Vascular Ultrasound Interpretation: Physics, Artifacts, and Spectral Analysis
Foundations
Every interpretation question ultimately rests on physics. Know the Doppler equation: the frequency shift is proportional to transmitted frequency, blood velocity, and the cosine of the Doppler angle, and inversely related to the speed of sound in tissue (~1540 m/s). Shifts increase with higher transducer frequency, so lower-frequency probes penetrate deeper but sacrifice resolution. Understand continuous-wave versus pulsed-wave Doppler, sample-volume placement, wall filters, and the aliasing threshold (Nyquist limit = PRF/2).
Spectral and Color Concepts
Candidates must read a spectral waveform: peak systolic velocity, end-diastolic velocity, resistive index [(PSV−EDV)/PSV], spectral broadening (turbulence), and waveform pulsatility. Recognize triphasic, biphasic, and monophasic arterial patterns and what each implies about downstream resistance. In color Doppler, understand the color map, variance maps that flag turbulence, and how gain, scale, and steering affect the display.
Traps
Common question traps: confusing increased velocity from stenosis with increased velocity from high cardiac output or vasodilation; mistaking aliasing for true flow reversal; overgaining color so that flash artifact or motion mimics flow; and forgetting that the Doppler angle must be corrected (ideally ≤60°) or velocities are invalid. Another trap is assuming a low resistive index always means stenosis when downstream vasodilation produces the same pattern.
Rule of Thumb
If a color signal appears in a region where anatomy says there should be no vessel, suspect artifact (flash, mirror image, or motion) before disease. Always verify a spectral finding with correct angle correction and sample-volume placement before calling a velocity abnormal. For angle: velocities rise steeply as the angle approaches 90°; keep correction consistent between serial exams.