Intraoral Technique, Errors & Localization

Paralleling vs bisecting angle

Feature	Paralleling	Bisecting angle
Receptor position	Parallel to the tooth's long axis (away from the tooth)	Close to the tooth, touching the crown
Central ray	Perpendicular to receptor and tooth	Perpendicular to the imaginary line bisecting the tooth–receptor angle
Accuracy	Least distortion; preferred technique	Some distortion; zygoma may overlap maxillary molar roots
Use	Routine periapicals and bitewings	Shallow palate, tori, children, gag reflex

Bitewings: about +5 to +10° vertical angulation, horizontal angle through the contacts.

Image error	Cause
Elongation (teeth too long)	Too little vertical angulation
Foreshortening (teeth too short)	Too much vertical angulation
Overlapped contacts	Incorrect horizontal angulation (beam not through the interproximal spaces)
Cone cut (clear unexposed area)	Beam not centered on the receptor
Apices missing	Receptor not placed deep enough, or too little vertical angle
Blur	Patient, tube or receptor movement
Herringbone or tire-track pattern	Film packet placed backward (lead foil facing the tube); light, underexposed image
Double image	Same receptor exposed twice
Bent or creased receptor lines	Film bent; PSP plate scratched or bent

Same lingual, opposite buccal (SLOB; buccal-object or Clark's rule): take two images with the tube shifted. If the object moves in the SAME direction as the tube, it is lingual; if it moves in the OPPOSITE direction, it is buccal.
Works with horizontal or vertical tube shifts (e.g., periapical plus bitewing or occlusal).
Right-angle technique: a periapical plus a cross-sectional mandibular occlusal shows buccolingual position directly.
CBCT gives three-dimensional localization when two-dimensional methods are not enough (e.g., impacted canines, third molar–canal relationship).

Swapping elongation and foreshortening.
Fixing overlap by changing vertical angulation: overlap is horizontal.
Reading SLOB backward when the tube moves mesially vs distally: compare the object's movement to the TUBE's movement.
Placing the receptor touching the teeth in paralleling technique.

SLOB: Same Lingual, Opposite Buccal (compare the object's movement with the tube's).

Pick an answer to see right away whether it is correct and why.

The teeth on a periapical image look too short. The likely error is:
Bitewing contacts overlap between the premolars. What should be changed?
A periapical film is light with a herringbone pattern. The cause is:
A second image is taken with the tube shifted distally; an impacted canine appears to move distally. The canine is:
Why is paralleling preferred over bisecting angle?

0 of 5 answered

Answer key: 1 A · 2 D · 3 D · 4 C · 5 C