Final answer:
To improve color Doppler signal detection of the main portal vein, one should decrease the system pulse repetition frequency, which is the correct adjustment to increase the sensitivity for slower blood flow.
Step-by-step explanation:
When performing a Doppler study of the hepatic venous system and you cannot obtain a color Doppler signal from the main portal vein, one appropriate step to take would be to decrease the system pulse repetition frequency (PRF). This adjustment can help increase the sensitivity of the color Doppler, making it easier to detect slower blood flow that might not have been registered at a higher PRF setting.
In contrast, increasing the color dynamic range or high pass filter could filter out low-velocity flow. Increasing the Doppler angle of incidence may lead to inaccurate measurements due to the cosine effect, and decreasing the color gain could further diminish the visibility of blood flow signals. Therefore, option C) Decrease the system pulse repetition frequency is the correct action to take in this situation.