CW Doppler & displays

Question 1

In a CW doppler instrument, what does the two crystals do?

Answer 1

one transmits beam and one recieves echoes.

Question 2

What is the advantage of a CW doppler instrument?

Answer 2

It has a large focal zone.

ie. large sample volume.

Question 3

What is the disadvantage of a CW doppler instrument?

Answer 3

It hears everything in its path

Question 4

What arre the typical transducers used?

Answer 4

4Mhz and 8Mhz

Question 5

Can CW doppler determine location?

Answer 5

Yes.

Question 6

What does a CW doppler measure?

Answer 6

It documents changes in velocity over time.

Question 7

What direction is the right speaker demonstrating?

Answer 7

Flow towards the brain.

Question 8

Which direction is the left speaker?

Answer 8

Flow away from the beam.

Question 9

Why isn’t the height of an analog wave not a siagnostic criteria on its own?

Answer 9

The waveform height shows the f shift, so the heigh of the waveform depends not only on the velocity of the blood flow, but also the transducer frequency, AND the angle the probe is held.

*lower angle= higher waveform*

*lower angle= high cosine*

Question 10

What are the 3 CW dopple controls?

Answer 10

1. Transducer choice (4 MHz vs 8MHz)
2. Gain/size on machine.
3. Wall filter.

Question 11

What is the limitation of CW Doppler?

Answer 11

Range Ambiguity.

Question 12

What does range ambiguity mean?

Answer 12

It means that thr location of flow cannot be determined due to no image.

Question 13

What does range resolution mean?

Answer 13

It means the location of flow is clear.

Question 14

What is range resolution also known as?

Answer 14

• Range specificity
• Freedom from range ambiguilty

Question 15

Why is range ambiguous with CW but not with PW?

Answer 15

Sample volume allows for specific placement for sampling doppler signal with PW. But there’s a large sample volume with CW and its size and depth cannot be changed.

Question 16

Which is ambiguous: PW or CW?

Answer 16

CW

Question 17

How is CW: analog tracing created?

Answer 17

It is created with a zero-crossing detector.

Question 18

What does a CW: analog tracing display?

Answer 18

It essentially displays an average of the change of frequency over time.

Question 19

In the CW: analog tracing, what is averaged in the display?

Answer 19

• Foward and reverse flow are averaged.
• turbulence is averaged.

Question 20

What is the zero-crossing detector?

Answer 20

IT is an instrument that creates the analog tracing for a CW Doppler.

Question 21

What does the zero-crossing detector count?

Answer 21

It counts the number of times an echo wave crosses the zero line.

Question 22

How is a zero-crossing detector displayed?

Answer 22

It is displayed in an analog format

Question 23

Why is it important to have a wall filter?

Answer 23

It is mportant because without a wall filter, the doppler would pick up motion from valves, wall motion, that cause low frequencies since their motion is slower than blood flow.

Question 24

What does a wall filter do?

Answer 24

It filters out low frequencies, leaving only frequencies from blood in display.

Question 25

what is the wall filter also known as?

Answer 25

“High Pass Filter” because only filters with high frequencies can pass through)

Question 26

What is the walll filter set at?

Answer 26

50 Hz or less.

Question 27

When is the wall filter usually needed?

Answer 27

It is usually needed in abdominal vasculature.

Question 29

What is the frequnecy of most echo signals?

Answer 29

10-1600 Hz

Question 30

What should you set the filter for venous (low)?

Answer 30

less than 50 Hz

Question 31

What should you set the filter to for arterial (high)?

Answer 31

50-100 Hz

Question 32

What effects wall filters?

Answer 32

transducer.

Question 33

What happens if the wall filter is set too low?

Answer 33

electronic saturation artifact may occur.

Question 34

High velocoties with spectral broadening often need ______ to see waveforms.

Answer 34

INCREASED GAIN

Question 35

What is spectural analysis?

(fourier analysis)

Answer 35

It is a computer process used to break down a complex signal (ie sound) into its frequency components.

Question 36

What is “Fast Fourier Transform”?

Answer 36

It is a devicee that performs spectral analysis.

Question 37

All echoes that return to the transducer from the sv are have ________.

Answer 37

Multiple frequencies.

Question 38

What is demodulation?

Answer 38

It is the process of calculating the frequency shift.

Question 39

What does demodulation extract?

Answer 39

It extracts the lower doppler shifted frequency off of the higher transducer frequency that is reflected.

ie. a 7.000 MHz transducer frequency is reflected as 7.004 MHz. The demodulation extracts the .004 MHz or 4 kHz.

Question 40

What is phase quadrature?

(aka quadrature detection)

Answer 40

It is a signal processing technique to seperate positive from negative doppler frequencies.

Question 41

Flow signals toward the doppler beam (positive) are displayed how?

Answer 41

Above the baseline.

Question 42

Flow signal away from the doppler beam (negative) are displayed as what?

Answer 42

Below the baseline

Question 43

What can control power output?

Answer 43

The operator.

Question 44

What does a large power do to the transducer?

Answer 44

It increased the power applied to the transducer.

Question 45

What does the large output power increase?

Answer 45

It increases the ampitude of the transmitted beam and of the echoes. It also increases the acoustic exposure to patient.