Final answer:
To increase the amount of color Doppler superimposed over a 2D image in ultrasound imaging, the threshold setting should be decreased, allowing visualization of lower velocity blood flows and thus enhancing Doppler sensitivity.
Step-by-step explanation:
In Color Doppler ultrasound imaging, increasing the amount of color Doppler signal superimposed over a 2D image can be achieved by adjusting various system controls. The correct choice for increasing the color display is decreased threshold. A lower threshold setting allows more color to appear on the image by including lower velocity blood flow, essentially making the Doppler sensitivity higher. This means more flow information, even from slower-moving blood, is displayed on the 2D image. This is opposite to increasing the threshold, which would filter out this low-velocity information, and increasing persistence or wall filter, which are adjustments that do not directly increase the visibility of color Doppler information over the 2D image. Persistence relates to the smoothness of the motion in the image and the wall filter is used to eliminate low-frequency high-amplitude signals from moving tissue rather than blood flow, thus neither would increase the color Doppler specifically.