c. Central Ray (CR) Angle. The second table factor to be adjusted is the angle
of the X-ray tube, more commonly referred to as the central ray (CR) angle. The beam
of rays, or central ray, should be protected onto the film at the desired angle. Accurate
positioning of the part and correct centering of the central ray are of equal importance in
obtaining a diagnostic radiograph. The central ray is centered to the film or portion of
the film used. You will need to adjust the CR angle according to the part to be X-rayed
and the views that are desired.
central ray (CR) angle: the angle and direction of the primary beam in relation to the
film or anatomical part.
d. The Source-to-Image Distance. The third table factor that must be set
is the source-to-receptor distance (SID). (The source is the tube focal spot; the image
receptor is the film.) The SID is the distance in inches between the tube local spot and
the film (figure 2-11). The SID is sometimes referred to as the anode-film distance or
the local spot film distance. The source-to-image receptor distance may be measured
using a digital read-out or by using the tape measure on the tube head. For a vertical
perpendicular central ray, use the automatic digital read-out of the SID. Located on the
overhead tube scale, it provides a digital readout of the distance between the tube focal
spot and the film as you raise or lower the table (figure 2-12). For an angled central ray,
use the tape measure located on the side of the collimator (figure 2-13). A cardinal rule
of radiation protection is that the distance between the radiation source and the
radiologic technologist should be as great as possible. For tabletop procedures, the
SID should be 40 inches from the tube focal spot to the tabletop (figure 2-14). For
Bucky procedures, the SID should be 40 inches to the Bucky tray (38 inches to the
tabletop and 2 inches from tabletop to Bucky) (figure 2-15). For upright procedures,
such as a chest X-ray, the SID should be 72 inches to the film (figure 2-16).
tube focal spot: the area on the anode target where cathode electrons strike to
produce X-rays.
image receptor: the radiographic film.
source-to-Image receptor distance (also referred to as anode-film, focal film, or
focal spot-film distance): distance in inches between X-ray tube focal spot and the
film.
MD0961
2-19
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