Doppler Technology and Technique Flashcards
How does doppler systems determine the velocity of blood flow?
Examination of the change in frequency of the ultrasound reflected from RBCs.
Term for alterations in the frequency of ultrasound signals caused by the doppler effect?
Modulation
What is the Doppler equation?
ft - fr = v x cos(theta) x 2ft/c
ft - transmitted frequency
fr - received frequency
v - blood viscosity
c - sped of sound in blood (1540 m/s)
theta - angle of incidence between ultrasound beam and blood flow.
What is quadrature demodulation?
a demodulation process done by comparing the returning echoes with internal reference signals that are in phase and 90 deg out of phase with the transmitted signal.
This is done to distinguish red cell modulated echoes from all other nonfrequency-shifted echoes created by reflections from tissue.
What is a spectral display?
Presenting doppler data as a time-velocity plot.
How is amplitude shown on a spectral display?
Frequencies with greater amplitude are marked with brighter pixels.
What is time gating? What kind of doppler is this used in?
signals associated with specific depth or location are selected for evaluation.
Single crystal is emitter and receiver. Time delay = 2d/c
This is used in Pulsed-Wave Doppler
What is the Pulse repetition frequency?
the rate at which the ultrasound repeatedly generates sound bursts.
Formula for Nyquist Limit?
1/2 the Pulse Repetition Frequency.
AKA the maximal frequency.
What is aliasing?
ambiguous signal for frequencies above the Nyquist limit.
Also known as wrap around.
How does decreasing the target distance for pulsed wave velocity measurements help?
increases the velocity that can be assessed.
What happens to nyquist limit with higher transmitted frequencies?
It is lower
So use lower transmitted frequency signals to increase the ability to capture higher velocites.
How does continuous wave doppler work?
One crystal is constantly transmitting and the other is continuously receiving.
What kind of doppler is used for color flow mapping?
Is red towards or away from transducer?
Pulse wave doppler
Red is towards transducer.
What happens to the PRF in color flow mapping relative to Pulse wave doppler?
What does this do to alia artifacts?
PRF is lower because a part of signal must be used for image generation.
Alias at a lower velocity than conventional pulsed-wave doppler
