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Doppler
Doppler section for SPI
| Term | Definition |
|---|---|
| Doppler Shift | a change or difference in the frequency of sound as a result of motion between the sound source and the receiver |
| Doppler frequency | difference between received and transmitted frequencies |
| Positive Doppler Shift | when the source and receiver are approaching each other. reflected frequency is higher than transmitted |
| Negative Doppler Shifts | when the source and receiver are moving apart. reflected frequency is less than transmitted frequency |
| Doppler measures: | frequency shift not amplitude |
| Doppler Units | Hertz, cycles per second |
| Typical Values | 20Hz-20kHz, audible. Created when sound reflects off of moving red blood cells |
| Demodulation | extracts the Doppler frequency from the transducer frequency and is performed by a demodulator |
| Bidirectional Doppler | analyzed with phase quadrature processing |
| Doppler shift equals | received frequency-transmitted frequency |
| Doppler Equation | 2x reflector speed x incident frequency x cos ( angle)/ propagation speed |
| Doppler shift is directly related to: | blood cell speed frequency of the transducer cosine of the angle between flow and sound beam |
| Doppler shift is inversely related to: | speed of sound in the medium |
| Doppler measures | velocity, not speed |
| Speed | magnitude only |
| Velocity | magnitude and direction |
| angle-cosine | 0 degrees= 1 60 degrees= 0.5 90 degrees = 0 |
| maximum Doppler shift | 0 degrees |
| no doppler shift | 90 degrees |
| Continuous Wave Doppler | two crystals in the transducer one crystal is continuously transmitting one crystal is continuously receiving |
| Advantage of CW Doppler | high velocities are accurately measured |
| Disadvantage of CW Doppler | echoes arise from entire length of overlap between the transmit and receive beams. called range ambiguity |
| CW Doppler | no damping narrow bandwidth High Q factor advantages are higher sensitivity and ease in detecting small doppler shifts 'jet sniffer' |
| Pulsed Wave Doppler | one crystal alternates between sending and receiving |
| Advantage of PW Doppler | Echoes arise only from area of interrogation. the sample volume or gate. Locate the gate( center of lumen, parallel to vessel walls) This is known as range resolution or range specificity |
| Disadvantage of PW Doppler | Aliasing, errors in measuring high velocities |
| Imaging and pulsed doppler | can be performed with a single crystal transducer. Simultaneous imaging & Doppler is known as duplex ultrasound |
| The horizontal axis (X axis) of a Doppler spectrum | time |
| The vertical axis ( Y axis) of a Doppler spectrum | Doppler shift or velocity |
| Aliasing | grows from the top or the bottom of a spectrum, never from the baseline |
| Wrap around aliasing | Pulsed doppler exhibits wrap around aliasing with extremely high velocities |
| Aliasing | high velocities appear negative. With Pulsed Doppler, high velocity measurements are inaccurate if the pulsed Doppler sampling rate ( PRF) is too low in comparison to measured Doppler shift |
| Nyquist frequency | The Doppler frequency at which aliasing occurs, equal to 1/2 the PRF |
| Nyquist Equation | Nyquist Limit= ( kHz) = PRF / 2 |
| Aliasing appears: | when the doppler shift exceeds the Nyquist limit |
| Aliasing can be eliminated | when the Doppler spectrum shrinks when the Nyquist limit is raised |
| Eliminating Aliasing | use continuous wave Doppler select a lower frequency transducer select a new view with a shallower sample volume increase the scale, same view, if the system is underdriven baseline shift |
| Continuous Wave Doppler | Aliasing cannot occur with CW Doppler |
| Low frequency transducers | reduce the Doppler shift and shrink the spectrum - when looking at the frequency |
| Shallow Sample Volume | PRF and therefore Nyquist frequency is higher |
| Increasing the scale | increases the PRF and thus the Nyquist limit, less aliasing decreases the ability to measure low velocities |
| What do you do to eliminate aliasing ( best answer) | zero baseline shift |
| Smaller sample volumes ) gates) | create Doppler spectra with clean spectral window |
| Larger sample volumes ( gates) | create Doppler spectra with filled- in spectra ( spectral broadening) |
| Gray Shades on a Doppler spectrum are related to: | amplitude or strength of the reflected signal or concentration (number) of blood cells creating the reflection |
| Color flow Doppler | black and white identifies anatomic structures color identifies blood flow velocities and function |
| Color Doppler is based on pulsed ultrasound and is subject to | range resolution or specificity aliasing |
| Color Doppler | provides information regarding flow direction |
| Average or Mean Velocity | Color flow measures mean velocities Pulsed and CW Doppler measures peak velocities |
| Color Maps | Top Color; is always flow toward or closer to the transducer Bottom Color: is always flow away from the transducer |
| Velocity Mode | colors on map do not vary from side-to-side |
| Variance Mode | colors on map vary from side-to-side left sided colors- the flow laminar or parabolic right sided colors- the flow turbulent or disturbed |
| Doppler Packets | multiple ultrasound pulses are needed to accurately determine red blood cell velocities by Doppler. these multiple pulses are called a packet or ensemble length |
| Small Packet | less accurate Doppler less sensitive to slow velocity or low flow higher frame rate , improved temporal resolution |
| Large Packet | more accurate Doppler more sensitive to fast velocity or flow lower frame rate, reduced temporal resolution |
| More pulses in the packet Advantages | greater accuracy of the velocity measurement sensitivity to low flows is also increased with many pulses in the packet |
| More pulses in the packet Disadvantages | more time is required to acquire the information frame rate and temporal resolution are reduced |
| Color Power Doppler aka energy mode, color angio | color Doppler information regarding direction and actual velocity is not calculated |
| Advantages of Color Power Doppler | increased sensitivity to slow flows, flow in small vessels and deeper vessels angle independent, not affected by Doppler angles, unless the angle = 90 degrees no aliasing or alias-free |
| Limitations of Color Power Doppler | Very low frame rates; poor temporal resolution no information regarding velocity or direction |
| Spectral Analysis | an echo returning after striking mass of moving blood cells is a complex signal with many varied Doppler shifted frequencies |
| Spectral Analysis is performed | to extract or identify the individual frequencies making up the complex signal. It is used to interpret individual velocities in the signal |
| Spectral Analysis techniques | pulsed and CW Doppler= Fast Fourier Transform (FFT) color flow= autocorrelation or correlation function both FFT and autocorrelation are digital techniques that are performed by computers |
| Autocorrelation | is used with color Doppler because of the enormous amount of Doppler information that requires processing. Autocorrelation is slightly less accurate, but substantially faster than FFT |
| Autocorrelation used for color Doppler | determines average velocities used for variance mode |
| Anemia & Doppler | a Doppler exam can always be successfully performed regardless of a patient's hematocrit |
| High Pass Filter | Eliminates low magnitude Doppler shifts that are created from moving anatomy rather than red blood cells |
| Wall filters | serve as a "reject" for Doppler. Wall filters remove low level Doppler shifts around the baseline. Wall filters are used to reject Clutter |
| Clutter | increasing the scale will eliminate clutter |
| Wall filter | determines whether low velocity flows are displayed. also called ' high pass filter' |
| Cross Talk | found in spectral Doppler only special form of "mirror image" where the Doppler spectrum appears above and below the baseline |
| Causes of Cross Talk | Doppler Gain is set too high incident angle near 90 degrees when flow is at focus |
| to create color in a normal vessel where none appears | steer color box away from 90 degrees increase color Doppler gain decrease scale reduce wall filter |
| to reduce 'confetti' | decrease color Doppler gain increase scale |
| to reduce ' rainbow' (aliasing) | increase scale |
| when you see only the top half of a vessel | image deeper |
| When you want to see more color Doppler in a horizontal vessel | make the color box wider this will decrease frame rate. image quality will not change |
| when imaging superficial structures | use a linear or expanded view transducer rather than a sector |
Created by:
Mesichm05
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