Ch. 29 Carlton

Assessing Distortion

• Geometric property of image quality.
• Always exists and can only be minimized.
• Two Types:
  • Size
  • Shape

Factors Affecting Distortion

• Size:
  • SID
  • OID
• Shape:
  • Alignment
  • Angulation

Factors Affecting Size Distortion

• Radiographic distances controlled by SID and OID.
• Magnification only.
• Digital systems can resize images post-processing.

SID

• Major factor for magnification.
• Longer SID yields less magnification.
• Decreased Entrance Skin Exposure (ESE).

OID

• Critical distance for magnification and spatial resolution.
• Minimize OID whenever possible.

Calculating Size Distortion

• Magnification Factor: M = \frac{SID}{SOD}
• M = \frac{Image \space size}{Object \space size}

Calculating Object Size

• O = \frac{I}{M}, where I = image size.
• Image size is always larger than object size in medical radiography.

Factors Affecting Shape Distortion

• Misrepresentation by unequal magnification.
• Types:
  • Foreshortening
  • Elongation

Factors Affecting Shape Distortion: Alignment

• Central Ray
• Anatomical Part
• Image Receptor
• Angulation: Degree & Direction

Central Ray Alignment Issues

• Off-centering of tube.
• Incorrect positioning of part.
• Off-centering of image receptor.

Evaluating Shape Distortion

• Effect cannot be calculated; subjective evaluation.
• Requires knowledge of normal anatomy and projections.