Figure 3-15. Effect of conventional and grid-controlled tubes.
(2) When a grid-controlled, short exposure is used, such as is demonstrated
in figure 3-15, the radiation dose to the patient is reduced. The reduction occurs
because the exposure does not encompass the portions of the wave that produce only
lower energy photons. The patient dose is also reduced when grid-controlled tubes are
used with cinefluorography (cine = motion picture) because x-ray production can be
synchronized with the cinecamera. The synchronization reduces the radiation dose to
the patient because x-rays are produced only when the shutter is open.
b. In addition to the two conventional elements, the cathode and anode, the grid-
controlled tube has a third element or grid. The grid, which is actually the focusing cup,
is electrically isolated from the filament. In operation, negative (bias) voltage is applied
to the grid, which makes the grid negative with respect to the filament. A "negative
electrostatic field" is set up which acts as a gate to stop electron flow by repelling the
negatively charged electrons. Therefore, when the bias voltage is applied, no electrons
reach the anode and x-rays are not produced. When the bias is removed, electrons
flow to the anode and x-rays are produced. Because the grid is closer to the filament, a
comparatively small change in the voltage to the grid will override a much larger positive
charge of the anode.