Angle of Doppler beam in relationship to direction of blood flow

The diagram illustrates a situation in which the interrogating or insonocating beam lies at an angle to the flow of blood. Ideally, the Doppler beam should be placed parallel to blood flow. When the beam does not lie parallel, it is possible to introduce a correction into the calculation of flow velocity by measuring the cosine of the angle of interrogation and introducing this value into the Doppler equation.