Deep vein thrombosis (DVT) occurs when a blood clot forms in a vein deep inside a muscle in your body. It usually happens in legs, but can also develop in your arms, chest, or other areas of your body. The blood clot can block your circulation or lodge in a blood vessel in your lungs, heart, or other area. Venous ultrasound is done to check for such condition and evaluate the functionality of the veins.