a. Amplitude. Amplitude is the distance, in inches, that the tube travels during
the exposure. A short amplitude will project a thick section; a long amplitude will project
a thin section. Amplitude is usually automatically set when a particular movement is
selected on polytomographic units. On rectilinear units, the amplitude must be set
manually.
b. Focus-film Distance. When the amplitude remains constant, the shorter the
SID, the thinner the section. This is true because more tube movement (and
consequently more blurring) occurs in the planes above and below the focal plane. On
the other hand, the greater the SID, the thicker the section.
4-19. MILLIAMPERE SECONDS
Because of the increase in part thickness due to tube movement, the mAs
(milliampere seconds) must be increased from 50 percent to 100 percent over the mAs
normally used in radiography for the same body part.
4-20. EXPOSURE TIME, AMPLITUDE, AND RATE OF TUBE TRAVEL
In polytomography, amplitude and rate of tube travel are automatically set when
a particular tube movement is selected. Exposure time for each movement is found in
the manufacturer's operating manual. In rectilinear systems, the three factors can
usually be adjusted to fit the situation.
a. Exposure Time. To find the exposure time when the amplitude and the rate
tube travel (in inches per second) are known, divide the amplitude by the rate. The
formula is expressed as follows:
Exposure = Amplitude (distance of tube travel)
Rate of tube travel
EXAMPLE: If the amplitude is 15 inches and the tube is to travel 10 inches per second,
then the exposure time would be 1.5 sec.
b. Amplitude. When the exposure time and the rate of tube travel are known,
multiply them to find the amplitude as follows:
Amplitude = Rate of the travel x exposure time
EXAMPLE: If the rate is 10 inches per second and the exposure time is 1.5 sec, the
amplitude would be 15 inches.
MD0959
4-18
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