Autocorrelation is the signal processing method that makes color Doppler imaging possible. Unlike spectral Doppler, which analyzes the full frequency spectrum of returning echoes, autocorrelation works by comparing consecutive ultrasound pulses and calculating the average phase shift between them to determine the mean velocity and direction of blood flow. This technique is much faster than full spectral analysis, which is why color Doppler can display real-time flow information across an entire region of the image simultaneously. The tradeoff is that autocorrelation provides only mean velocity estimates rather than the detailed velocity spectrum that spectral Doppler offers, but its speed makes it indispensable for quickly identifying vessels and flow abnormalities.
Sonography Term