Term
| What format is the signal received with? |
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Definition
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Term
| What format does the AD converter convert the signal into? |
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Definition
| The AD converter converts it into digital format. |
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Term
| What does the number of sampling depend on? |
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Definition
| The amount of memory available. |
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Term
| What is the Nyquist limit (theorem)? |
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Definition
| The Nyquist theorem states that for accurate reconstruction of a waveform, each cycle must be sampled at least two times. |
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Term
| Explain how the Velocity of blood can be measured low. |
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Definition
- In the AD converter, the signal is sampled to be re-created in digital format. The rate of sampling = the PRF.**
- Later, the DA converter reconstructs the waveform. The Nyquist limit of 2 samples per cycle should be met to accurately reconstruct the wave.
- If velocity of blood is very high, then the doppler shift will be very high as well. There may not be adequate sampling per cycle to recreate a high doppler shift.
> The recreated doppler shift will be lower than the actual doppler shift.
> Velocity of the blood will be measured low (Vblood ~ doppler shift) |
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Term
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Definition
| - Aliasing is an ARTIFACT due to an INSUFFICIENT SAMPLING RATE. It occurs when the frequency shift is greater than 1/2 the PRF (Nyquist frequency). |
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Term
| What can be done to avoid aliasing? |
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Definition
- Increase the PRF (scale)
- Increase the doppler angle (no bigger than 60 degrees)
- Decrease depth if possible
- Decrease frequency of the transducer if possible.
- Decrease baseline.
- Use CW doppler |
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Term
| What is the catch 22 with obtaining a doppler signal from a deep vessel? |
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Definition
- You want to use a lower frequency transducer (if you can)
- But a high frequency transducer:
> better resolution
> low scale and penetration |
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Term
| Aliasing Vs. Spectral Broadening |
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Definition
- Often confused
- Often occur at the same location. |
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