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Intraoral Radiography: Principles, Techniques and Error Correction

Gail F. Williamson, RDH, MS

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To better understand x-ray imaging, it is helpful to consider a radiograph as a shadow image of the teeth. Whether using rigid or phosphor plate digital sensors or conventional film, the purpose of the receptor is to record the shadow image. In discussing shadow-casting principles, the source of x-ray photons is the focal spot on the anode target inside the x-ray tube. Shadow-casting principles can be summarized as follows:

Principle One: X-rays should be emitted from the smallest source of radiation as possible. 

As electrons strike the focal spot, x-rays are emitted. The smaller the focal spot inside the tubehead, the greater the detail on the resultant image. Manufacturers govern the size of the focal spot, and it cannot be changed by the operator. However, the focal spot can become enlarged over time due to continuous use. When focal spot enlargement does occur, the resultant radiographic image becomes less sharp. The focal spot must be monitored through a quality assurance program. Resolution test devices will determine any change in the focal spot size.

Principle Two: The x-ray source-to-object distance should be as long as possible. 

The x-ray source-to-object distance refers to the distance between the focal spot and the receptor. The use of a long position indicating device (PID or cone) will enable the x-ray photons to emerge in a straighter line therefore producing a truer image and more accurate shadow. The straighter the x-ray photon line, the less divergent the x-ray beam. The resulting image will be a sharper, more accurate presentation of the structure being radiographed and demonstrate less image magnification.

Principle Three: The object-to-receptor distance should be as short as possible.

The object in this principle refers to the tooth or structures being radiographed. Placing the object close to the receptor reduces magnification and increases image sharpness. The bisecting angle technique follows this principle more so than the paralleling technique. The bisecting angle technique is more prone to shape distortion and is not recommended as a primary technique. Shape distortion is defined as deviation from the true shape of the object.

Principle Four: The receptor and long axis of the tooth should be parallel to each other.

When the receptor and the long axis of the tooth are parallel (as in the paralleling technique), the distortion of the radiographed image is decreased. Receptor to structure parallelism improves anatomic accuracy and reduces shape distortion.

Principle Five: The x-ray beam should be directed perpendicular to the receptor. 

The x-ray beam must be directed perpendicular to the long axis of the tooth, which ideally is also perpendicular to the receptor. When this principle is not followed, an error in vertical angulation is seen, and the resultant image will appear either foreshortened or elongated. Right angle entry of the x-ray beam improves anatomic accuracy and reduces shape distortion. The central ray (CR) is the center of the the x-ray beam and is often used to indicate the beam entry as well as centering the x-rays over the receptor.

These same shadow-casting principles can be demonstrated by using a flashlight and projecting the shadow of an object on a wall in a dark room. Completing this simple exercise may help the novice radiographer to better understand the need to apply these principles to produce an accurate image.

The "perfect" radiographic technique incorporates all five principles of shadow casting. Unfortunately, an ideal technique which meets all the requirements for accurate shadow casting has not been found. However, the paralleling technique which typically utilizes film-holding devices is the preferred intraoral technique because it follows most of the shadow-casting principles listed above.

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