Reference

Radiolucent vs radiopaque: dark and light on a dental radiograph

On a dental radiograph, radiolucent areas look dark and radiopaque areas look light. Here is what that means, which structures show which way, and what angulation errors do to the picture.

RadiolucentLooks dark

RadiopaqueLooks light

Source: MD0512 ¶3-24.

Why some areas look light

Where two teeth overlap, the manual describes a light area forming because the X-ray beam cannot get through the two dense surfaces.

Source: MD0512 ¶4-8b.

MD0512 was written for film, so it says “on the film”.

Two panels: on the left a radiograph with a solid white mass in a molar, on the right a photograph of the molar. — Left, a restoration shows as a solid white mass on the radiograph. Right, the tooth itself.Image: DRosenbach · Public domain · source

Which structures look dark, and which look light

Panoramic dental radiograph of a full adult dentition, with dark areas above the upper back teeth on both sides. — A panoramic radiograph: the maxillary sinuses are the large dark areas above the upper back teeth, on each side.Image: Roquex · CC0 · source

Dark on the radiograph
Structure	How it looks	Source
Maxillary sinus	A very prominent radiolucent structure, sometimes as overlapping lobes or a single area with a radiopaque border	MD0512 ¶3-25a
Incisive foramen	A dark area between and above the central incisors	MD0512 ¶3-25b
Nasal fossae	Roughly elliptical radiolucent areas of various sizes	MD0512 ¶3-25d
Mandibular canal	A dark band with radiopaque borders	MD0512 ¶3-27b
Mental foramen	A dark area below and between the bicuspids	MD0512 ¶3-27c

Light on the radiograph
Structure	How it looks	Source
Nasal septum	A radiopaque band between the nasal fossae	MD0512 ¶3-25d
Zygomatic arch	Commonly a well-defined radiopaque area	MD0512 ¶3-26c
Border of the mandible	A heavy white line	MD0512 ¶3-28a
External oblique ridge	A white line of variable density	MD0512 ¶3-28b

This page lists only what its sources state. Tooth decay, the pulp, air and tooth-colored fillings are missing because none of these sources says how they look.

Dental radiograph with a large dark oval area above the roots of a tooth. — A radiolucent area that is not a normal landmark: the dark oval above the roots.Image: “Periapical radiolucency” by Shaimaa Abdellatif · CC BY-SA 4.0 · source

Angulation errors

Pointing the beam at the wrong angle changes the image, in three named ways.

Foreshortening: The vertical angulation is larger than necessary, and the teeth look shorter than normal. (MD0512 ¶4-8a)
Elongation: The vertical angulation is less than necessary, and the teeth look longer than normal. (MD0512 ¶4-8a)
Overlapping: The horizontal angulation is incorrect, so the proximal surfaces of adjacent teeth are superimposed, leaving a light area where they overlap. (MD0512 ¶4-8b)

Practice reading radiographs

Start the free Radiation Health and Safety practice test About the exam

The app

Study radiography on your phone with Dentalingo

Dentalingo has 1,520 questions across the RHS®, ICE®, GC, AMP and CP exams, every answer explained and checked against a cited source, and timed mock exams with each exam’s question count and time limit.

Lessons in a path, a few minutes a day, with streaks and daily goals, spaced review of the questions you missed, and readiness by exam domain.

The first 7 questions are free. After that, full access is a Study Camp subscription; the App Store shows the price in your country. More about the app

Scan to get Dentalingo on your iPhone.

Dentalingo's Practice screen: daily quests and the questions due for spaced-repetition review.

Dentalingo's question screen: a chairside question about a blood pressure reading, answered correctly, with the explanation below the options.

Sources

All of these are U.S. government works, in the public domain. Images are credited under each one.

Dental Radiography (subcourse MD0512)U.S. Army Medical Department Center and School · U.S. government work, public domain