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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 picture of the projected image of the teeth and surrounding structures similar to a photograph.  Whether using rigid or phosphor plate digital receptors or conventional film, the purpose of the receptor is to record the projected image or area of interest.  In discussing the principles of accurate image projection, the source of x-ray photons is the focal spot on the anode target inside the x-ray tube within the x-ray head.  The principles of accurate image projection 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 is inside the x-ray tubehead, the greater the detail or resolution of 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 machine use.  When focal spot enlargement does occur, the resultant radiographic image becomes less sharp.  The focal spot should be monitored through a quality assurance program.  Resolution test devices will determine any change in the focal spot size and can indicate when an x-ray head may need to be replaced.  These tests may be part of state-mandated x-ray machine inspection.

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 object to be recorded.  The use of a long open-ended position indicating device (PID or cone) will enable the x-ray photons to emerge in a straighter line therefore producing a more dimensionally accurate image.  The straighter the x-ray photon line, the less divergent the x-ray beam.  The resultant image will be a sharper, more accurate representation of the structures 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 single principle more so than the paralleling technique.  However, the bisecting angle technique does not conform to the other principles of accurate image projection.  As a result, it 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 radiographic 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 Tooth and Receptor.

The x-ray beam must be directed perpendicular or at a right angle 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 or length is seen, and the resultant image will appear either foreshortened (shorter than the actual object) or elongated (longer than the actual object).  Right angle entry of the x-ray beam improves anatomic accuracy and reduces shape distortion.

These same principles of accurate image projection can be demonstrated by using a flashlight and projecting the shadow of an object onto a wall in a dark room.  Completing this simple exercise may help the novice radiographer to better understand how to apply these principles and their role in producing an accurate image.  Also, it provides insight into how and why technical errors occur when the principles are violated.

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

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