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Paralleling Radiographic Exposures: The Preferred Method

XCP

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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• Film exposure

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Principles of Paralleling Technique

A. The film is placed in the mouth parallel to the long axis of the tooth being radiographed

B. The central ray of the x-ray beam is directed perpendicular to the film and long axis of the tooth

C. A film holder must be used to keep the film parallel with the long axis of the tooth

Correct Incorrect

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Different Film Holding Devices

• Rinn XCP

• Stabe Bite-Block

Anterior Posterior

Parallel to the Long Axis of Tooth

Define long axis of tooth- *teeth to do not sit in the mouth straight up & down

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CORRECT

Long Axis of Tooth

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Object and film/receptor are not parallel = distortion

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Principles of Paralleling Technique

Remember:• To achieve parallelism, the film must be placed away from the tooth and toward

the middle of the cavity• Object receptor (or film) distance must be increased to keep the film parallel to

the long axis of the tooth• Because of the distance from the film to the tooth, magnification and loss of

definition result. To compensate, the target receptor distance must be increased (16 inch cone used-known as “long cone technique”). This results in less magnification and increases definition.

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Object-Film-Distance v/s Target-Film-Distance

Compare/Contrast object-receptor-distance & target-receptor-distance

(tooth to film) (anode/focal spot to film)

*Both must be increased when using the paralleling technique*

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REVIEW- Protection Measures for the Patient

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.

Challenges with Placement of Holder

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Conditions (i.e. crowding, mal-aligned teeth, missing teeth, torus, size of mouth, vault of palate, presence of unerupted 3rd molars, & variations in occlusal relationship) affect ideal placement

If the holder cannot be placed correctly- the beam will not be aligned correctly = errors (distortion, elongation, foreshortening, etc…)

Patient conditions & cooperation do not always match what textbook teaches!

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Modifications of Placement (see text)

Shallow palate (p. 193- <than 20- degrees tilt acceptable; if more -must modify)

Maxillary/Mandibular tori (p. 207-209)

Muscle attachments (p. 208-210)

Missing teeth (use cotton rolls to stabilize)

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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

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Film Placement for each area- *review handout & text

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9-10

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6 7-8 11

27 23- 26

ANTERIOR

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Film Placement for Anterior

7 Size #1 films used for anteriors Maxillary (4) - canine (both sides) - lateral/central incisors (both sides)

Mandibular (3) - canine - lateral/central incisors

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Film Placement for Posterior

8 (Size #2) films are taken in the posterior

Maxillary (4)- premolar- molar

Mandibular (4)-premolar

-molar

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Must see distal of canine on premolar PACenter 2nd molar

Center canines

Center central incisors

Need to see wisdom teeth

Center central/lateral contact

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Order for FMS

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!

This prevents Errors Loss of timeExposing the patient unnecessarily

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Order for FMS

I. Start with anterior exposure sequence (Size #1 film used)– Seven films are taken– Begin with maxillary right canine-expose all

maxillary right maxillary left– Move to mandibular area-begin with mandibular

left canine and expose remaining films left to right

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Order for FMS Cont’d2. Posterior exposure sequence (Size #2 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

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Order for FMS Cont’d

3. Lastly, take Bitewing radiographs (Size #2 film used)- 4 radiographs are usually taken; horizontal BW unless otherwise indicated

- BW part of FMS

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Advantages of Paralleling Technique

• Produces images with dimensional accuracy• Is simple and easy to learn and use• Is easy to standardize and can be accurately

repeated

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Disadvantages of Paralleling Technique

• Accurate film placement may be difficult at times for the radiographer to achieve

• Film placement can sometimes be difficult for the patient to tolerate (it hurts!)

• Not disposable- most holders have to be sterilized

• Rings are a “guide” only- radiographer must have skill to access accuracy

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Don’t forget to put the dot at the incisal edge now!

Review helpful hints page 211!