Doppler Ultrasound
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