Extraoral Techniques & Error Correction
Mastering the Panoramic & Cephalometric View
🎯 Learning Objectives
- Identify the Focal Trough and its importance in panoramic imaging.
- Recognize the “Smile” and “Frown” errors caused by chin positioning.
- Understand how to prevent Ghost Images and lead apron artifacts.
- Differentiate between Midsagittal and Frankfort plane alignment.
1. The Focal Trough
Extraoral imaging relies on a Focal Trough (image layer). This is a three-dimensional curved zone where the structures are captured clearly. If the patient’s teeth are too far forward or too far back from this “sweet spot,” the image will be blurred.
2. Panoramic Positioning Errors
The DANB exam frequently uses visual descriptions of these errors. Use this chart to link the Effect to the Cause:
| The Visual Result | The Error (Cause) |
|---|---|
| “Exaggerated Smile” | Patient’s chin was too low. |
| “Reverse Smile” (Frown) | Patient’s chin was too high. |
| Skinny Anterior Teeth | Patient was too far forward (ahead of the focal trough). |
| Fat/Wide Anterior Teeth | Patient was too far back (behind the focal trough). |
| Dark shadow over maxillary roots | The tongue was not against the palate. |
3. The “Ghost Image”
A ghost image is a radiopaque (white) artifact that looks like a larger, blurred version of a real object. It appears on the opposite side of the image and higher up than the original object.
- Common Causes: Earrings, necklaces, dentures, or hearing aids.
- Prevention: Always ask the patient to remove all “jewelry from the neck up.”
🚨 DANB EXAM ALERT: The “Shark Fin”
If a large, triangular radiopaque (white) shadow appears in the bottom center of a panoramic image, it is called a Shark Fin artifact. The Cause: The lead apron was placed too high on the back of the neck or a Thyroid Collar was used. Remember: Never use a thyroid collar for panoramic imaging!