b. Viewing. The image intensifier tube creates a fluoroscopic image that is
many times brighter than a conventional fluoroscopic screen, but this image is too small
to be of value. Therefore, a means of magnifying and viewing the image is necessary.
(1) An efficient magnification and viewing system should meet the following
Enlarge the part to approximately life size or greater.
Provide an image for both eyes simultaneously (binocular).
Place minimum restrictions on the user.
Not degrade the resolution of the image.
Not degrade the contrast of the image.
Provide viewing for two or more people simultaneously.
(g) Efficiently gather the light generated at the output phosphor.
(2) Magnification is now controlled by the image intensifier with the ability to
control the field size. Observation is now open to anyone that can see the screen of the
digital monitor; and there is often a monitor in the control-room as well as the
(3) To permit TV monitors, cine cameras, or videotape recorders to be used
with an image intensifier, a beam-splitting device is used. It is made of plate glass
coated with a special layer that reflects part of the incident light and allows the
remainder to be transmitted. The beam splitter channels the image to different devices;
therefore, the intensity of each image is reduced. To maintain the same light intensity,
the technique must be increased, which will result in increased patient exposure. If only
one medium of visualization is used, the beam splitter can be moved out of the way.
c. Focusing. Focus depends upon the voltage applied across the electrodes of
the image-intensification tube. Poor electrostatic focus may cause unsatisfactory image
detail. Focus may be checked by observing the resolution obtained from a suitable test
object, such as an area of the fine-meshed wire. The resolution capability of an image
tube is a measure of its ability to show physiological detail. To do so, it must depict
discrete parallel lines of equal width, spaced by an amount equal to the width of each
line. These are called line pairs and consist of one black line and one space. To
perceive the border of a certain area, this border must be darker or lighter than its
surroundings. The contrast between two areas must have a certain minimum value to
be visible by the human eye. The contrast of the image is not improved by
amplification. If greater contrast is desired, contrast media are used.