an 11-inch radius is needed to cover the 14 x 17 area. To determine x-ray coverage,
use the following formula: the tangent of the target angle (tan) times the focus-film
distance (FFD) equals the radius of the area covered (RC) or tan X FFD = RC.
Example: If the target angle is 20 and the FFD is 40 inches, the radius of the covered
area would be 14 inches, since the tangent of 20 is 0.364.
Figure 3-14.
Target angle and x-ray coverage.
3-13. GRID-CONTROLLED TUBE
a. To prevent relay damage due to arcing, an x-ray exposure is normally
synchronized to the line voltage so that it begins and ends when the sine wave is at
zero value. With this system, in a single-phase generator the shortest exposure
possible is 1/120 second since the sine wave reaches zero value every 1/120-second.
With a grid-controlled tube, the exposure is also synchronized with the line voltage, but
it does not necessarily begin and end at zero value. It can be synchronized to a
particular portion of the sine wave.
(1) Figure 3-15 shows two waveforms with exposures of 1/120 second on a
conventional times tube and 1/360 second on a grid-controlled tube. Since the
conventional exposure should begin and end at zero value, it must encompass one
complete pulse. On the other hand, the grid-controlled exposure indicated here
includes only the middle third of the pulse. Even shorter exposures are possible with
grid-controlled tubes, and this makes them desirable for examinations requiring very
short exposures.
MD0950
3-19