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Spi Chapter 19
Doppler
| Question | Answer |
|---|---|
| Doppler shift is? | A change in frequency. |
| Doppler shift is also called: | Doppler frequency. |
| What creates a frequency change? | When the sound source and receiver move closer or father apart. |
| Frequency does not change when? | When the distance between the sound source and receiver remain constant. |
| Doppler shift is what type of frequency? | A low frequency that "rides" on top of the much higher transducer frequency. |
| What is demodulation? | Extracts low doppler frequency from the transducer carrier. |
| What occurs when the doppler shift is positive? | Blood cells are traveling towards the transducer. |
| Doppler shifts formula: | Doppler shift (Hz)= reflected frequency- transmitted frequency |
| How are doppler shifts created? | When sound waves strike moving blood cells. |
| When doppler shift its positive, that means what? | The reflected frequency is higher than the transmitted frequency. |
| What occurs when the doppler shift is negative? | Blood cells are traveling away from the transducer. |
| Doppler frequencies only indicate: | Velocity |
| Speed is a: | Magnitude & indicates the distance a red blood cell moves in 1 second. |
| The unit for speed is: | cm/s or any distance divided by time |
| Velocity is: | Magnitude & a direction. |
| What is the relationship between doppler shift and velocity? | Direct. |
| How many doppler shifts are there in a clinical exam? | 2 |
| The first shift is: | Occurs when a sound wave strikes moving red blood cells. The bloods reception is enough to create a doppler shift. |
| The second shift is: | After striking, reflection returns to the transducer. |
| Modern ultrasound systems measure what between the received & transmitted waves? | frequency difference |
| Velocity unit is: | m/s |
| X-axis of a doppler spectrum represents: | Time |
| Y-axis of a doppler spectrum represents: | Doppler shift or velocity |
| What is the relationship between doppler shift & frequency of the transmitted sound? | Direct |
| How is velocity measured? | When the blood cells are moving parallel to the sound beam |
| How is the percentage of velocity measured? | Depends on the cosine of the angle between the sound beam & the direction of the motion |
| What is the relationship between Cosine 0 & the doppler shift? | Direct |
| If the cosine 0 is halved then? | Doppler shift is halved |
| How is flow parallel to the sound beam? | When the angle between the direction of motion and sound is 0 or 180 |
| Cosine 0 is | 1 & indicates flow towards transducer |
| Cosine 180 is | -1 & indicates flow away from transducer |
| Doppler shift velocity can not be measured because of? | Perpendicular incidence |
| Cosine 90 is | zero |
| Cosine 60 is | 0.5 |
| The measured velocity at 60 is | one-half the actual velocity |
| What is the most common clinical device in doppler? | Bidirectional |
| Bidirectional distinguishes: | Direction of flow (away or towards transducer) |
| Audio of bidirectional doppler requires: | stereo headphones or speakers |
| Sound from each speaker indicate? | flow away and towards transducer |
| Flow towards the transducer is displayed where in the doppler spectrum? | Above the baseline |
| Flow away the transducer is displayed where in the doppler spectrum? | Below the baseline |
| Arterial velocities are higher than: | Venous velocities |
| How many crystals are in continuous wave doppler? | two |
| What is the greatest advantage of CW doppler? | Accurately measures very high velocities |
| What is the primary disadvantage of CW doppler? | Exact location of moving blood cells can not be determined. |
| What its range ambiguity? | Display of the structure of interest in the wrong location |
| CW doppler has a lack of? | Time gain compensation |
| How many crystals are in pulse wave doppler? | One |
| What is the primary advantage of pw doppler? | Able to select the exact location where velocities are measured |
| Pw doppler is also called: | Range resolution, range specificity or freedom from range ambiguity |
| With pw doppler what does the sonographer position ? | A small marker called the sample volume or gate on a two-dimensional image |
| What is the primary disadvantage of pw doppler? | Inaccurate measures of high velocities |
| What is aliasing? | High velocity flows in one direction is incorrectly displayed as traveling in the opposite direction |
| Aliasing is referred to as: | A false identity |
| Pulsed doppler transducers characteristics are? | Low quality factor, lower sensitivity & wide bandwidth |
| What is the Nyquist limit? | The maximum doppler shift frequency that can be sampled without aliasing |
| Nyquist limit is : | One-half of the PRF (PRF/2) |
| Aliasing only occurs with? | Pw doppler |
| Aliasing occurs when? | The doppler sampling rate is too low in comparison to the measured blood velocities |
| Two ways to avoid aliasing are: | Raise the Nyquist limit or reduce the Doppler shift |
| When the sample volume is deep: | The PRF is low and the Nyquist limit is low |
| When the sample volume is shallow: | The PRF is high & the Nyquist limit is high |
| What creates more aliasing? | High frequency transducers |
| Aliasing is less common with what type of transducer? | Low frequency |
| How many techniques are there to avoid aliasing artifact? | 5 |
| First technique is | Adjust scale to its maximum |
| Second technique is | Select a new ultrasonic view with a shallower sample volume |
| Third technique is | Select a low frequency transducer |
| Fourth technique is | Use Baseline shift |
| Fifth technique is | Use CW doppler |
| Gray shades on a doppler spectrum are related to: | Amplitude of the reflected signal or number of blood cells creating the reflection |
| Color doppler characteristics are: | Pulsed ultrasound technique, has range resolution & is subject to aliasing |
| Color doppler reports: | Average or mean velocities |
| Pulsed and continuous wave report: | Peak velocities |
| Spectral doppler reports: | Peak velocity |
| Color doppler uses what to convert measured velocities ink to colors that appear on the image? | "Dictionary" or look up table |
| This look up table is displayed as: | A vertical bar of various colors with a black region in the center |
| The two most common dictionaries are: | Velocity mode and variance mode |
| Velocity mode mode colors display: | Information on flow direction and velocity |
| The black region in the middle of the color doppler indicates: | no doppler shift |
| The colors above the black stripe indicate: | Postive doppler shifts. Blood is moving towards the transducer |
| The colors below the black stripe indicate: | Negative doppler shift. Blood is moving away from the transducer |
| When multiple colors appear below the black stripe, it means? | Slower velocity |
| With velocity mode maps: | the color change is always up & down |
| Variance mode distinguishes: | Laminar flow from turbulent flow |
| Variance mode displays colors how? | side to side as well as up & down |
| Colors on the right side of the map indicate: | Turbulent flow |
| Colors on the left side indicate: | Laminar flow |
| The "homeline" is the: | Imaginary location of the transducer |
| What is a packet/ensemble? | Multiple pulses that help determine blood velocities |
| Advantages of large packets are: | More accurate velocity measurement and increased sensitivity to low flow. |
| Disadvantages of large packets are: | More time needed to acquire data, reduced frame rate and decreased temporal resolution |
| Power mode only identifies: | The presence of a doppler shift |
| Power mode does not evaluate: | Speed or direction |
| Power mode is: | Non-directional color doppler |
| Power doppler is also called: | Energy mode or color angio |
| Three advantages of power mode are: | Increased sensitivity to low flow or velocity Unaffected by doppler angles No aliasing |
| Three disadvantages of power mode are: | No measurement of velocity or direction Lower frame rates May result in burst of color or flash aritifact |
| What is clutter? | On spectral displays, it is a noise artifact that appears as diffuse echoes overly signals of interest |
| What is Ghosting artifact? | In color doppler, it is produced when refraction of a beam occurs in one part of a scanning plane |
| What is Wall filter? | Used to eliminate low frequency doppler shifts from moving anatomy |
| Wall filters are also called: | High pass filters |
| What is crosstalk? | A special form of "mirror image" artifact. |
| Crosstalk only arises with: | Spectral doppler |
| Crosstalk appears as: | An identical doppler spectrum above and below the baseline |
| Crosstalk results from: | Doppler gain set to high or the incident angle is near 90 between the sound beam & flow |
| What is spectral analysis? | A tool that breaks the complex signal into its basic building blocks & identifies the individual velocities that make up the reflected doppler signal |
| What are two methods of spectral analysis? | Fast Fourier transform & Autocorrelation |
| What is Fast Fourier transform? | A computer based technique used to processed both pulsed and continuous wave doppler signals |
| What are two advantages of FFT? | exceedingly accurate & displays all individual velocity components that make up a reflected signal |
| The spectral display of an FFT distinguishes? | Laminar flow from turbulent flow |
| What is spectral broadening? | A wider range of velocities & doppler shifts within the sample volume |
| What is autocorrelation? | A computer based technique used to analyze color flow doppler |
| Autocorrelation is faster to perform than? | Fast Fourier transform |
Created by:
Montesmayan