Where \(f_D\) is the measured Doppler frequency shift, \(f_0\) is the nominal ultrasound frequency, \(c\) is the speed of sound and \(\theta\) is the angle between the ultrasound beam and the flow direction at the location of the velocity measurement. Equation 1 is used to convert the measured frequency shift to velocity. As equation \ref{velocity_measurement} indicates, it is important to have precise knowledge of the angle between the ultrasound beam and the flow direction in order to calculate the velocity. This angle is commonly referred to as the “Doppler angle”. In current clinical practice, the ultrasound operator (sonographer) would manually specify the angle between the Doppler ultrasound beam and the vessel orientation while performing the ultrasound exam in order to ensure that the correct flow velocities are estimated. This is clinically referred to as angle correction, and is one of multiple steps in the clinical workflow for Spectral Doppler. Angle correction refers to adjustment of the Doppler angle and is used to calibrate the velocity scale for the angle between the US beam and the blood flow being measured.