Doppler Imaging uses the change in observed frequency of the ultrasound signal to calculate the speed of flow:

The angle needs to be known if v is to be calculated accurately. This is not easy and calculations of v are rarely performed. Rather comparisons are made between v in comparable arteries and veins to detect blockages.

The factors that should be considered when selecting a frequency for Doppler ultrasound imaging are:-

• resolution (better for larger frequency (about 1mm in soft tissue for 5MHz))
• depth of penetration (large frequency cannot probe as deeply).

Click here for an article on its use