Term
| What's a big difference between imaging studies and doppler studies? |
|
Definition
- Imaging can only use PW (requires listening time)
- Doppler studies can use both PW and CW. |
|
|
Term
CW doppler:
A ______ electrical signal is produced by the pulser. |
|
Definition
|
|
Term
| How often is the crystal stimulated in CW doppler? |
|
Definition
| - The crystal is stimulated all the time. |
|
|
Term
| The frequency of the electrical signal = ? |
|
Definition
| The frequency of the sound. |
|
|
Term
| Explain the communication between crystals in a CW doppler. |
|
Definition
Since one crystal cannot talk and receive at the same time:
- One crystal is always "talking"
- The second crystal is always "listening" |
|
|
Term
| What is done to allow overlap between the lines of sight? |
|
Definition
| The two crystals are slightly angled. |
|
|
Term
| What is the area of overlap called between the two lines of sight from the crystals in a CW pulser? |
|
Definition
| - The area of overlap is called the FOCAL VOLUME or SENSITIVITY. |
|
|
Term
| How big is the focal volume for CW transducers? |
|
Definition
| - The focal volume for CW transducers is VERY LARGE, and because of this there is virtually NO DEPTH LIMITATIONS. |
|
|
Term
T or F
CW has a big PRFmax. |
|
Definition
False:
CW does not have a PRFmax at all due to the fact it is on all the time and there is no interruption between sending of pulses. |
|
|
Term
| What are the limitations of scale for CW? |
|
Definition
| There are no limitations of scale for CW and therefore NO ALIASING. |
|
|
Term
| How do we tell what depth an echo is returning from with CW doppler? |
|
Definition
| - There is NO way to tell what depth the echo is coming from with CW doppler because there is NO LISTENING TIME INVOLVED. |
|
|
Term
| What are the "pros" to CW doppler? |
|
Definition
- No Aliasing
> Due to the fact that there are no limitations for the scale (PRFmax) and you only get aliasing if the returning echo's have a frequency higher than the scale.
- Cheap and simple
> You only need a minimum of 2 crystals
- Good Sensitivity (Focal Volume)
> Large focal volume = good sensitivity |
|
|
Term
| What are the "cons" of CW doppler? |
|
Definition
- Unknown Depth
> Because the wave is continuous there is no listening time; listening time is required to know the depth.
- No imaging
> Only PW doppler can produce an image
- Detects any motion
> Because it has such good sensitivity it detects any motion and this can work against you since it can pick up the smallest movements such as bowel gas, wall motion, etc. |
|
|
Term
| How does a PW doppler transducer work? |
|
Definition
| - A PW doppler has a single transducer that first sends the ultrasound pulse and then receives the echoes. |
|
|
Term
| Where is the doppler shift received from? |
|
Definition
| - In a PW transducer the doppler shift (change in frequency due to a change in motion between the source and receiver) is only received from a specific location known as the SAMPLE VOLUME or GATE. |
|
|
Term
| Explain the steps in sending a pulse from a PW transducer. |
|
Definition
1) An electrical spike is generated by the pulser.
2) A single crystal is stimulated to produce an acoustic pulse.
3) The frequency of the sound wave depends on the thickness of the crystal
> Fpw = Vcr/2th
> A large crystal produces smaller frequency
> A smaller crystal produces a higher frequency
4) A Long pulse (5-10 cycles) is produced to get uniform frequency. |
|
|
Term
| After the pulse is sent out to the _____, the system waits for the ______ to return from a ______ ______. |
|
Definition
- tissues
- echoes
- certain depth |
|
|
Term
| What determines the location of the sample volume (AKA gate)? |
|
Definition
- Location (Waiting)
- The pulser sends out the signal and the time it takes before it receives the echo back determines the location or depth of the sample volume (gate).
- Ex. The pulser sends a signal and WAITS 26 microseconds before it hears the echo. At what DEPTH is the gate?
> 2cm |
|
|
Term
| What determines the size of the sample volume (gate)? |
|
Definition
- Size (listening)
- The size of the gate is determined by how long the pulser is listening to the returning echo for.
For example: The transducer is listening to the returning echo for 13 microseconds. What is the size of the sample volume (gate)?
> 1cm |
|
|
Term
| What is another name for Range Ambiguity? |
|
Definition
|
|
Term
T or F
Range Ambiguity never exists. |
|
Definition
False
Range Ambiguity ALWAYS exists. |
|
|
Term
|
Definition
1) 1st pulse is generated and travels to the location of the gate (true gate)
2) Once there part of the pulse is reflected back to the probe and a 2nd pulse is generated.
3) The other part of the pulse continues on and is reflected at an interface twice as deep as the original gate location. This name of this second gate is the ambiguous gate.
4) The reflection from the ambiguous gate returns to the transducer simultaneously with the second pulse.
5) The echo from the ambiguous gate is much weaker due to attenuation. |
|
|
Term
| What can create Range Ambiguity artifacts? |
|
Definition
- There is motion at the ambiguous gate locations (2d, 3d, etc.)
- The true, and therefore, ambiguous gates are relatively shallow the echoes have to travel less of a distance between the ambiguous gates and therefore attenuate less. |
|
|
Term
| What determines the appearance of a range ambiguity artifact? |
|
Definition
| - The appearance of a range ambiguity artifact depends on the MOTION at the ambiguity gate. |
|
|
Term
| How should one check if range ambiguity artifact is present? |
|
Definition
- To check if range ambiguity artifact is present they should change the scale:
> If changing the scale increases/decreases the size in the spectral waveform, but the characteristics of the waveform doesn't change then no artifact is present.
> If changing the scale causes major changes in the characteristics of the waveform, then artifact is present. |
|
|
Term
| Summary of avoiding range ambiguity and aliasing |
|
Definition
To avoid aliasing: 2deltaf < PRFmax
To avoid range ambiguity: PRFmax < v/2d
Therefore:
2deltaf < PRFmax < v/2d |
|
|
Term
|
Definition
| - HPRF or High PRF is a function that utilizes the range ambiguity phenomenon to avoid aliasing. |
|
|
Term
|
Definition
- Let's assume the velocity in the blood vessel is very high and generates aliasing (big doppler shift is created due to high velocity which makes it bigger than the scale creating aliasing.
- You max out the scale, but aliasing is still present.
- Further increase of the PRF will not provide adequate listening time to receive the echoes from a desirable depth.
- When the HPRF is in use, PRF is increased AS IF the sample volume is at 1/2 of the desirable depth (we'll call this the true gate). The vessel of interest is then at the ambiguous gate.
- The echoes are received from the true gate and the ambiguous gate due to range ambiguity.
- High velocities are resolved w/o aliasing due to an increase of PRF. |
|
|
Term
CW Doppler PW Doppler
Min # of crysals:
Range Ambiguity (Depth Uncertainty):
Aliasing:
Imaging: |
|
Definition
CW Doppler PW Doppler
Min # of crysals: 2 1 (will suffice)
Range Ambiguity (Depth Uncertainty): Large Small
Aliasing: Never Sometimes
Imaging: Not possible Possible |
|
|
Term
1. Imaging 2. PW Doppler
Pulse Duration:
Optimal Angle:
Frequency:
Sensitivity: |
|
Definition
1. Imaging 2. PW Doppler
Pulse Duration: 1. short 2. long
Optimal Angle: 1. 90 degrees to ensure max reflection and no refraction 2. 0 degrees is optimal to ensure maximum doppler shift.
Frequency: 1. High f is desirable to improve LARD. 2. Low f is desirable to avoid aliasing.
Sensitivity: 1. Not very sensitive to low amplitudes that are associated with "noise." 2. Very sensitive to low amplitude signals. RBC are poor reflectors. |
|
|
