Vascular Technology: Instrumentation, Doppler, and Duplex Measurement
Vascular Technology tests how ultrasound instruments generate, detect, and display blood-flow signals, and how technical choices change the numbers you report. Expect calculations, Doppler artifact recognition, duplex optimization, and waveform interpretation.
Core Doppler rules
- The Doppler equation is fd = (2 f0 v cos θ)/c, where fd is Doppler shift, f0 transmitted frequency, v blood velocity, θ the angle between beam and flow, and c about 1540 m/s in soft tissue. Always angle-correct; keep θ at or below 60°. At 60°, cos θ = 0.50, so an uncorrected or wrongly corrected angle can halve or double velocity.
- Continuous-wave Doppler detects flow anywhere along the beam but lacks depth resolution. Pulsed-wave Doppler has a sample volume and range gate; PRF determines the Nyquist limit. Aliasing occurs when the Doppler shift exceeds PRF/2; raise scale/PRF, lower frequency, adjust baseline, or increase angle toward 60°.
- Color Doppler maps mean velocity and direction; power Doppler is angle-independent and more sensitive to low flow but gives no direction or velocity. Spectral gain too high creates mirror artifact; too-low gain loses low-velocity signal. Wall filter removes high-amplitude low-velocity clutter but can erase slow flow.
- Waveform shape reflects downstream resistance: triphasic high-resistance limb arteries have sharp systolic upstroke, early diastolic reversal, and late diastolic forward flow; low-resistance organs such as the ICA, renal, and hepatic arteries have continuous diastolic flow. Tardus-parvus indicates proximal stenosis.
Worked example
A 5 MHz probe insonates flow at 60° and records a 2 kHz Doppler shift. Convert: v = fd c /(2 f0 cos θ) = (2000 × 1540)/(2 × 5,000,000 × 0.50) = 0.616 m/s, or 62 cm/s. If the technologist forgets angle correction (cos θ = 1), the calculated velocity becomes 31 cm/s, half the true value, because the denominator doubles. Report the angle and avoid comparing velocities obtained at different angles.
Traps that cost marks
- Confusing peak systolic velocity with mean velocity or time-averaged maximum.
- Interpreting aliasing as true high velocity without checking scale and baseline.
- Using a sample volume so large that it includes vessel wall or adjacent vein.
- Assuming power Doppler can quantify stenosis; it cannot.
- Forgetting that velocity varies with angle, hematocrit, and beam frequency.