Vascular Physics and Instrumentation
Vascular Physics and Instrumentation
This domain tests the physical principles behind B-mode, spectral, color, and power Doppler, plus the instrument controls used to obtain valid vascular measurements.
Core rules
- The Doppler equation is Δf = (2 f0 v cosθ)/c. With c ≈ 1540 m/s in soft tissue, shift rises with transmit frequency and velocity and falls as the angle increases.
- Keep the Doppler angle at or below 60° and align the cursor with flow. A 60° angle increases velocity error from small angle misalignment compared with 0°.
- Aliasing occurs when the Doppler shift exceeds half the pulse repetition frequency, the Nyquist limit. Raise PRF/scale, lower frequency, adjust baseline, or use continuous wave (CW) if depth resolution is not needed.
- Wall filters remove low-velocity clutter but can erase low-flow signals. Sample-volume size and gain affect spectral broadening and velocity estimation.
- Axial resolution improves with a shorter spatial pulse length; lateral resolution depends on beam width and focus. Color frame rate falls as color box size, depth, or line density increases.
Worked example
A 5 MHz probe insonates a 1.0 m/s vessel at 60°. Δf = (2 × 5,000,000 × 1.0 × 0.5)/1540 ≈ 3,250 Hz. At 0° the shift would be ≈6,500 Hz. If the displayed PRF scale is 8 kHz, the 0° shift exceeds the 4 kHz Nyquist limit and aliases; the 60° shift fits. The corrected velocity is valid only if the cursor is truly parallel to flow.
Traps
Do not report a velocity obtained at 75-80° as accurate, and do not fix aliasing by increasing gain. CW Doppler has no range gate, so it cannot localize the highest velocity. Mirror artifact, ghosting, and twinkling are not stenosis criteria.