Frequency or phase shift when using pulsed wave doppler

[mkh004]

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?

 

[MagicianT]

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

 

[jim hardy]

Are you measuring distances less than a wavelength?

 

[olivermsun]

You typically are trying to measure position shifts of less than a wavelength between successive pulses.

 

[alphy]

Hi there,

First of all, it's great that you're interested in understanding the physics behind pulsed wave doppler measurements. I can provide some insight on the frequency and phase shift in this technique.

In pulsed wave doppler, a high frequency sound wave is transmitted into the body and then reflected back by moving blood cells. This reflected wave has a slightly different frequency due to the Doppler effect, which is caused by the motion of the blood cells. This change in frequency is measured and used to calculate the velocity of the blood flow.

However, in order to measure the Doppler shift accurately, the transmitted and reflected waves must have a consistent phase relationship. In other words, the peaks and troughs of the waves must line up in order for the frequency to be accurately measured. This is because the Doppler equation uses the difference in phase between the transmitted and reflected waves to calculate the velocity.

If there is a phase shift between the waves, it can lead to errors in the velocity measurement. This is why in pulsed wave doppler, the focus is on measuring the phase shift rather than the change in frequency. By measuring the phase shift, any errors caused by changes in frequency can be corrected for.

I hope this helps clarify why phase shift is used in pulsed wave doppler measurements. Keep up the curiosity and good luck with your studies!