Vascular Sonography: What Actually Works in Practice
Most vascular sonography textbooks present a clean, ideal workflow. Real patients rarely cooperate. You will encounter calcified plaques that create shadowing so dense you cannot see through them. You will find patients with bilateral femoral deep vein thrombosis where distinguishing acute from chronic thrombus becomes a matter of millimeters of compressibility and signal augmentation response. The technique matters far more than knowing the textbook criteria. Before you touch the transducer, set up your machine correctly. Gain should be balanced across the sector. A common mistake is setting color scale too high to avoid aliasing, which then makes low-velocity venous flow invisible. Set your velocity scale for venous studies around 15 to 20 cm per second. For arterial work, start at 60 to 80 cm per second and adjust as needed. Pulse repetition frequency matters here. If your PRF is too low, you get aliasing. If it is too high, you miss low-flow signals entirely. I spent three years doing carotid duplex scans before I learned to trust my eigenvalues over my instincts. The Nanda criteria and the Vancouver consensus guidelines give specific velocity thresholds. A peak systolic velocity above 125 cm per second with an end-diastolic velocity above 40 cm per second typically indicates 50 to 69 percent stenosis. Above 230 cm per second peak systolic velocity with an EDV above 100 cm per second suggests greater than 70 percent stenosis. But these numbers shift depending on your machine, your insonation angle, and the patient's hemodynamic status.
There was a case where a patient's internal carotid artery showed a peak systolic velocity of 180 cm per second. The numbers suggested moderate stenosis. I followed up with a post-stenotic spectral broadening assessment and noticed turbulent flow patterns consistent with near-total occlusion. The velocity was actually lower because the flow volume was severely reduced, not because the stenosis was mild. This is what we call a hemodynamically insignificant velocity reading masking a critical lesion. The workaround is always to examine spectral morphology, not just the numbers. Look at the waveform shape. A damped, tardus-parvus waveform distal to a stenosis tells you something the peak velocity alone cannot. For venous studies, compression ultrasonography remains the gold standard for deep vein thrombosis detection. Non-compressibility is the primary diagnostic criterion. You apply gentle pressure with the transducer until the vessel walls approximate. If they do not, you have a thrombus. The common femoral vein, femoral vein, popliteal vein, and calf veins are your standard examination sites. I once missed a soleal vein thrombosis because I stopped compressing at the popliteal level. The patient presented two weeks later with a pulmonary embolism. I do not bring that up to make myself feel bad. I bring it up to remind you that extending your examination to include the calf veins when clinical suspicion is high can prevent exactly this kind of outcome. B-mode imaging gives you anatomy. Doppler gives you physiology. Spectral Doppler provides velocity measurements. Color flow Doppler provides spatial localization of flow. Power Doppler is more sensitive to slow flow but does not provide directional or velocity information. You use each mode for different purposes. Color Doppler is essential for confirming vessel identity and detecting flow within a thrombus. Power Doppler can help visualize vessel patency in obese patients where signal penetration is poor.
Practical Workflow Considerations
Start with a B-mode survey. Identify the vessel of interest. Assess wall characteristics. Look for plaques, intimal thickening, or wall irregularities. Measure the lumen diameter at multiple points. Then apply color Doppler to confirm flow direction and presence. Finally, use spectral Doppler at a 60-degree insonation angle to obtain velocity measurements. The 60-degree rule exists because the Doppler equation uses the cosine of the angle. At 60 degrees, the cosine is 0.5, and small angular deviations produce relatively small errors in velocity calculation. At angles greater than 60 degrees, error increases exponentially. One thing beginners consistently get wrong is angle correction. They try to get the cursor perfectly parallel to flow and end up with an angle near zero, which introduces massive error because the cosine of zero is one, and the machine essentially loses accuracy. Keep the angle between 30 and 60 degrees. If you cannot maintain that range due to anatomical constraints, note it in your documentation and acknowledge the limitation. Arterial duplex scanning for peripheral arterial disease follows a similar protocol. The superficial femoral artery, popliteal artery, posterior tibial artery, and dorsalis pedis artery are standard landmarks. Ankle-brachial index measurements should accompany your duplex study for a complete assessment. A resting ABI below 0.9 indicates peripheral arterial disease. Below 0.5 suggests severe disease. Above 1.3 indicates non-compressible vessels, often due to medial calcification in diabetic patients.
Get the Full Details
There is a specific edge case with post-intervention follow-up. After a stent placement in the superficial femoral artery, you need to monitor for in-stent restenosis. The velocity ratio across the stent is your primary metric. A ratio greater than 2.5 suggests significant restenosis. But stent artifact can produce reverberation that mimics elevated velocities. I learned to distinguish artifact from true turbulence by comparing multiple sampling sites along the stent length and by using a lower frequency transducer to reduce noise. A 5 MHz curvilinear probe usually penetrates better through the stent material than a high-frequency linear array. Venous reflux evaluation requires specific maneuvers. The Valsalva maneuver assesses valve competence in the saphenofemoral junction. Manual compression and release assesses valvular function in the superficial and deep systems. You look for reverse flow lasting longer than 0.5 seconds after release. For deep venous systems, you assess augmentation with distal compression. Lack of augmentation suggests proximal obstruction. The limitations of vascular sonography are straightforward to list but easy to underestimate. Operator dependence is the primary limitation. Two sonographers can produce different velocity measurements on the same patient. Body habitus significantly affects image quality. Obesity reduces penetration and degrades resolution. Bowel gas obscures iliac vessels in most patients. You simply cannot visualize the common iliac arteries through a layer of bowel gas. When that happens, you document what you can see and recommend cross-sectional imaging such as CTA or MRA for the segments you cannot visualize.
Calcified plaques create acoustic shadowing that prevents accurate degree-of-stenosis determination. In these cases, you report the pre-stenotic and post-stenotic velocities, describe the appearance of the plaque, and note the limitation in your impression. You do not fabricate a percentage when you cannot measure it. That is not diligence. That is negligence.
Documentation Standards
Your documentation should include B-mode images of each segment, spectral waveforms with velocity measurements, and color flow images where applicable. Record the peak systolic velocity, end-diastolic velocity, and velocity ratio for each arterial segment. For venous studies, document compressibility, spontaneous phasic flow, augmentation response, and reflux duration. Include your insonation angle for all spectral measurements. Reference ranges vary by vessel. Normal common carotid peak systolic velocity is below 125 cm per second. Internal carotid artery normal PSV is below 100 cm per second. Vertebral artery forward flow should be present throughout the cardiac cycle with a peak velocity typically below 60 cm per second. Renal artery stenosis is assessed with a renal-aortic ratio. A ratio greater than 3.5 suggests hemodynamically significant stenosis. The field has evolved significantly over the past two decades. Contrast-enhanced ultrasound has improved visualization in difficult cases. Elastography is beginning to have applications in plaque characterization. Automated measurement software is reducing inter-observer variability, though it has not eliminated it. What has not changed is the fundamental requirement for a solid understanding of hemodynamics and hands-on scanning experience. Machines can measure velocities, but they cannot interpret waveforms. That part still requires a trained human being looking at the screen.
