Final answer:
The operator control used to adjust image contrast in medical imaging is the Overall gain, which modifies image brightness and affects contrast perception.
Step-by-step explanation:
The operator control that can be used to adjust the image contrast in medical imaging, such as ultrasound, is the Overall gain. This function allows for the modification of the brightness of the image, which impacts how contrast is perceived. Unlike the focal zone, which adjusts the resolution at a specific depth, or the dynamic range, which controls the range of shades between black and white, the overall gain affects the amplitude of the echo signals received, directly influencing the contrast of the entire image. The output power affects the strength of the sound waves that are emitted, but this is more about the signal strength than contrast itself.