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An Image Intensifier (II) is an electronic device used in fluoroscopy to convert low-intensity X-ray photons into a bright, visible image.
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It amplifies the image brightness up to 5000–30,000 times, allowing real-time visualization of dynamic processes at much lower radiation doses compared to early screen–film fluoroscopy.
Diagram of an X-ray intensifier. The X-ray image passes through the input window and interacts in the input screen, converting the X-rays to a visible light image. The photocathode converts this light image to an electron beam image. The electrodes focus and accelerate the electrons that strike the output screen, producing the amplified light image of a much reduced size.
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| Input Window | Thin aluminum, allows X-rays to enter with minimal absorption. |
|---|---|
| Input Phosphor | • Usually cesium iodide (CsI). |
| • Converts incident X-rays into visible light photons. | |
| • CsI crystals are needle-shaped → channel light, reduce scatter, improve resolution. | |
| Photocathode | • Directly bonded to the input phosphor. |
| • Made of photoemissive material (antimony–cesium). | |
| • Converts visible light photons into photoelectrons. | |
| Electrostatic Focusing Lenses | Accelerate and focus the electrons towards the output phosphor. |
| Anode | • Positively charged (≈ +25–35 kV). |
| • Attracts photoelectrons, accelerating them to the output phosphor. | |
| Output Phosphor | • Made of zinc cadmium sulfide. |
| • Converts the accelerated electrons back into visible light. | |
| • Produces a bright, minified version of the input image.v |
Cross-section of the II system illustrates the function of the input phosphor (convert X-rays to light), the photocathode (convert light to electrons), the cylindrical focusing lens and applied voltage (accelerate and focus electrons onto the output phosphor), the output phosphor (create very bright light image from electrons impinging on output phosphor), the conjugate lens system and aperture (refocus light onto light-sensitive TV camera target and adjust intensity of light), the TV camera (produce a variable video voltage amplitude corresponding to the incident X-ray flux), and the analog to digital converter to convert the continuous video voltage into a corresponding digital value stored in a matrix
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