Principles of Accurate Image Projection

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 capture 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 (Figure 2). The principles of accurate image projection can be summarized as follows:

Principle One: X-rays Should be Emitted from the Smallest Source of Radiation 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.³ This enlargement is often referred as “blooming” of the focal spot. When focal spot enlargement does occur, the resultant radiographic image becomes less sharp or fuzzy in appearance.³ X-ray machines in high-use operatories may exhibit signs of blooming before other intraoral x-ray machines in the office as a consequence of the greater workload and excessive heat generation. 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. Specialized equipment is required to conduct this test.³ This test may be part of periodic state-mandated x-ray machine inspections.

Figure 2: Focal Spot

Figure 2: Focal Spot

Principle Two: The X-ray Source-to-Object Distance Should be as Long as Possible.

The x-ray source-to-object distance (Figure 3) 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.

Figure 3: Source-to-Object Distance

Figure 3: Source-to-Object Distance

Principle Three: The Object-to-Receptor Distance Should be as Short as Possible.

The object in this principle refers to the tooth or anatomical structures being radiographed. Placing the receptor as close as possible to the object reduces magnification and improves image sharpness (Figure 4). The bisecting angle technique follows this single principle more 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 the primary technique. Shape distortion is defined as deviation from the true shape or dimension of the object. The bisecting angle technique is considered a secondary method but may be a necessary compromise in certain clinical situations such as a shallow plate or presence of tori.

Figure 4: Object-to-Receptor Distance

Figure 4: Object-to-Receptor Distance

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 to each other (as in the paralleling technique), the distortion of the radiographic image is minimized. When this alignment is achieved, the x-ray beam is directed perpendicular (right angle) to both the object and receptor (Figure 5). Receptor to structure parallelism improves anatomic accuracy and reduces shape distortion.

Figure 5: Object-Receptor Parallelism

Figure 5: Object-Receptor Parallelism

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 (Figure 6). 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. Table 1 provides a summary of these principles and their impact on radiographic imaging.

Figure 6: CR Entry

Figure 6: CR Entry

These same principles of accurate image projection can be demonstrated by using a flashlight and projecting the shadow of an object or objects onto a wall in a dark room. Completing this simple exercise may help the novice radiographer 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 these 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 identified. The paralleling technique, which typically utilizes receptor-holding devices, is the preferred intraoral technique because it follows four of the five principles of accurate image projection listed above.