Frequency or phase shift when using pulsed wave doppler

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Discussion Overview

The discussion revolves around the physics of pulsed wave Doppler imaging, particularly focusing on the measurement techniques involved, specifically the distinction between measuring frequency changes and phase shifts in ultrasound applications for cardiac strain assessment.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification

Main Points Raised

  • One participant questions why pulsed wave Doppler measurements are said to measure phase shifts rather than frequency changes.
  • Another participant explains that measuring frequency requires counting pulses over a specific time period, which introduces limitations and potential errors, suggesting that phase measurement allows for greater precision.
  • A participant raises a question regarding the measurement of distances less than a wavelength, implying a relationship between measurement techniques and the wavelength of the ultrasound waves.
  • Another participant notes that the goal is to measure position shifts that are less than a wavelength between successive pulses, indicating a technical aspect of the measurement process.

Areas of Agreement / Disagreement

The discussion presents multiple viewpoints regarding the measurement techniques in pulsed wave Doppler imaging, with no consensus reached on the primary method of measurement or its implications.

Contextual Notes

Participants express varying assumptions about the limitations of frequency measurement versus phase measurement, as well as the implications of wavelength in the context of ultrasound imaging.

mkh004
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Hi,
I'm a medical student who is using ultrasound to measure cardiac strain. I'm trying to read up a bit on the physics behind doppler imaging (I did AP physics in high school), and I have a question for some genius out there. I keep reading that in pulsed wave doppler measurements, one does not measure the change in frequency, but a phase shift of the waves. Why?
 
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To measure a freq. you have to count pulses over specific period of time , "gate" time. This period can't be too long, I'd think at least no more than 100 milliseconds for biological process, to keep update rate 10 Hz or so. This implies error in measurements, +-10 Hz. But if you measure a phase, there is no such limitation, and precision in estimation of deviation a frequency 360 times better for phase error 1 degree
 
Are you measuring distances less than a wavelength?
 
You typically are trying to measure position shifts of less than a wavelength between successive pulses.
 

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