Bushongs Ch. 21

Medical Image Quality

• Refers to the fidelity with which the anatomy under investigation is rendered on the image receptor.

Spatial Resolution

• The ability to image small objects that have high subject contrast.
• Improves with smaller pixel size.

Contrast Resolution

• The ability to distinguish anatomical structures of similar subject contrast.

Noise

• The random fluctuation of x-ray interaction on the image receptor.
• Quantum mottle: reduced with use of high-mAs, low kVp.

Speed

• Fast image receptors have high noise and low contrast resolution.
• Low noise accompanies slow image receptors and high contrast resolution.
• Spatial resolution is limited to pixel size.

Geometric Factors

• Magnification: affected by SID and OID
  • MF = \frac{Image\ size}{Object\ size}
  • MF = \frac{source-to-image\ receptor\ distance (SID)}{source-to-object\ distance (SOD)}
• 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.
  • Foreshortening.
  • Elongation.
  • Spatial distortion.
• Focal-spot blur
  • 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
• 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.