Medical Image Viewing Notes

Photometry

  • Science of the response of the human eye to visible light.
  • Measures and controls brightness, contrast, and color.

Vision

  • Photopic vision: Bright-light vision with cones (day vision).
  • Scotopic vision: Dim-light vision with rods (night vision).

Measures of Light

  • Luminous Flux: Total visible light emitted by a monitor.
  • Luminance Intensity: Total intensity of light from a source.
  • Luminance: Intensity of light emitted from a surface.

Cosine Law

  • Luminance intensity is at its maximum when viewed straight on.
  • Luminous intensity reduces at larger viewing angles.

Soft Copy Viewing

  • Performed on LCD or LED displays.

Liquid Crystal Display (LCD)

  • A pixel is a basic picture element on a display.
  • LCDs use light-polarizing filters to control light intensity and color.
  • Advantage: Reduces ambient light influence.
  • Disadvantage: Less image contrast at increasing viewing angles.
  • Each pixel has three filters: red, green, blue.
  • Aperture ratio measures the image luminance of LCDs.

Light Emitting Diode Display (LED)

  • Phosphor emits light in response to stimulation (luminescence).
  • Luminescence involves electrons raised to excited energy levels, emitting light photons when returning to their normal state.
  • Types of Luminescence:
    • Fluorescence: Emission during stimulation.
    • Phosphorescence: Emission during and after stimulation.
    • Electroluminescence: Emission when electric current is passed through.
  • LED has electroluminescence characteristics.
  • LED provides the backlight; does not replace LCD.

LCD vs. LED

  • LCD uses fluorescent backlights.
  • LED uses light-emitting diodes.

Advantages of LED Backlight

  1. Thinner with a larger active area.
  2. Used in curved video screens.
  3. Longer life.
  4. Lower power consumption.
  5. Less heat.

Preprocessing

  • Designed to produce artifact-free digital images.
  • Corrects signal interpolation, lag, and noise.
  • Provides electronic calibration to reduce response differences.

Signal Interpolation

  • Corrects defective pixels by averaging surrounding pixel values.

Latent Image/Image Lag

  • Image lag occurs as a variation in voltage.
  • Corrected by offset voltage.

Line Noise

  • Caused by fluctuations in electrical current, resulting in linear artifacts.
  • Solution: Apply correct voltage from unirradiated areas.

Preprocessing Calibration

  • Flatfielding: Reduces noise and improves contrast.
  • Offset Images: Generated many times daily.
  • Gain Images: Generated every few months.

Post Processing

  • Done after image acquisition.
  • Allows visualization of all shades of gray.

Annotation

  • Adding text to an image.

Dynamic Range

  • Digital images have ranges up to 16-bit (2^{16} = 65,536 gray levels).
  • Human eye can visualize approximately 30 shades of gray.

Matrix Size

  • Large matrix size = better spatial resolution.

Image Flip

  • Flipping images horizontally or vertically.

Image Inversion

  • Inverting an image from white to black, also known as digital subtraction.
  • Enhances image contrast.

Pixel Shift

  • Post-processing technique to assist with better resolution.
  • Used in digital subtraction angiography.

Region of Interest (ROI)

  • Area of an anatomical structure defined by the operator.

Edge Enhancement

  • Alters pixel values to make edges more prominent.
  • Effective for fractures and small high contrast tissue.

Highlighting and Navigation

  • Highlighting is used for diffuse.
  • Pan allows for careful, precise regions of an image.