The compression ultrasound study will be performed by post-doctoral research fellows, who will be formally trained for lower extremity ultrasound, under direct supervision. Diagnostic Criteria and Interpretation · The gray scale compression sonographic findings of acute DVT are based on direct visualization of the thrombus and lack of venous compressibility. Altered luminal echogenicity and alterations in flow characteristics are secondary signs. · Visualization of thrombus is variable, depending on the extent, age, and echogenicity (ranging from anechoic to complex) of the clot. · in case of thrombosis, Each Doppler image should demonstrate spontaneous and phasic flow. · Loss of phasic variation suggests proximal obstruction (intrinsic or extrinsic). · Prominent pulsations can be due to right sided heart failure and may be a manifestation of tricuspid valve regurgitation. Normal - Vein is totally compressible. Duplex Doppler reveals spontaneous phasic waveform Acute DVT - Vein is non compressible, but deformable and smooth. Vein is generally distended, with or without luminal echoes. The thrombus can be free floating. Spectral Doppler may be normal, continuous, or not present