Medical Image Descriptors - Notes

Chapter 21: Medical Image Descriptors

Objectives

  • Define medical image quality, resolution, noise, and speed.
  • Distinguish the geometric factors that affect medical image quality.
  • Analyze the subject factors that affect medical image quality.
  • Examine the tools and techniques available to create high-quality images.

Definitions

  • Medical Image Quality: A term describing how accurately an image reproduces the anatomic structure.
  • Spatial Resolution: The ability to image small objects that have high subject contrast, such as a bone-soft tissue interface, a breast microcalcification, or a calcified lung nodule.
    • Spatial resolution improves with smaller pixel size.
  • Contrast Resolution: The ability to distinguish anatomical structures of similar subject contrast.
    • Controlled by bit depth.
  • Noise: The random fluctuation of x-ray interaction on the image receptor.
    • Quantum mottle: Reduced with use of high-mAs, low kVp.
    • Lower noise leads to better contrast resolution.
    • SNR (Signal-to-Noise Ratio).
  • Speed:
    • Fast image receptors have high noise and low contrast resolution.
    • Spatial resolution is limited to pixel size.
    • High speed means more sensitive and less dose.

Image Receptor Factors

  • Pixel size.
  • Dynamic range.
  • Intensity response.

Factors Affecting Image Quality

  • Geometric Factors
    • Magnification
    • Distortion
    • Blur
  • Subject Factors
    • Contrast
    • Thickness
    • Density

Geometric Factors

  • Magnification: Affected by SID (source-to-image receptor distance) and OID (source-to-object distance).
    • MF = \frac{source\text{-}to\text{-}image\ receptor\ distance (SID)}{source\text{-}to\text{-}object\ distance (SOD)}
    • MF = \frac{Image\ size}{Object\ size}
  • Distortion: Unequal magnification of different portions of the same object.
    • Depends on object thickness, position, and shape.
    • Thick objects are more distorted than thin objects.
    • Types of distortion:
      • Foreshortening
      • Elongation
      • Spatial distortion
  • Focal-Spot Blur:
    • Occurs because the focal spot is not a point.
    • Focal\ spot\ blur = effective\ focal\ spot \times \frac{OID}{SOD}
  • Heel Effect:
    • The focal-spot blur is small on the anode side and large on the cathode side of the image receptor.

Subject Factors

  • Subject Contrast:
    • Radiographic\ contrast = Image\ receptor\ contrast \times Subject\ contrast
    • Affected by:
      • Patient thickness
      • Tissue mass density
      • Effective atomic number
      • Object shape
  • Motion Blur:
    • Patient motion is usually the cause of motion blur.

Tools for Improved Radiographic Image Quality

  • Patient positioning.
  • Image receptors.
  • Selection of technique factors.
  • Keep exposure time as short as possible.

Summary

  • Using appropriate language to describe images is essential in professional practice of radiology.
  • Understanding how your system, the patient, and geometric factors affect the image outcome will allow radiographers to optimize their imaging technique and image evaluation.