MODEL RELEASED. Doppler ultrasound of pregnant woman. A doctor holds an ultrasound emitter while scanning a pregnant woman. The video screen shows blood flow in the chest cavity of the unborn baby. The vertebrae and ribs can be seen, and blood flow appears red the area of the heart. The doppler technique was refined by linking ultrasound equipment to a numeric colour video to obtain such high-definition images. Doppler ultrasound uses high-frequency sound waves to detect body tissues of different density, diagnose disease, and register moving particles like blood.