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Paralleling Radiographic Exposures : The Preferred Method. XCP. Key Terms to Know. Parallel Intersecting Perpendicular Right angle Long axis of the tooth Central ray. Basic Rules of Paralleling- Know. Film placement Film position Vertical angulation Horizontal angulation
Basic Rules of Paralleling- Know
B. The central ray of the x-ray beam is directed perpendicular to the film and long axis of the tooth
Define long axis of tooth- *teeth to do not sit in the mouth straight up & down
Object and film/receptor are not parallel = distortion
object-receptor-distance & target-receptor-distance(tooth to film) (anode/focal spot to film)
*Both must be increased when using the paralleling technique*
Source–film distance.The longer the source–film or receptor distance, the more parallel the rays and the less tissue exposed. Note the beam size at the patient’s skin entrance is 2.75 in. (7 cm) for both source–film distances; it is the exit beam size that increases to expose a larger area when using the shorter source–film distance. Remember from safety module- 16” cone preferred due to less divergence of x-ray beam as exits.
When holder tilted more than 20-degrees due to patient conditions (and therefore NOT parallel to long axis) VERTICAL ANGULATION needs to be INCREASED 5-15 degrees more than indicated (therefore NOT lined up with circle of XCP)
**However, if angulation too excessive = incisal edges missing from image & foreshortening**If angulation inadequate (not enough) = apices missing & elongation
7 Size #1 films used for anteriors
- canine (both sides)
- lateral/central incisors (both sides)
- lateral/central incisors
8 (Size #2) films are taken in the posterior
Center 2nd molar
Must see distal of canine on premolar PA
Center central/lateral contact
Center central incisors
When using the paralleling technique, an exposure sequence, or definite order for periapical film placement and exposure, must be followed.Keep track of exposures on tray cover!
I. Start with anterior exposure sequence (Size #1 film used)
-always expose the premolar first in all areas
-8 films are taken
-begin with maxillary right premolar then molar
-move to mandibular left –premolar then molar (you do not have to reassemble XCP for this area)
-reassemble XCP and move to maxillary left-premolar then molar
-finish with mandibular right area
- 4 radiographs are usually taken; horizontal BW unless otherwise indicated
- BW part of FMS
Don’t forget to put the dot at the incisal edge now!