Doppler Flashcards
CW Doppler
DF=1,100%. CW can measure very high velocities but there is depth(range) ambiguity. Probe doesn’t know what depth the sound is coming from. NO ALIASING.Converts Doppler shift to audible sound. Detects any flow in the path of the beam.
Non-directional Doppler
Is like the handheld Doppler in the OBs office. Only detects the presence of flow, not the direction
Bi-directional Doppler
Can detect which direction the flow is flowing. Can distinguish between a positive and negative Doppler shift
Phase quadrature detector
The component used for bi-directional Doppler. Determines direction of flow (forward or reverse)
CW Doppler advantages
Increase sensitivity simple & inexpensive devices can be small no aliasing
Disadvantage - no range resolution
PW disadvantages
Depth and frequency limit lower sensitivity complex and expensive equipment
Advantage-range resolution which outweighs all the disadvantages
Increasing the PRF of a pulsed Doppler system will?
Decrease the potential for aliasing
The doppler shift signal heard on the loud speaker of a CW or pulsed doppler unit is the
Difference between the transmitted and received frequencies
A negative Doppler shift means
Relative flow away from the transducer
spectral analysis
Allows us to identify the individual components that make up the returned signal. Done by Fast Fourier Transform (FFT)- slow &accurate. FFT breaks down the complex signal (many frequencies) into individual frequencies (velocities)
Parts of the spectrum
1 Time- the phase of the cardiac cycle the spectrum appears in (systole diastole) 2. Duration- how long does the event occur 3. Direction- is blood flowing toward or away from probe 4. Amplitude- how many RBCs are there? 5. Broadening- range or frequencies present (window & envelope)
Spectral Broadening
Presence of many different velocities cause a thickened envelope. Turbulent flow.
Laminar flow will have a thin clean envelope
Peak systolic velocity
Highest velocity during systole
End diastolic velocity
Usually measured just before beginning of next systole. Not always the lowest velocity
Pulsatility Index (PI)
PSV-EDV\ mean velocity. PI=A-B/Mean
Resistive Index (RI)
PSV-EDV/PSV. RI=A-B/A
Increasing the operating frequency increases the frequency shift
Therefore increasing the operating frequency DOES NOT CHANGE the calculated velocity
Doppler controls
Higher frequency more sensitive to flow but with less penetration. The higher the frequency the higher intensity of the scatter. Also the higher the frequency the higher the likelihood of aliasing
Sample Volume (SV)
Important to be in the center of the vessel. Too wide and you risk spectral broadening. Too small and you risk missing the fastest component of the flow. Use large SV when “searching” for flow.
Angle Correction
The STEER of the Doppler cursor and the ANGLE CORRECTION we apply (not. Cardiac) to accurately measure velocities. With linear probes you can steer the Doppler cursor left or right. We change the steer of the cursor to avoid being perpendicular to flow.
Doppler controls 2
Must be <60 or less! Cardiac always at 0 & general & vascular typically between 30 & 60. We convert the frequency shift info to velocity info using Doppler equation. We need to flow angle in order to accurately calc the velocities. WE MEASURE THE FREQUENCY SHIFT BUT CALC VELOCITIES.
T or F
The lower the angle, the more accurate the velocity estimation?
True
Scale/PRF (same thing)
This is the sampling time/rate. High speed flow requires a high sampling rate for accurate measurement. Slow flow requires lower sampling rates. The deeper the sample volume the slower the flow we can accurately sample.
Flow sensitivity
Use low scale/PRF for slow flow. Lower PRF= higher sensitivity. Use higher PRF settings for fast flow. Increase PRF in presence of aliasing. Aliasing means that your sampling rate is too slow for the flow speed. Flow really fast = increase PRF. Having trouble finding flow=decrease PRF
When to increase & decrease PRF/scale typically
Increase to make waveform smaller. Decrease to make waveform larger.
Typically- higher PRFs for arteries or high flow veins. Lower PRFs for veins or slow-flow arteries
Sweep Speed
How man waveforms will be displayed on the screen at one time. Stretch out waveform for more accurate measurements. Used for renal arteries also if you want to see if fetus has heart arthermia.
Wall Filter
Used to eliminate low frequency signals in the spectral signal that appear from wall or valve motion. Cardia uses high WF. General uses low WF. Less noise from wall motion. Also called wall-thump or high pass filter. It’s the black line under the waveform.
Baseline
Controls where the bottom of the waveform will be displayed. The baseline can be lowered in the presence of aliasing. Doppler shift must be less than 1/2PRF
Spectral Gain
Overall brightness of the dots. Avoid over or under measurements of spectral waveform velocities
Invert
Inverting the spectral scale to show forward flow always above the baseline
Spectral analysis measures ____ over ____
Velocity; time
Diastolic info is missing along the region of a PW spectral baseline. What control is used to correct for this?
Decrease wall filter
The waveform appears very small compared to the spectral window. What would you do to make this more optimal?
Decrease the scale
What is the max Doppler angle correction that should be used?
60 degrees
Color Doppler
Color represents the direction of the flow. The shade signifies lower and higher velocities. The color represents the mean if the velocities in the waveform. Not the max as in spectral Doppler. CDI is a PW device & is bound by the same limitations. Size and angle of the gate controlled by operator- same as spectral
Ensemble length or “packet size”
How many pluses per scan line. May be as low as 3 pulses per scan line but typically 10-20. Color Doppler with large gates causes a significant decrease in frame rate.
INCREASE ensemble length INCREASE flow sensitivity DECREASE frame rate
Aliasing with Color Doppler
Color Doppler aliases when Doppler shift exceeds 1/2PRF(same as spectral) results in incorrect flow direction on color Doppler image. Can increase flow speed range ;decrease baseline decrease frequency to reduce aliasing.
Velocity Mode
Averages out the measured velocities for each gate. Colors appear up and down on the color map.
Variance mode
Average velocities calculated but the colors are displayed side to side. Used in cardiac imaging for turbulence.
Hue
Saturation
Brightness
Hue is the perceived color. Red blue green etc
Saturation is the shade of an original color.
Brightness (luminance intensity) from dull –> brilliant
Color gain
Too low the vessel isn’t filled in. If its too high the vessel “bleeds” out color.
Color persistence
Also called “smoothing” or temporal compounding. Same as 2d persistence. Averages frames of color info over time. Trade off-decrease in temporal resolution.
Color priority
Low priority means that grayscale will be displayed preferentially over color information. High priority means color will be displayed even if there is grayscale info. Helps reduce tissue/color overlapping.
Power Doppler
Aka:color Doppler energy (CDE), power angio,amplitude Doppler. Uses the amplitude/intensity of the signal to produce color flow imaging.evaluates the strength of the shift but not the velocity. Relatively independent of angle. Very sensitive. The density or construction of RBCs determines the amplitude.
Directional Power Doppler
Overlays PDI info with CDI info to display direction of flow
Tissue Doppler Imaging
Blood flow is low amplitude, high velocity signal. Wall motion is high amplitude low velocity signal. TDI uses a low pass filter to filter out the low amplitude RBC signals. Still limited by 90degrees. Can be performed in PW spectral and color Doppler modes
Doppler Cheat Sheet
Fast Fourier Transform vs. autocorrelation
FFT: CW & PW Doppler ; spectral waveform
Autocorrelation: color Doppler
Mirror image
Occurs with spectral & color Doppler. Can be caused by too- high Doppler gain or angle is too close to 90.
Clutter
Tissue motion also causes a Doppler shift not just from the RBCs. This appears on screen as color not related to blood flow. Eliminated by wall filters.
Reverberation
Not specific to Doppler . Can mimic vessel dissection/tear.
Twinkle
Commonly caused by calcifications/stones. Can be reduced by decreasing the color priority. To see twinkling use high color priority and low grayscale gain.
Spectral Doppler artifacts
Depth ambiguity high PRF setting to eliminate aliasing. PRF too high to allow for proper timing of returning echoes. Multiple range gates denote possible depth ambiguity
Color Bruit
A stenosis, AV fistula or pseudo aneurysm can cause a thrill or tissue vibration. Usually when something is abnormal.. Look further
Increasing the packet size has what effect?
Decreases temporal resolution
Color pixels appear to be outside of the vessel. What needs to be adjusted?
Decrease color gain
Which of the following is the processing method for color Doppler?
Autocorrelation
What does power Doppler rely on?
Number of red blood cells RBCs
