(4) The total amount of image intensification or total gain is based on
electronic intensification and minification. A high positive voltage (25 to 30 kV) applied
to the accelerating electrodes speeds up the electrons emitted to the photo cathode.
This accounts for a gain in light intensity of approximately 35 to 45 times. Minification is
determined by the ratio of the input phosphor to the output phosphor. For example, if
the input area of a 9-inch tube is 9 by 9 inches, or 81 square inches, and the output
diameter of this same tube is 1 inch (with an area of 1 square inch), the minification
factor is 81:1. The total intensification or total gain is equal to the amount of electronic
intensification multiplied by the minification factor. A tube having an electronic
intensification of 40 and a minification factor of 81 would have a total gain of 3,240;
meaning the image would be 3,240 times brighter than a fluorescent screen excited with
the same amount of radiation.
(5) A lens system, which is actually part of the viewing system, collects and
collimates the light emitting from the output phosphor.
c. Image intensifiers are rated at 5, 8, 9, and 11 inches. This does not relate to
the field size covered, but to the diameter of the input phosphor. The 8-inch and 9-inch
image intensifiers are generally preferred for fluoroscopy and/or cinefluorography.
d. Another development is the dual field tube. In effect, it provides a 6-inch or
9-inch input phosphor at the same time so that the fluoroscopist may use either the
6-inch or 9-inch mode at will. By changing the focusing voltages, the minification factor
is changed. When using the 6-inch mode, the center 6-inch square of the 9-inch input
phosphor is transmitted to the same output phosphor as used for the 9-inch mode.
Since the input/output ratio is 6:1, the image will be larger, but part of the amplification is
lost. This image enlargement pertains to the dual field tube. Standard image tubes
amplify the intensity of the light image, but do not enlarge it.
1-15. BASIC OPERATIONAL CONSIDERATION
a. Installation. A typical image intensifier unit is usually mounted upon a
motorized stand balanced with counter-weights that allows easy maneuverability (figure
1-5). To set up for the procedure, the intensifier unit is moved into position, the
shielding in moved into place, and the spotfilm device is loaded. After the necessary
connections and adjustments have been made, image-intensified fluoroscopy can be
accomplished. The x-ray table unit may be titled as for conventional fluoroscopy (figure
1-6).
MD0959
1-11
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