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X-Ray Image Intensifiers by Mind Map: X-Ray Image Intensifiers
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X-Ray Image Intensifiers

Convert X-Rays into visible light

First the x-ray hits the input phosphor

The phosphor is actually a tubular crystal, making a light pipe to the photocathode (aluminum)

The light creates photoelectronics that are guided through the tube and concentrated into the smaller output opening via EM fields created by electrodes along the walls of the tube

It is in this stage that the distortions occur through influence of external magnetic fields on the guidance fields

The photoelectron is converted back to visible light by the (photoanode or phosphor)

Contrast

Defined as the brightness of the periphery of the window vs the center of the window with the center blocked by a lead disk.

Two types of contast

large area contrast (10%), 10:1 to 30:1 is typical

small detail contrast (10mm), 15:1 to 35:1 is typical

Brightness Gain

Magnification

Uses smaller input phosphor area to same output area, giving the effect of zooming in

Dose increases

For same noise level, dose quadruples for doubling of magnification

Smaller the field size, larger the magnification, larger the dose to the patient

Spatial resolution

4-6 line pairs/mm (lp/mm)

Distortions

Lag

Persistance of lumenescne after x-ray has disappeared, Not as big a deal these days - older II's had 30-40ms lag, new ones have 1ms., "lag" in modern fluoroscopic systems is more likely caused by the CCTV system than the XRII

Vignetting

Center is brighter than periphery, --> Center has higher resolution, less distortion, increased brightness

Veiling Glare

Scattering of light, defocusing of photons within the XRII, X-ray glare, phosphor spreading, electron scatter all contribute

Pincushion

Geometric distortion, New node

Nonlinear magnification across the image

S Distortion

Caused by external magnetic fields affecting photoelectron trajectories in the vacuum tube

Affects the perimeter of the image more than the center

Larger XRII's are more sensitive to this distortion

Manufacturers include a highly conducting metal shield around the vacuum tube