Chapter 4: Doppler Imaging Concepts Flashcards
What is the formula for Ohm’s law?
Flow (I) = Voltage (V) / Resistance (R)
or
Blood flow (Q) = Pressure change (P) / Resistance (R)
What are the following doppler artifacts and how are they corrected?
Clutter
A heartbeat with a bunch of crap underneath. Acoustic noise eliminated with high-pass wall filters.
Increase the wall filter.
Aliasing
When the top of the heartbeat wave is on the bottom of the screen. (wraparound) Caused by insufficient sampling of flow.
Increase PRF, decrease frequency shift, or use CW
Mirror image
Reproduction of spectral or color information opposite a strong reflector.
Decrease color/spectral gain or decrease angle to <90 degrees
Color bleeding
Overgained color or spectral waveform causing signal to be larger than it should be.
Decrease gain or transmit power
High PRF/Depth ambiguity
HPRF setting in PW Doppler means the PW is emulating a CW probe and is unsure where echoes came from.
Change depth or Doppler angle
Flash artifact
Excessive color signal in surrounding tissue caused by movement during power Doppler.
Eliminate the source of motion if possible (breath hold)
What is the point at which Reynolds number predicts turbulence?
2000
Oscillator
The component of a continuous-wave Doppler device that produces the voltage that drives the transducer.
fast Fourier transform
A mathematic process used for analyzing and processing the Doppler signal to produce the spectral waveform
Phase quadrature
The component of the Doppler device that determines positive opposed to negative frequency shifts and, therefor, direction of blood flow.
What is the Resistive Index and how is it calculated?
Doppler measurement used to quantitate the resistiveness of the distal bed.
(Peak sys velocity - End dias velocity) / PSV
What is the pulsatility index and how is it calculated?
Doppler measurement used to determine how pulsatile a vessel is over time.
(Peak sys velocity - End dias velocity) / Mean
mean = average of the velocities between peak sys and end dias
Power doppler
aka Amplitude doppler
Doppler mode in which the signal is determined by the amplitude (strength) of the shift, not the frequency. Amplitude is directly proportional to the number of RBC. No aliasing. Can use 90 degree angle of incidence.
Continuous wave doppler
Doppler device that uses CW US transmission. Needs 2 elements.
Color doppler
Doppler shift information presented as a color superimposed over the grayscale image.
Can measure mean velocity, but not peak or trough.
Uses autocorrelation for signal processing
Pulsed wave doppler
Doppler technique that uses pulses of sound to obtain Doppler signals from a user-specified depth.
Frequency shift
The difference between the transmitted and received frequencies
What factors would increase frequency shift?
Decrease?
F = 2fv cos0 / c
above the line: frequency of transducer, velocity of blood, and cosign of the doppler angle
below the line: propagation speed through soft tissue
*note: as doppler angle increases, cosign of angle decreases
Nyquist limit
The highest frequency, after which you would get aliasing.
Equal to 1/2 the PRF.
If PRF is 2500 Hz, then a doppler shift >1250 Hz will alias.
Ensemble length/Packet size
The number of pulses per scan line in color doppler. As low as 3. Typically 10-20
Higher length = more pulses per scan line, higher sensitivity to slow flow, more accurate mean velocity, slower frame rate, worse temporal resolution
What signal processing technique is used for color Doppler?
For spectral doppler?
Autocorrelation
Fast Fourier transform
