shift formula?
The angle between sound and vessels
What does bidirectional Doppler distinguish?
The direction of flow toward or away from the transducer
What does a spectral tracing below the baseline indicate?
A negative shift or flow away from the transducer
What does a spectral tracing above the baseline indicate?
A positive shift or flow toward the transducer
What is Continuous Wave (CW) Doppler?
A commonly used signal processing technique for bidirectional Doppler
What are the advantages of CW Doppler? List at least two.
- Ability to accurately measure very high velocities
- Very small Doppler shifts can be detected
What is a disadvantage of CW Doppler?
Cannot determine the exact location of moving blood cells due to range ambiguity
What does range ambiguity refer to?
The inability to determine depth of reflections due to overlap of transmit and receive beams
Fill in the blank: Simultaneous anatomic imaging and Doppler fusion is achieved with _______.
2D US + MRI/CT
What does duplex imaging/scanning refer to?
Velocities sampled every second in blood circulation
What are the two Doppler shifts in the Doppler shift formula?
- Reception of sound wave by moving RBC
- Transducer’s reception of the sound wave from the moving RBC
What is the significance of the number 2 in the Doppler shift equation?
Represents that there are actually 2 Doppler shifts involved
What does PW Doppler stand for?
Pulsed Wave Doppler
PW Doppler is a type of ultrasound that measures blood flow velocities.
What is range resolution in the context of PW Doppler?
The ability to select the exact location where velocities are measured
This is achieved by placing a small marker called the gate or sample volume on a 2D image.
What is aliasing in Doppler ultrasound?
Inaccurate measurement of high velocity signals
It can result in high velocity flow being displayed as traveling in the opposite direction.
What is the Nyquist limit?
The highest Doppler frequency or velocity that can be measured without aliasing
It is defined as Nyquist limit (Hz) = PRF / 2.