Overview
This lecture introduces computed radiography (CR), focusing on the components of CR cassettes and how X-ray energy creates and stores a latent image.
Introduction to Computed Radiography
- Computed radiography (CR) is a digital imaging technique using cassette-based systems similar to traditional film radiography.
- CR cassettes record and store X-ray energy as a latent, unprocessed image.
- The captured latent image requires further processing by a plate reader to become a usable radiograph.
CR Cassette Structure
- CR cassettes have protective outer cases and a photo stimulable phosphor (PSP) plate.
- Main components of the PSP plate: protective layer, phosphor layer, conductive (anti-static) layer, support (base) layer, and reflective (light shielding) layer.
- Cassettes are made from hard plastic, light metals, and carbon fibers.
- The outer protective case only shields the plate and does not affect imaging.
Functions of Cassette Layers
- Protective layer shields the underlying phosphor layer.
- Phosphor layer is the active component where the latent image is formed.
- Conductive layer grounds the plate and reduces electrostatic charges.
- Reflective layer directs emitted light during image processing.
- Support layer provides mechanical stability to the cassette.
Phosphor Layer and Image Formation
- The phosphor layer contains small particles capable of storing and releasing energy.
- Most common phosphors are europium-activated barium fluorohalide bromides and iodides.
- X-ray photons interact with the phosphor via photoelectric absorption, ejecting inner shell electrons.
- Ejected electrons, or photoelectrons, excite others into a conduction band; europium acts as electron traps (F-centers).
- Trapped electrons store energy, creating the latent image until released by a laser in the plate reader.
- The number of trapped electrons is proportional to the amount of X-ray exposure.
Key Terms & Definitions
- Computed Radiography (CR) — Digital X-ray imaging using reusable cassettes and PSP plates.
- Latent Image — An invisible image formed by X-ray exposure, later processed into a visible radiograph.
- Photo stimulable Phosphor (PSP) Plate — The plate inside a CR cassette that stores X-ray energy.
- Phosphor Layer — Active PSP layer where X-ray energy is captured and stored as a latent image.
- Photoelectric Absorption — Process where X-ray photons transfer energy by ejecting electrons from atoms.
- F-centers — Electron traps in the phosphor layer created by europium activators, storing image information.
Action Items / Next Steps
- Review cassette layer functions and the image formation process.
- Prepare for the next lecture on plate reader processing in computed radiography.