Figure 1-4. An image intensification tube.
b. In the main, the functional aspects of a typical image intensifier tube are as
(1) X-rays, having passed through the patient, impinge on a fluorescent
element, or image input phosphor, at the input end of the image intensifier tube. This
fluorescent layer then emits light proportional to the x-ray beam impinging on it. A
photoelectric layer (photocathode), which is in direct contact with the input phosphor,
emits electrons proportional to the light image produced by the fluorescent layer, thus
converting the light image into an equivalent electron image. The primary reasons for
the conversion of x-rays to light and light to electrons are that x-rays cannot be focused,
and they, by themselves, cannot be amplified or accelerated. Once the image is
converted into an electron image, it can be electronically amplified and focused.
(2) The electrons comprising the image are accelerated by the accelerating
and focusing electrodes to high speeds by the application of high voltage placed across
the highly evacuated tube. The photoelectron current is then focused by a low potential
on the inside metallic coating of the tube so that it passes through the anode aperture.
(3) The paths of the high-energy electrons flowing from the image-input
phosphor converge at a point and are electrostatically focused on the face of the output
phosphor, forming bright image on the output phosphor. The output phosphor consists
of materials similar to the front of the ordinary television picture tube that will give off
light when struck by high-energy electrons. This conversion of electrons to light is
necessary for visualization of the image.