Doppler Signal Processing, Tissue Characterization

Notes

Cards

• basicI.M-001
  What is the role of a demodulator in a Doppler system?
  → Extracts the low-frequency Doppler shift (~kHz) from the high-frequency transducer carrier (MHz). Bidirectional Doppler uses phase quadrature processing to preserve direction.
• basicI.M-002
  What signal-processing algorithm generates the spectral Doppler display?
  → Fast Fourier Transform (FFT) — decomposes the complex received signal into its constituent frequencies (velocities).
• basicI.M-003
  What do the gray shades in a spectral Doppler display represent?
  → Signal amplitude — proportional to the number of red blood cells moving at that velocity. Brighter shade = more RBCs at that velocity.
• basicI.M-004
  What is tissue Doppler imaging (TDI)? How does it differ from conventional PW Doppler?
  → TDI records myocardial motion instead of RBC flow. TDI signals are high-amplitude, low-velocity — so power output/gain are lowered and velocity range is narrowed.
• basicI.M-005
  What is second-harmonic imaging?
  → An imaging mode that receives signals at 2× the transmitted frequency. Improves lateral resolution 20–50% and reduces artifact, but degrades axial resolution 40–100%.
• basicI.M-006
  Is dynamic range higher or lower for improved black-white contrast?
  → Lower dynamic range → higher black-white contrast (fewer gray shades, more binary).
• basicI.M-007
  Contrast resolution — what does it describe?
  → The minimum difference in a displayed parameter (usually gray shades) that can be visualized as distinct pixel intensities.