b. Fluoroscopic Shutters. The fluoroscopic shutters limit the area of the
screen illuminated by x-rays. They not only reduce the area of irradiation, but also
lessen the effect of SR (scattered radiation or secondary radiation) on patient, staff, and
fluoroscopic image. TB MED 521 specifies that the useful beam must be restricted to
an area that is less than the lead barrier. When the shutters are open to their fullest
extent, they should leave a margin of at least 1/4 of an inch of illuminated fluorescent
screen with the screen at its greatest practical distance from the tube.
c. Image Intensifier. The image intensifier replaced the fluoroscopic screen, a
sheet of leaded glass with zinc-cadmium sulfide coating. The image intensifier is a
complex device that receives the remnant x-ray beam, converts it into light (figure 1-1)
and increases the light intensity. It is usually contained in an evacuated glass envelope
for structural support. The image intensifier is usually mounted with in a metal container
to protect it from rough handling.
Figure 1-1. An image intensification tube.
PATIENT, FOCAL SPOT, AND SHUTTER RELATIONSHIPS
Starting from the focal spot, the path of the x-ray beam passes through the filters
(inserted between the exit portal of the tube and the shutter), the "open" portion of the
shutter, the table top, the patient, the grid (if used), and the image intensifier, the beam
is stopped and captured at the input phosphor. The lead glass (which stops most of the
x-ray beams) provides protection from the CR for the specialist and radiologist. In the
typical fluoroscopic assembly, the tube, shutter, and fluoroscopic screen are arranged to
always move together, in any direction, at right angles to the path of the CR. The
relationships of the tube-to-screen and of the part-to-screen distances affect
magnification and distortion of the image, just as in radiography.