Lecture Notes: Direct and Indirect Conversion Digital Radiography (DR)

Overview of Digital Radiography (DR)

• Digital Radiography is different from computed radiography as it does not use cassettes or standalone cassette readers.
• Image processing follows image acquisition immediately, converting radiographic images into digital data without an operator's involvement.
• Faster and produces higher-quality images than computed radiography.

Methods of X-ray Image Acquisition

1. Direct Conversion
   • Converts X-rays directly to electronic signals.
2. Indirect Conversion
   • Converts X-rays to visible light, then to electronic signals.

Direct Conversion Digital Radiography

• Components:
  • Photoconductor made of amorphous selenium.
  • Thin-film transistor (TFT) array.
• Process:
  • Voltage applied to detector before exposure.
  • X-ray interacts directly with amorphous selenium, releasing electrons.
  • Storage capacitors in DELs collect the charge.
  • TFT switches release charges to analog-to-digital converter.
• Advantages:
  • Higher image quality as it skips conversion to visible light.
• Use Cases:
  • Primarily used in mammography for high image quality and because detectors can't be large enough for general radiography.

Indirect Conversion Digital Radiography

• Components and Process:
  • Scintillation Layer: Converts X-ray photons to visible light photons.
  • CCD Systems:
    • Scintillation layer coupled to CCD sensor chip via lenses or fiber optics.
    • Preferred material: Cesium iodide for better spatial resolution.
    • Light converted to electrical signals, then to digital signals.
  • TFT Systems:
    • Scintillation layer followed by photodiode and TFT layers.
    • Converts light photons to electrical signals passed to a computer.
• Use Cases:
  • General-purpose radiography, angiography, and fluoroscopy.
• Advantages and Disadvantages:
  • Cost less and easier to repair than direct conversion.
  • Extra step reduces spatial resolution.

Summary

• DR Types: Direct (higher spatial resolution, more expensive) and Indirect (less cost, easier maintenance).
• Direct Conversion:
  • Directly converts X-rays to electronic signals.
  • Superior spatial resolution.
• Indirect Conversion:
  • Two types: CCD and TFT.
  • Involves conversion via scintillation layer.
• Each system suits different radiographic needs and cost considerations.