Residency · Residency · Diagnostic Radiology

Doppler Ultrasound: Principles and Vascular Applications

Introduction

Doppler ultrasound exploits the Doppler effect -- the shift in frequency of a sound wave reflected from a moving target -- to provide real-time hemodynamic information noninvasively. It is fundamental to vascular imaging, echocardiography, and numerous abdominal and obstetric applications. Understanding the physics and potential artifacts is essential for accurate interpretation.

Physics of the Doppler Effect

When a sound wave encounters a moving reflector (red blood cells), the reflected frequency shifts proportionally to the reflector's velocity. The Doppler equation is fd = 2 f0 v * cos(theta) / c, where fd is the Doppler frequency shift, f0 is the transmitted frequency, v is the blood flow velocity, theta is the angle between the ultrasound beam and the direction of flow (the Doppler angle), and c is the speed of sound in tissue (1540 m/s). The cosine relationship means that at 0 degrees (beam parallel to flow) the Doppler shift is maximal, while at 90 degrees it is zero. In practice, a Doppler angle of 60 degrees or less is required for accurate velocity measurements because errors increase dramatically above 60 degrees.

Doppler Modes

Color Doppler

Color Doppler displays mean velocity and direction of flow as a color map overlaid on the grayscale image. By convention, red indicates flow toward the transducer and blue indicates flow away (mnemonic: BART -- Blue Away, Red Toward). It provides qualitative information about flow presence, direction, and turbulence. Color Doppler energy (power Doppler) displays the amplitude of the Doppler signal rather than mean velocity, making it more sensitive for detecting slow flow but losing directional information.

Spectral (Pulsed-Wave) Doppler

Spectral Doppler plots velocity (y-axis) versus time (x-axis) at a specific sample volume location. It allows quantitative measurement of peak systolic velocity (PSV), end-diastolic velocity (EDV), and derived indices. The resistive index (RI) equals (PSV - EDV) / PSV, and the pulsatility index (PI) equals (PSV - EDV) / mean velocity.

Continuous-Wave (CW) Doppler

CW Doppler transmits and receives simultaneously and has no range resolution but can measure very high velocities without aliasing. It is used primarily in echocardiography for quantifying high-velocity jets such as those in aortic stenosis.

Artifacts and Pitfalls

Aliasing occurs in pulsed-wave and color Doppler when the Doppler shift exceeds the Nyquist limit (half the pulse repetition frequency), producing a wraparound artifact. It is corrected by increasing PRF, shifting the baseline, using a lower frequency, or switching to CW Doppler. Blooming overestimates the vessel lumen on color Doppler due to excessive gain. Wall filter artifact occurs when high wall filter settings eliminate low-velocity venous flow signals. Angle dependence renders velocities unreliable when the Doppler angle exceeds 60 degrees.

Vascular Applications

Carotid Doppler

Carotid Doppler is the primary screening tool for extracranial carotid stenosis. The key measurement is the internal carotid artery (ICA) PSV: normal is less than 125 cm/s; 50-69% stenosis corresponds to PSV of 125-230 cm/s; 70% or greater stenosis corresponds to PSV greater than 230 cm/s; near-occlusion shows variable and may decreased velocity (the "string sign"); and complete occlusion shows no detectable flow. The ICA/CCA PSV ratio provides an additional grading criterion, with a ratio of 4.0 or greater suggesting 70% or greater stenosis. Plaque morphology should be assessed because ulcerated, calcified, and echolucent (soft) plaques carry different embolic risk.

Lower Extremity Venous Doppler

Lower extremity venous Doppler is the primary modality for diagnosis of deep vein thrombosis (DVT). The technique involves compression ultrasound, where a normal vein is fully compressible and a thrombosed vein is not. The common femoral, femoral (superficial femoral), and popliteal veins are evaluated with augmentation by distal calf compression. Acute thrombus appears hypoechoic with a distended, non-compressible vein, while chronic thrombus is echogenic with thickened vein walls and may show collateral formation. Spectral Doppler shows loss of respiratory phasicity when proximal obstruction is present.

Renal Artery Doppler

Renal artery Doppler screens for renal artery stenosis (RAS). A main renal artery PSV greater than 200 cm/s or a renal-to-aortic ratio (RAR) greater than 3.5 suggests 60% or greater stenosis. A tardus-parvus waveform in the intrarenal arteries (delayed systolic upstroke, low RI) suggests proximal stenosis. An elevated RI greater than 0.80 in intrarenal arteries may indicate intrinsic parenchymal disease and predicts poor outcome after revascularization.

Hepatic and Portal Venous Doppler

Normal portal vein flow is hepatopetal (toward the liver) with a gentle continuous or mildly phasic waveform. Hepatofugal (reversed) portal flow indicates severe portal hypertension. Normal portal vein velocity is 15-40 cm/s, with less than 15 cm/s considered sluggish. Hepatic veins normally show a triphasic waveform reflecting right atrial pressure changes. Loss of phasicity (monophasic, flattened waveform) occurs in cirrhosis and hepatic congestion.

Transplant Doppler

Post-transplant renal and hepatic Doppler surveillance monitors for arterial thrombosis, stenosis, and rejection. Absent diastolic flow or reversal of diastolic flow in a transplant renal artery is an emergency finding suggesting thrombosis or severe rejection.

Key Clinical Pearls

Always angle-correct to 60 degrees or less for velocity measurements; small angle errors at high angles produce large velocity errors. In the setting of aliasing, do not mistake the wraparound signal for bidirectional flow; increase PRF or use CW Doppler. A tardus-parvus waveform in the intrarenal arteries may be the only Doppler clue to proximal renal artery stenosis when the main renal artery is obscured by bowel gas. Power Doppler is preferred over color Doppler for detecting slow flow in the testis, thyroid, and musculoskeletal applications. In carotid near-occlusion, PSV may paradoxically decrease; always correlate with grayscale and color Doppler findings.

References

1. Zwiebel WJ, Pellerito JS. Introduction to Vascular Ultrasonography. 7th ed. Elsevier; 2020.
2. Grant EG, Benson CB, Moneta GL, et al. Carotid Artery Stenosis: Gray-Scale and Doppler US Diagnosis -- Society of Radiologists in Ultrasound Consensus Conference. Radiology. 2003;229(2):340-346.
3. Rubens DJ, Bhatt S, Nedelka S, et al. Doppler Artifacts and Pitfalls. Radiol Clin North Am. 2006;44(6):805-835.
4. AbuRahma AF, Bergan JJ, eds. Noninvasive Vascular Diagnosis. 4th ed. Springer; 2017.

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