Medical Image Descriptors Notes

Image Quality Factors

Spatial Resolution

  • Ability to image small objects with high subject contrast (e.g., bone-soft tissue). Improves with smaller pixel size.
  • Smaller focal spot = better spatial resolution.
  • Cannot image objects smaller than pixel size.

Contrast Resolution

  • Ability to distinguish between anatomic structures of similar subject contrast (e.g., gray vs. white matter).
  • Radiographic contrast is most important in post-processing.

Noise

  • Random fluctuation of x-ray interaction on the image receptor (IR).
  • Inherent in the imaging system (electronic noise).
  • Lower noise = improved contrast resolution.
  • Quantum mottle is the main cause of radiographic noise.

Speed

  • Connected to noise, spatial and contrast resolutions.

Medical Imaging Quality Rules

  • Fast IRs have high noise with low contrast resolution.
  • Low noise = slow image = high contrast resolution.
  • Spatial resolution is limited to pixel size.

Factors Affecting Radiographic Quality

  • Image Receptor Factors: Pixel size, dynamic range, signal to noise, postprocessing.
  • Geometric Factors: Distortion, magnification, blur.
  • Subject Factors: Contrast, thickness, density, atomic number, motion.

Geometric Factors

  • Magnification: Images larger than the object.
    • MagnificationFactor=image sizeobject sizeMagnification Factor = \frac{image \ size}{object \ size}
    • Minimize magnification by using large SID and small OID.
  • Distortion: Unequal magnification of different parts of an object.
    • Causes: object thickness, position, shape.
  • Focal-Spot Blur: Blurred region on the radiograph (penumbra).
    • Caused by large effective focal spot, short SID, and long OID.
    • Greatest towards the cathode end (Heel Effect).

Subject Factors

  • Subject Contrast: Determined by size, shape, and x-ray attenuating characteristics.
  • Patient Thickness: Thicker sections attenuate more x-rays.
  • Tissue Mass Density: How tightly atoms are packed (bone > soft tissue).
  • Effective Atomic Number: Enhanced by contrast media (increased photoelectric absorption).
  • Motion Blur: Blurring from patient or x-ray tube movement.

Reducing Motion Blur

  • Use shortest possible exposure time.
  • Restrict patient motion.
  • Use a large SID and small OID.
  • Suspend respiration.

Tools for Improved Radiographic Quality

  • Patient positioning.
  • Image receptors.
  • Selection of technical factors.