The CR Image Plate (IP)

Composition

The CR plate resembles a conventional film cassette but contains a phosphor imaging layer rather than photographic film.

Main components (top to bottom):

1. Protective layer: transparent polymer coating to protect the phosphor.
2. Phosphor layer: photostimulable phosphor (BaFBr:Eu²⁺ or BaFI:Eu²⁺).
3. Reflective or light-shielding layer: enhances light collection efficiency.
4. Support base: flexible polyester substrate.
5. Anti-static / backing layer: prevents dust and static artefacts.

Step-by-step mechanism of latent image formation

When the CR plate is exposed to X-rays, energy from the photons is absorbed and stored within the phosphor crystal lattice in a stable, invisible form. This trapped energy represents the latent image, which can later be released and converted into light during the readout process.

Step 1: X-ray photon absorption

• The phosphor layer in the CR plate contains barium fluorobromide (BaFBr) crystals doped with a small amount of divalent europium (Eu²⁺), the activator element.
• When an incident X-ray photon is absorbed by the BaFBr crystal (mainly by photoelectric effect), its energy is transferred to electrons in the lattice.
• These electrons are excited from the valence band (ground state) to the conduction band (free energy state).

X-ray photon energy → electron excitation (BaFBr:Eu²⁺)

Step 2: Creation of electron–hole pairs

• Each absorbed X-ray photon typically liberates one or more electron–hole pairs:
  • The electron is elevated to the conduction band.
  • The hole (positive charge) remains in the valence band or near the activator centre.

At this point, the excited electron is free to move within the conduction band.

Step 3: Electron trapping at F-centres

• The BaFBr crystal contains small lattice defects called F-centres (from the German Farbe = colour).
• These are fluorine ion vacancies that can trap electrons.
• Some of the excited electrons fall into these traps instead of recombining immediately.

This trapped electron is now at a metastable (slightly higher) energy level. Stable enough to remain there for several hours, but not permanently. The number of trapped electrons is proportional to the local X-ray intensity. This is the latent image.

Step 4: Role of the activator (Europium)

• The europium activator (Eu²⁺) plays a critical dual role:
  1. Energy absorber: participates in the initial excitation process.
  2. Luminescence centre: during later readout, Eu²⁺ is oxidised to Eu³⁺ as trapped electrons are released, producing light (photostimulated luminescence).
• In simplified terms, each absorbed X-ray slightly alters the oxidation state of the europium atoms surrounding the traps, marking those regions as “exposed.”

Step 5: Charge equilibrium and storage

• The crystal now contains:
  • Trapped electrons in metastable F-centres.
  • Positive holes near Eu³⁺ centres.
• These two populations remain spatially separated, so no recombination (light emission) occurs yet.
• This separation of charge is stable for minutes to hours. Long enough for the plate to be read later.

At this stage, the image is invisible but physically encoded within the crystal as a distribution of trapped charges.

Again, this is the latent image.

Now we need to convert this latent image into a readable signal to generate a digital image.

This is done with the IMAGE READOUT process.

Image Readout: Photostimulated Luminescence

After exposure, the cassette is placed in a CR reader. This is a common exam question.

Step-by-step process:

1. Laser scanning
   • A finely focused red laser beam (λ ≈ 600–700 nm) scans the plate in a raster pattern.
   • The laser provides photons with just enough energy to release trapped electrons from the metastable sites.
2. Emission of light
   • As electrons return to the ground state, they emit blue–violet light (λ ≈ 400 nm).
   • This process is called photostimulated luminescence (PSL).
3. Light detection
   • The emitted light is collected by a light guide and directed to a photomultiplier tube (PMT) or photodiode.
   • The PMT converts the light signal into an electrical signal proportional to local exposure.
4. Signal digitisation
   • The analogue signal is amplified, then converted into digital pixel values via an analogue-to-digital converter (ADC).
   • Each pixel represents one scanned region of the plate (matrix typically 2,000–4,000 pixels across).
5. Image erasure
   • After readout, residual trapped electrons are cleared by exposure to bright white light, erasing the plate for reuse.

 

Workflow summary

Stage	Process	Outcome
1	X-ray exposure	Latent image stored as trapped electrons
2	Laser scanning	Electrons released → PSL light
3	Light detection (PMT)	Signal converted to electrical output
4	Digitisation (ADC)	Image matrix created
5	Erasure	Plate cleared for reuse

Key Takeaways and Exam Tips:

• CR uses photostimulable phosphors (BaFBr:Eu²⁺) to store energy from X-ray exposure as trapped electrons.
• Laser readout releases this energy as light (photostimulated luminescence).
• The emitted light is converted to an electronic signal, digitised, and displayed.
• Plates must be erased before reuse.
• Advantages: wide dynamic range, compatibility with film workflows.
• Disadvantages: slower readout, lower DQE, potential artefacts.
• Common exam question: “Describe the mechanism of image formation in computed radiography and explain the role of photostimulable phosphors.” “Explain the readout process in CR.”

Up Next

Next, we’ll move on to Direct Digital Radiography (DR), covering both indirect (CsI/Gd₂O₂S) and direct (a-Se) flat-panel detectors, including TFT readout, signal conversion, and detector performance characteristics.