What does 'red' indicate on an ultrasound image?

In ultrasound imaging, color Doppler techniques are used to represent the direction of blood flow. The use of color on ultrasound images provides critical information about the motion of structures being examined. When 'red' appears on the ultrasound, it typically indicates that blood or another substance is moving toward the ultrasound probe. This color coding is particularly useful in vascular studies, as it helps the clinician assess the direction of blood flow in arteries and veins, contributing to the evaluation of cardiovascular health.

The other options do not accurately describe the meaning of 'red' on an ultrasound image. Movement away from the probe is generally indicated by a different color, while the presence of air and fluid accumulation would not specifically correspond to 'red' in the context of color Doppler. Thus, interpreting 'red' as movement toward the probe is a fundamental aspect of understanding ultrasound imaging and its application in medical diagnostics.