c. Rate of Tube Travel. When the amplitude and the exposure time are known,
divide the amplitude by the exposure time to find the rate of tube travel, as follows:
Rate of tube travel =
EXAMPLE: If the amplitude is 15 inches and the exposure time is 1.5 sec, then the rate
would be 10 inches per second.
There are still equipment sets in hospitals around the world where these
calculations are needed. Most tomographic equipment today is computerized
and performs the calculations for the technologist.
4-21. GENERAL PRINCIPLES
a. Introduction. For a tomographic examination, the patient is positioned on
the table as for ordinary radiography. The part is measured and the basic exposure
technique selected. Then a fulcrum point is selected on the basis of a lateral exposure
to correspond to the level of the focal plane in centimeters. Usually, several exposures
correspond to the level of the focal plane in centimeters. Usually, several exposures or
cuts are made at different planes until the desired plane is demonstrated. In sections
where tomographic units are available, precise operating instructions depend upon local
policy. However, certain general instructions apply in most cases.
b. Control of Image Detail. Because of the blurring effect, the image is likely to
have reduced sharpness unless you:
Use the minimum source to image distance (SID).
Select an SID of at least 30 inches.
Use a small focal spot.
Select an exposure technique that will provide adequate penetration.
Ensure that the section is not more than 2 to 3 mm thick.
(6) Give the patient breathing instructions just before the exposure. Do not
forget that tomographic exposures are longer than normal radiographic exposures. This
gives the patient more time in which to breathe, thereby, increasing the possibility of