Image intensifiers (II) are used to convert low energy x-radiation into visible light images. Frequently the detector portion of an x-ray C-arm used in operating theaters, the image intensifier has a low scatter input portion made of low absorption substances such as titanium or aluminum 1,2. Image intensifiers are several thousand times more sensitive compared to standard 400-speed screen-film combinations, and in practice can produce images using several thousand times less radiation 3,4.
The biggest advantage of image intensifiers in medical imaging is the synergy of high detector efficiency and high conversion efficiency to effectively utilize fluoroscopy while adhering to the radiation protection principle of dose optimization.
Overview
After the x-ray beam emerges from the patient, it enters the image intensifier tube through the input window and is partially absorbed by the fluorescent input screen (entrance phosphor) creating a number of light photons 5.
The light photons strike the photocathode of the input screen and are absorbed by photoelectric interactions, causing it to emit photoelectrons (via the photoelectric effect) 5.
The electrons are accelerated towards the output fluorescent screen by an electric field produced between the photocathode and anode. Focussing and distortion minimization is accomplished by the focussing electrodes 5.
The electrons hit the output phosphor and cause large numbers of light photons to be produced, which subsequently may be captured by various imaging devices 3,4.
The output of the image intensifier can be evaluated by brightness gain and conversion factor. Brightness gain is the product of minification gain and flux gain. Minification gain is the ratio of the input area to output area of phosphor. It makes the image brighter but does not improve the quality and contrast of the image. Meanwhile, flux gain is the number of photons generated at the output phosphor, comparing with photons generated at input phosphor 5.
Brightness gain from minification does not improve the statistical quality of the image because same number of photons is reaching output phosphor whether the input screen is larger or smaller. Thus, the total light output remains the same although minification gain results in increased brightness in the output screen 6.
Conversion factor is defined as output luminance of the image intensifier divided by entrance exposure rate, measuring the efficacy of conversion from x-rays to light. It has units of candela per square meter per milliroentgen per second ([cd/m2]/[mR/sec]) 5.
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