Ultrasonography is considered to be the most non-invasive and reliable diagnostic method for evaluating deep vein thrombosis. Compression ultrasonography (compression method) is the simplest ultrasonic technique for diagnosing venous thrombus. Recent advanced ultrasonic examinations can reliably diagnose both proximal deep vein thrombosis and calf vein thrombosis. In flow ultrasonography, color Doppler and pulsed Doppler signals are used to produce the images. Color Doppler results in a display of flowing blood as a color overlay to the gray-scale ultrasound image, which makes it easier to identify the veins. The flow in normal veins has a phasic pattern with respiration and can be augmented by a manual milking technique. When the
phasic pattern or response to milking is absent, it indicates the presence of venous obstruction. The defects of color signals indicate venous thrombus.