Characterization and Measurement

Once segmentation is achieved, another common tasks is quantitative or qualitative characterization and measurement of the areas segmented in processing the image. Quantitative characterization may include:

  • Object or area count , counting the areas (objects) segmented
  • Characterization of signal intensity, color or other information related to volume of signal channels
  • Dimensional characterization such as area, perimeter, equivalent spherical diameter, maximum and minimum projections
  • Morphological characterization including elongation or spherisity
  • Spatial Characterization includes parameters such as orientation, XY(Z) coordinates or center of mass , distances between objects, dispersion or uniformity measurements and more.

Image Analysis Considerations. What are you measuring?

There is wide variety of measurements that may be obtained from the image after features of interested are identified  through segmentation and processing steps. Some measurements provide characterization of the entire imaging field of view and some describe individual features on the image.

Measurements that are field based

  • Area fraction
  • Average linear intercept
  • Mean free path
  • Spacing
  • Intercept density
  • Other

Measurements that are object based

  • Diameter
  • Length
  • Width or breadth
  • Perimeter
  • Projections
  • Feret lengths
  • Other

Field Measurements

Common Field Measurements

The field measurements should be averaged, and the standard deviation can be used to assess the relative accuracy of the measurement, as described in ASTM E 562.

The volume fraction was first estimated by areal (relating to area) analysis by Delesse, French geologist, in 1848. He showed that area fraction was an unbiased estimate of the volume fraction.

  • Area
  • Area Fraction
  • Count
  • Chord Lengths
  • Field Perimeter
  • Intercepts
  • Intensity
  • Derived
  • Others

Feature or Object Measurements

Common Feature Measurements

 

 

 

 

 

 

 

Stereology

Example: intercept count for grain size determination

Other Feature Measurements

Derived Feature Measurements

  • Roughness = Perimeter/Convex Perimeter
  • Compactness = 4xPiexArea/Convex Perimeter
  • Circular Diameter = 2/Area/Pie
  • Aspect Ratio = Length/Breadth or  Longest Feret/Shortest Feret
  • Many Others

Errors and their sources

Any measurement system will produce measurement errors. Understanding the degree of errors and their sources is very important for determining applicability and limitation of the system

Field Measurement Errors

  • Holes in Objects
  • Extraneous Features
  • Frame Edge Effects
  • Statistical Sampling Effects

Feature Measurement Errors

  • Acquisition
    • System resolution
    • Geometric distortion
    • Selection of thresholds
    • Feature discrimination
  • Measurement
    • Fractal Effect in Ferets/Perimeters
  • Edge Effects
  • Sampling

Threshold Errors

Feature Resolution Effect

                                         728x524                                        1024x1024

Conclusions

  • Type of image is important
  • High frequency features are more sensitive to compression
  • 50% compression is acceptable while retaining good accuracy
  • Image processing can be used to correct for reconstruction inaccuracies