Doppler Shift = (2 x Velocity of blood x transducer frequency x cos θ)/ Propagation speed
*θ = Angle of Insonation (angle of incidence between the ultrasound beam and the direction of flow)
So the Doppler shift is mainly related to TWO things:
- The Velocity of the blood cells
- The Angle of Insonation
Below is a figure detailing how the Doppler Shift is used and how the angle of insonation is extremely important in what the transducer will detect as the amount of flow/movement. For any type of Doppler you want the flow/movement to be going directly towards your probe (zero degrees) as you move more towards a 90 degree angle there will be no flow detected by the ultrasound machine.
(Note: I’m using the velocity of blood as the example here. But the same principles apply if you are measuring muscle movement using tissue doppler.
So the most important thing you can do to improve your Doppler technique for any mode is to make sure that the movement of whatever you are measuring is parallel to your ultrasound probe as much as possible (zero degrees). Anything above 25-30 degrees will significantly underestimate your measurements. And if you are perpendicular, the cosine of 90 degrees = 0 and the ultrasound Doppler will read no flow or movement.