The frame rate in color flow imaging is influenced by several factors, one of the most significant being line density. Line density refers to the number of ultrasound lines used to create an image. Increasing line density improves spatial resolution but requires more time to acquire each frame, thereby reducing the frame rate. Other factors such as filter selection, dynamic range, and variance map selection can affect the quality of the color flow image, but they do not have as direct an impact on frame rate as line density does.
Reference: ARDMS Sonography Principles and Instrumentation, Chapter on Color Doppler Imaging.