If there is difficulty differentiating artery from vein, color Doppler may be helpful, because it will demonstrate pulsatile arterial flow in comparison with the continuous flow of the vein. Pulse-wave Doppler can also be useful, because it demonstrates the peak and trough quality of arterial waveforms compared to the continuous undulating wave form of veins.

• Compression technique: At each point, apply firm compression perpendicularly to achieve complete collapse of the vein. The lumen of the vein must disappear completely in order to exclude the presence of a clot. (See image 3, image 4.) A clot may be seen as echogenicity within the lumen. However, in many instances the only evidence of a DVT will be the inability to compress the vein fully.
• Other signs of patency: Color-flow Doppler may reveal a persistent filling defect or thrombus in the color column of the lumen. (See image 5, image 6.) Squeezing the calf will produce the augmentation of venous flow to complete color filling of the lumen. However, using color Doppler and augmentation techniques is time-consuming and has not been shown to improve the detection of DVT significantly over venous compression.⁷
• Scanning the common femoral vein: The inguinal area is generally proximal to the junction of the great saphenous and common femoral veins. Overlying tissue or pannus may have to be lifted or moved to expose this most proximal site. Move the probe distally until you identify the great saphenous vein joining the common femoral vein medially and the common femoral artery coursing laterally. (See image 7.) Apply firm pressure downward to compress both the common femoral and proximal great saphenous veins. Although the great saphenous vein is a superficial vein, a clot identified within its proximal portion can propagate into the deep venous system and will require the same treatment as a deep vein thrombus. A patent vein will collapse completely.
• Scanning the proximal superficial femoral and deep veins: After compression of the common femoral and great saphenous veins, slide the probe distally to the femoral triangle, following the femoral vein. The image on the screen will show the common femoral vein medially and the superficial and deep femoral arteries laterally. (See image 8.) Continue to scan distally until you see the common femoral vein split into superficial femoral and deep femoral tributaries. Apply firm pressure downward to compress both veins.
• Scanning the popliteal vein: Place the probe behind the knee high in the popliteal fossa, where the popliteal vein and artery are located. The popliteal vein lies superficial to the artery. You may need to steady the outside of the knee with your nonscanning hand to stabilize the leg and facilitate compression. Apply firm pressure to collapse the popliteal vein. (See image 9, image 10.)

Pitfalls

• Failing to place the probe perpendicular to the skin to achieve direct pressure in compressing vessel walls.
• Mistaking an acute clot with chronic clot. Consider MRI to distinguish acute from chronic DVT.
• Assuming that visualization of the vessel lumen equals absence of DVT. An acute thrombus can be anechoic. Confirm patency by complete apposition of the anterior and posterior walls of the vessel.
• Technical difficulty in obese patients or those with significant lower extremity edema.
• Scanning the vessel longitudinally only. During compression the probe may slide off of the vessel wall and may lead to false negative results.
• Ilio-femoral DVT cannot be evaluated using ultrasound.
• Confusing a lymph node or Baker’s cyst with DVT. The transducer can be moved proximally and distally to identify the edges of the lymph node or cyst.
• Failure to understand the limitations of the three-point compression technique—calf vein thrombi and the rare segmental clot may be missed. Use Wells clinical prediction rule and d-dimer assays to determine your pre-test clinical probability. (See clinical algorithm.) If you have moderate to high suspicion for DVT but a negative initial screening exam, have the patient return within 1 week for a repeat ultrasound or perform contrast venography.⁸

For best results, optimize the patient position and scan the unaffected extremity for comparison. Consider scanning the legs of the patient presenting in cardiac arrest with pulseless electrical activity, because a dislodged clot resulting in a pulmonary embolism may direct emergent resuscitative care.