Final answer:
In color Doppler ultrasound with a sector-shaped screen, users can modify the steering, size, location, and shape of the color box but cannot modify the velocity of the moving blood cells.
Step-by-step explanation:
When using color Doppler ultrasound and the screen is sector shaped, what can be modified includes the steering of the color box (a), the size and location of the color box (b), and the shape of the color box (c). These modifications help in optimizing the Doppler effect visualization of blood flow within the vessels. The Doppler effect accurately measures the velocity of the moving blood cells by analyzing the shift in frequency of the ultrasound waves reflected off the moving red blood cells. It is important to note that the user does not modify the velocity of the blood cells; this value is measured by the Doppler ultrasound. Adjusting the steering angle can alter the region of interrogation of the vessel, while changing the size and location of the color box can focus on specific areas within the vessel that are of interest. The shape adjustment allows for a more efficient angle of insonation corresponding to the shape of the vessel or anatomical area being examined.