Final answer:
The sonographer should assume that the angle of incidence is 90 degrees and adjust it to less than 90 degrees relative to the blood flow direction to detect color flow with Doppler ultrasound. Alternative actions such as using Power Doppler or considering equipment issues should only be explored after angle adjustment has been tried.
Step-by-step explanation:
If the sonographer does not see any color flow when color Doppler is applied, but could see movement of blood cells within the vessel on grayscale imaging, the operator should first assume that the angle of incidence is 90 degrees, and needs to be changed. Doppler ultrasound is highly angle-dependent and when the transducer is perpendicular to the blood flow (90 degrees), no color flow will be detected because the Doppler shift is virtually zero at this angle. The sonographer should adjust the transducer to create a more favorable angle (less than 90 degrees) relative to the direction of blood flow to improve the color Doppler signal.
Choosing an alternative like Power Doppler, assuming a blood vessel obstruction, or calling maintenance should only be considered after repositioning the transducer fails to resolve the issue. Power Doppler can be more sensitive to low flow or flow at suboptimal angles, but it does not depict flow direction. However, if movement is observed in grayscale, it is less likely to be an obstruction or a sensitivity issue with the equipment.