Paralleling Technique: Periapical Radiographs

What Is the Paralleling Technique?

The paralleling technique is the preferred method for taking periapical radiographs because it produces images with the least geometric distortion. The core principle is simple: the image receptor (film, PSP plate, or digital sensor) is placed parallel to the long axis of the tooth, and the central ray of the x-ray beam is directed perpendicular (at a right angle) to both the receptor and the tooth. This geometry minimizes elongation and foreshortening, giving the dentist an accurate picture of tooth length, root morphology, and surrounding bone.

Compare this to the older bisecting-angle technique, which angles the beam toward an imaginary line between the tooth and the receptor. The bisecting technique is more prone to distortion and is used only when the paralleling technique is not possible.

Film-Holding Instruments

Because the receptor must be held away from the tooth (often near the center of the mouth), patients cannot simply bite on it. Standardized film holders — commonly the Rinn XCP (Extension Cone Paralleling) system or equivalent — hold the receptor in the correct position and attach to an aiming ring and indicator rod that guide the tube head to the right angulation. Each component has a role:

• Bite block: holds the receptor and gives the patient something to close on
• Indicator rod: extends from the bite block toward the patient's face
• Aiming ring: attaches to the indicator rod; the x-ray tube head (Position Indicating Device, or PID) is positioned flush against the ring

XCP kits include color-coded components for anterior (blue) and posterior (yellow) regions. Some manufacturers use a third set (red) for bitewing radiographs. Learn your clinic's specific system.

Receptor Placement by Region

A standard full-mouth series (FMS) consists of 18 periapical images covering every region of the dentition. Placement guidelines:

• Maxillary anterior: receptor placed vertically, centered behind the teeth being imaged, parallel to the long axes of those teeth. The patient's chin is slightly elevated.
• Maxillary posterior: receptor placed horizontally in the palate, parallel to the upper premolar or molar roots.
• Mandibular anterior: receptor placed vertically in the floor of the mouth, parallel to the lower incisors or canines.
• Mandibular posterior: receptor placed horizontally in the floor of the mouth, parallel to mandibular premolar or molar roots.

Always place the receptor so the active surface faces the tube head. With PSP plates, the white side faces the beam; with sensors, follow the manufacturer's orientation mark. Reversed placement causes a severe underexposure with a herringbone artifact (for film/PSP) or a blank/reversed image (for sensors).

Tube-Head Alignment

Once the holder is in place, slide the PID until it rests against the aiming ring. The ring ensures:

1. Correct horizontal angulation — the beam passes through the contact areas of the teeth, avoiding overlapping

2. Correct vertical angulation — the beam is perpendicular to both the receptor and the tooth, preventing elongation or foreshortening

3. Correct centering — the beam covers the receptor fully, preventing cone cuts

Never guess the angulation freehand when using an XCP system. Trust the ring.

Common Errors and Corrections

| Error | Cause | Correction |

|---|---|---|

| Elongation | Vertical angulation too flat (not enough) | Increase vertical angulation |

| Foreshortening | Vertical angulation too steep (too much) | Decrease vertical angulation |

| Cone cut (clear unexposed area) | PID not centered on aiming ring | Re-align PID flush with ring |

| Overlapping contacts | Incorrect horizontal angulation | Adjust PID horizontally through contacts |

| Blurry image | Patient or receptor movement | Instruct patient to hold still; secure holder |

| Light image (underexposed) | Low kVp/mA, short time, or reversed receptor | Check unit settings; correct receptor orientation |

| Dark image (overexposed) | High exposure settings | Reduce exposure factors |

Radiation Safety in the Lab

Every exposure, even on a phantom, requires the same safety habits you will use in clinic.

• Place a lead apron with thyroid collar on the phantom as you would on a patient. Practice this habit now so it becomes automatic.
• Stand at least six feet from the tube head, or behind a barrier, during exposure. Never hold the film or receptor in the patient's mouth during exposure.
• Confirm the x-ray unit is properly set (kVp, mA, exposure time) before pressing the exposure button.
• Announce "X-ray" before each exposure so others in the room can step back.
• Minimize retakes: each unnecessary exposure, however small, adds dose. Correct your technique before re-exposing.

Connecting to Your Credentials

The DANB Radiation Health and Safety (RHS) component exam — one of the exams that make up the CDA credential — tests your knowledge of paralleling technique, receptor placement, error identification, and radiation protection. Your state dental radiology license exam tests similar content with a legal emphasis: operating an x-ray unit without a license is a violation of state law. Mastery of paralleling technique is therefore both a clinical skill and an exam objective.