Image intensifier systems

The image intensifier (II) is an electronic device that converts low-intensity X-ray images into bright, visible light images suitable for real-time viewing.

It achieves this through a sequence of energy conversions and electronic amplification, enabling dynamic imaging with manageable radiation doses.

Purpose and Principle

Fluoroscopic imaging requires bright images under low X-ray exposures.
The image intensifier increases brightness by:

1. Converting X-rays to light (input phosphor).
2. Converting light to electrons (photocathode).
3. Accelerating and focusing electrons (electrostatic lenses).
4. Converting electrons back to light (output phosphor).

Overall effect → a small, bright image visible on a monitor or camera.

Structure of the Image Intensifier

A. Input Window and Phosphor

• Thin aluminium or titanium window allows X-rays to enter.
• Input phosphor: typically caesium iodide (CsI), needle-shaped crystals channel light to minimise scatter.
• Converts X-ray photons into visible light photons.

B. Photocathode

• Coated directly onto the input phosphor.
• Emits electrons when stimulated by light (photoelectric effect).
• The emitted electron pattern mirrors the light intensity pattern, preserving spatial information.

C. Electrostatic Focusing Lenses

• Series of electrodes at increasing positive potential (≈ +25–35 kV).
• Accelerate and focus the electron image toward the output phosphor.
• Focus adjusted by manufacturer to ensure geometric fidelity across field.

D. Output Phosphor

• Usually zinc cadmium sulphide (ZnCdS:Ag).
• Converts accelerated electrons back into visible light.
• The output image is inverted (upside-down and reversed) but much brighter than the original.

Brightness Gain

Brightness gain quantifies how much brighter the output image is compared to the input X-ray image.

Total brightness gain = Minification gain × Flux gain

Minification Gain

Occurs because the output phosphor is much smaller than the input.

Minification gain = (​D_input/D_output​​)²

Typical example:
Input = 25 cm, Output = 2.5 cm → Gain = (25/2.5)² =100

Flux Gain

Each electron accelerated across the potential difference (~25–35 kV) produces hundreds of light photons at the output phosphor.

Typical flux gain ≈ 50–100.

Overall Brightness Gain

Typical total gain = 100 × 50 =5,000

Values range from 2,000–8,000, depending on design.

This large gain allows fluoroscopic images to be viewed under normal lighting conditions.

Multi-Field (Magnification) Modes

• Modern IIs allow switching between different input field sizes (e.g. 25 cm → 17 cm → 12 cm).
• Achieved by increasing focusing voltage, so only the central portion of the input phosphor contributes to the image.

Magnification mode	Effect
Smaller input field	Magnified image
Same number of output electrons spread over smaller area	↓ Minification gain → image dimmer
ABC compensates by ↑ exposure factors	↑ Patient dose

Thus, magnification mode increases resolution but also dose.

Common Image Intensifier Artefacts

Artefact / Limitation	Cause	Appearance / Effect
Pincushion distortion	Electron focusing geometry	Outward bowing of straight lines
S-shaped distortion	External magnetic fields	S-like warping of image
Vignetting	Electron focus non-uniformity	Darkening at periphery
Veiling glare	Internal light scatter	Reduces contrast
Lag	Phosphor persistence	Smearing of moving objects
Limited dynamic range	Saturation at high exposures	Loss of bright detail

These artefacts were major limitations of image intensifiers, leading to the adoption of flat-panel detectors.

Advantages	Disadvantages
High brightness gain	Geometric distortion (pincushion)
Real-time dynamic imaging	Bulky and heavy
Wide field of view	Limited resolution vs FPD
Established technology	Increased dose in magnification mode
Cost-effective	Lower DQE, lag and glare issues

Image Coupling and Display

• The bright optical output can be:
  • Viewed directly (older systems).
  • Captured by a video camera or CCD sensor for digital display.
  • Split to secondary monitors or recording systems.
• Modern systems integrate the II with a digital camera chain for video processing and storage.

Key Takeaways and Exam Tips:

• Image intensifier converts X-rays → light → electrons → light.
• Brightness gain = minification × flux gain.
• Magnification mode improves resolution but increases dose.
• Automatic Brightness Control (ABC) maintains constant brightness.
• Common artefacts: pincushion distortion, vignetting, veiling glare, lag.
• Common exam question: “Explain how brightness gain is achieved in an image intensifier and describe its main limitations.”

Up Next

Next, we’ll move on to Flat-Panel Detector Fluoroscopy, describing the design, operation, and advantages of modern digital fluoroscopy systems that have replaced image intensifiers.