Term
| What is the appropriate gate size for arterial studies? |
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Definition
| For arterial studies the gate size should be approximately 2/3 - 3/4 of the vessel diameter. |
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Term
| What should the gate size be for venous studies? |
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Definition
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Term
| What happens if the gate size is too large? |
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Definition
| If the gate size is too large, it picks up slower velocities on the sides of the vessel and creates window filling. |
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Term
| Doppler angle is the angle between.... |
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Definition
- The line of insonation (indicated by DOPPLER LINE and changed by STEERING)
and
- The flow (indicated by the GATE and adjusted by ANGLE CORRECT) |
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Term
| What is absolutely essential that the sonographer does about setting the gate in a certain direction> |
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Definition
| It is absolutely essential that the sonographer sets the gate PARALLEL to the direction of the flow. |
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Term
| How is appropriate doppler angle achieved? |
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Definition
Appropriate doppler angle is achieved by:
- Angling the vessel with the heel-toe maneuver.
- Steering the incident beam (available on linear probes)
- Changing the Doppler angle. |
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Term
| What should the doppler angle be kept at, but never at the expense of what? |
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Definition
| The doppler angle should always be kept low but NEVER at the expense of the cursor with the flow alignment. The angle should never be more than 60 degrees to avoid overestimating the velocity. |
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Term
What is the scale?
What are the units for scale? |
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Definition
- The scale is the maximum and minimum values displayed on the spectral analysis graph.
- The units are cm/sec or KHz |
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Term
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Definition
- The PRF and depth of the gate
- The doppler frequency
- The doppler angle |
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Term
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Definition
| Scale = 1540 m/s^2 / 8xFoxDxCos@ = PRFx 1540 m/s / 4xFoxCos@ |
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Term
| Scale control affects _____ and hence the _____ of the graph. |
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Definition
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Term
| If the scale is too low, what will you see? |
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Definition
| - If the scale is too low, the waveform will be huge and aliasing will occur. |
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Term
| If the scale is too high what will you see? |
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Definition
| - The waveform is very small. |
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Term
| If the scale is adequate the waveform will take up about ____ of the graph. |
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Definition
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Term
What is doppler frequency?
How does it affect the scale? |
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Definition
- The doppler frequency, Fo is the frequency the system generates to interrogate the vessel.
- Think of the formula for scale: PRFx1540m/s / 4xFoxCos@
By keeping the Fo low you keep a higher scale which avoids aliasing. |
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Term
What is acoustic power?
What does it look like on a waveform for high and low? |
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Definition
- The acoustic power regulates the applied voltage, and hence the amplitude of the generated sound wave.
- Recall that brightness of a pixel is determined by the amplitude, High amp = bright pixels. With a low acoustic power, the waveform will be dim, with a high acoustic power, the waveform will look just like the waveform with a low acoustic power, however the pixels will be MUCH BRIGHTER. |
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Term
| What does a low doppler gain look like on spectral analysis? |
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Definition
| - A low doppler gain is very dim, and shows poor visualization of flow, especially with small quantities of blood. |
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Term
| What does high doppler gain look like on spectral analysis? |
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Definition
| - Very bright waveform, with lots of window filling. |
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Term
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Definition
1) The signal is split into two different channels and mixed with the electrical signal of the Fo.
2) In channel one the signal is mixed with the Fo in-phase.
3) In channel two the signal is mixed with the Fo out of phase ( a 90 degree shift or a cosine function instead of sine.) |
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Term
| Explain Doppler shift detection |
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Definition
- The received echoes contain shifted frequencies from the moving target RBC.
- The other component of the received echo frequencies contain the frequencies of stationary objects such as the vessel wall and tissues. These frequencies are the same frequency of the Fo of the transducer and of no interest for calculating velocity. LOW PASS FILTER removes these frequencies from the stationary source(s) through a process called DEMODULATION or HETERODYNING. |
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Term
| What are the old techniques of Doppler shift detection? |
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Definition
- Zero crossing detector
- Time Interval Histogram |
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Term
| Explain how a Zero Cross Detector method works. |
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Definition
- The system counts each time the signal crosses the zero line, estimating the drop in frequency.
- Zero frequency detector cannot estimate different frequencies simultaneously.
- Not very accurate.
- Zero crossing detector cannot determine sign of the drop in frequency (no directional information) |
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Term
| What is the function of the quadrature detector? |
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Definition
- Allows us to determine the direction of the flow whether the shift is positive (moves toward the probe) or negative (moves away from the transducer).
- This property is only present in bi-directional doppler machines (modern). Old units did not have this function and so all uni-directional machines only displayed the doppler shift above the baseline. |
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Term
T or F
Both channels contain positive and negative doppler shift. |
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Definition
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Term
| If the doppler shift is positive then the _____ signal will be leading and the _____ signal will be lagging. |
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Definition
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Term
| If the doppler shift is negative then the _____ signal will be leading and the _____ signal will be lagging. |
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Definition
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Term
| What are the old techniques of flow direction detection? |
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Definition
- Single-sideband detector
- Heterodyne detector |
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Term
| Explain Wall Filter (aka High pass filter) |
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Definition
1) RBC's move very fast, but they are poor reflectors due to Rayleigh's scattering. (High doppler shift, low amp)
2) The vessel wall or cardiac valve leaflets move very slow, but are great reflectors (small doppler shift, high amp)
3) To void them from the spectral analysis, the Wall filter (high pass filter) filters out all low doppler shift, high amplitude signals. |
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Term
| In a very tight stenosis you would _____ the wall filter. |
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Definition
- Decrease
- You would decrease the wall filter, because in a very critical stenosis the flow is very slow moving, so you want to catch that slow velocity. |
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Term
| What does the spectral analysis waveform look like with a high wall filter? |
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Definition
| You just see the top of the triangles above the baseline, with nothing connecting the triangles to the baseline since the lower velocities are filtered out. Also since all high amplitude signals are filtered out the waveform is a bit dim. |
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Term
| What is the purpose of the FFT? |
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Definition
| The Fast Fourier Transformer (FFT) converts complex doppler signal into individual frequencies that are later used to calculate individual velocities. |
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Term
| What are the steps in FFT? |
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Definition
1) Signal is seperated into individual frequency components
2) Signals are converted from time domain into frequency domain
3) blood velocities are calculated for each frequency and signal is converted into time doman. |
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Term
FFT
There are many __1____ velocities within the _2______ __3___ (___4____ or __5___ flow)
Therefore the ___6___ carries multiple __7______ ___8__, each with a unique ___9____. |
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Definition
1. Different
2. Sample
3. Volume
4. Parabolic
5. Turbulent
6. signal
7. doppler
8. shifts
9. amplitude |
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Term
| What did Jean Fourier prove and when? |
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Definition
- In 1822, Jean Fourier proved that any complex function can be broken into sine and cosine functions, known as FOURIER COMPONENTS.
- FFT makes this conversion of a complex function into fourier components (different sets of waves with different frequencies and velocities) |
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Term
FFT:
- Note that each frequency component has different amplitude, where... |
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Definition
- A drop in frequency represents velocity of the relative motion
- Amplitude represents the scattering capabilities of the RBC's (or the amount of blood) |
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Term
FFT:
How are frequency and amplitude related? |
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Definition
- Amplitude and frequency are not related. Therefore low amplitude from the RBC (amt of RBC's) does not affect the drop in f (velocity).
- Essentially just saying low amplitude doesn't mean a low velocity. |
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Term
| What is the second type of conversion made in a FFT? |
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Definition
- The FFT converts the domain on the graph from A, (t) ----> A, (?f)
- Each drop in f is used to calculate the velocity of the blood. |
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Term
| What is the final conversion in the FFT process? |
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Definition
- The FFT then converts the signal from:
> a frequency domain: A (?f) grpah
> into time domain: delta f (t) or vblood (t) graph |
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Term
| Amplitude is represented by what after an FFT conversion? |
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Definition
| Amplitude is represented by the BRIGHTNESS of a pixel. |
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Term
| Explain compression (dynamic range) |
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Definition
- Compression (dynamic range) of the signal is often done to decrease the range from the brighest to the dullest.
- When high A is not compressed, it is hard to differentiate between a High and medium A because both appear white.
- When the signal is adequately compressed the high a is white and the medium A is grey. |
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Term
| What does a spectral analysis waveform look like when there low Dynamic range (compression)? |
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Definition
| Waveform is black and white. |
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Term
| What does a spectral analysis waveform look like when there is high Dynamic range (compression)? |
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Definition
The waveform appears to be quite gray.
(A good way to think of it is like all the ranges of colors black,white, and all their shades have been compressed into just one color, grey) |
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Term
| What does the post process (gamma) function allow us to do? |
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Definition
- Allows us to choose different grayscale curves.
(Different shades of gray) |
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Term
| What does sweep speed do? |
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Definition
| Adjusts the scale long the x axis. |
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Term
| What would increasing the sweep speed do? |
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Definition
- By increasing the sweep speed you're making the scale smaller to display fewer cardiac cycles.
- Good to view waveforms from patients with fast heart rates such as tachycardia or babies. |
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Term
| What would decreasing the sweep speed do? |
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Definition
- By decreasing the sweep speed, the scale becomes larger, and will display more cardiac cycles.
- Good for Bradycardia |
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Term
| What does Doppler map do? |
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Definition
| - Changes the color of the spectral analysis waveform. |
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Term
| What does the invert function do? |
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Definition
| - Allows the sonographer to choose whether the flow towards to the probe is displayed above or below the baseline. |
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Term
| What direction is the flow towards the probe displayed after the invert function has been activated? |
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Definition
| - Flow towards the probe is displayed under the baseline with the invert function on. |
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Term
| What are you adjusting when you adjust the baseline? |
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Definition
| - When you adjust the baseline you are adjusting the fraction displayed towards or away. |
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Term
| What will you see with a high baseline? |
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Definition
| A high aliasing artifact. |
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Term
| What does pixel ratio affect? |
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Definition
- The pixel ratio affects the height and width of a pixel.
> Increase the pixel ratio for a SMOOTHER image.
> Decrease the pixel ratio for better ACCURACY. |
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Term
| 5 things you should adjust first: |
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Definition
1) Get vessel in long axis
2) Gate positioning
> The angle should be no more than 60 degrees and parallel to and in the direction of the flow.
> Gate size should be no more than 2/3 - 3/4 of the vessel diameter.
3) Gain
4) Scale (PRF)
5) Filter |
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