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← Section I · Physical Principles, Instrumentation, Examination Principles
I.L

Digital Image Processing

5 cards

Notes

Pre- vs post-processing

  • Preprocessing - applied before storage: gain, TGC, dynamic range compression, filtering. Changes what is stored.
  • Postprocessing - applied after storage: gray-scale map, edge enhancement, colorization. Does NOT change stored data; can be reapplied non-destructively.

Dynamic range

  • Range of signal amplitudes (from smallest to largest) represented in the display.
  • Lower dynamic range → higher contrast (black-white image, fewer gray shades).
  • Higher dynamic range → smoother, more gray-shade tissue characterization (softer image).

Frame averaging (persistence)

  • Averages consecutive frames to reduce noise.
  • Trade-off: reduced temporal resolution (blurs rapid motion).

Scan conversion

  • Converts polar (radial) beam-line data into a rectangular pixel grid for the display.
  • Interpolation fills in pixels between sparse scan lines.

Zoom types

  • Write zoom - reacquires the ROI at higher line density / pixel density → true resolution gain.
  • Read zoom - magnifies stored data (no resolution gain).

Image storage / DICOM

  • Modern echo systems store as DICOM files (metadata + image or cine loop).
  • Compression may be lossy (JPEG) or lossless.

Frame rate and depth relationship

  • Frame rate ↑ with: narrow sector, shallow depth, few focal zones, low line density.

Cards

  • basicI.L-001
    Preprocessing vs postprocessing — which one changes the stored data?
    → Preprocessing (gain, TGC, dynamic range, filtering) alters signals before storage. Postprocessing (gray-scale maps, colorization, edge enhancement) is applied after storage and can be changed non-destructively.
  • basicI.L-002
    Effect of a lower dynamic range on the displayed image?
    → Higher contrast — fewer gray shades between black and white, more binary appearance.
  • basicI.L-003
    Effect of frame averaging (persistence)?
    → Averages consecutive frames to reduce noise, at the cost of temporal resolution — motion appears blurred.
  • basicI.L-004
    What is scan conversion?
    → The process of interpolating radial (polar) beam-line data onto a rectangular pixel grid for display.
  • basicI.L-005
    Which zoom improves resolution — read or write?
    → Write zoom (reacquires with more scan lines / pixels in the ROI). Read zoom only magnifies stored data.