Summary of Rotational Panoramic Radiography

Theory of Rotational Panoramic Radiography Panoramic Radiography –Unique in that the focus of the horizontal projection is different from the focus of the vertical projection. In the horizontal dimension, the rotation center of the beam serves as the functional focus. In the vertical dimension, the X-ray source serves as the functional focus, just as a conventional radiographic projection.

In intraoral radiography, –X-rays diverging from the focal spot project the image of the structures of interest onto a film placed inside the mouth : Central Projection –If the X-ray source is placed inside the mouth and the image of a perfectly circular segment of the jaws is recorded on a curved extraoral film concentric to this segment, the result is likewise a central projection. Principle of Projection in the Plane of Rotation (stationary)

In rotational panoramic radiography, –Using an extraoral X-ray source –The intraoral X-ray source is removed and that, instead, a beam of very narrow width is allowed to rotate about the point where the X-ray source was previously located.. (stationary)

–In the horizontal dimension, the X-rays will now appear to diverge from an intraoral focus just as before and an identical projection results. In this case, the object is successively projected on the film as it is scanned by the rotating beam.. (stationary)

The vertical dimension is unaffected by the rotation of the beam in the horizontal plane. In this dimension, the X-ray source serves as the functional focus of the projection. (stationary) Principle of Projection in the Vertical Dimension

If the film is stationary, this leads to a discrepancy in the horizontal and vertical magnification factors. Magnification : the ratio of the focus-to-film (FFD) and focus-to-object (FOD) distances FFD FOD Role of the Moving Film

In the vertical dimension, the magnification is calculated by dividing the distance from the X- ray source to the film by the distance from the X-ray source to the object. In the horizontal dimension, since the focus is different, it is calculated by dividing the distance from the intraoral rotation center to the film by the distance from the rotation center to the object. This makes the magnification factor much greater in the horizontal than in the vertical dimension and leads to the peculiar image.

horizontal vertical

In rotational panoramic radiography, the film is not stationary. Instead, the film is attached to the rotating system, and moves in the same direction as the beam although at a slower speed.

Although this does not affect the central projection of the object, it does affect the length of the image recorded on the film, so that the registered image is foreshortened in the direction of movement. By carefully choosing the speed of the moving film, it is possible to reduce the horizontal magnification until it just matches the vertical magnification for one particular curved plane within the object.

Not difficult to show mathematically that the speed of the film, relative to the beam, required to maintain equal vertical and horizontal magnification factors in a curved plane at a given distance from the rotation center is directly proportional to that distance. –Only one drawback : the technique works perfectly only for this one curved plane. Details at other depths within the object inevitably appear distorted because the horizontal and vertical magnification factors match only for the specific curved plane.

A beam of finite width gives rise to a successive accumulation of unsharpness on a stationary film since any point in the object would be successively projected onto the film by each ray in the beam. Principle of Image Layer Formation.

These rays do not project the point at the same spot, and, as a result, the projection at the film plane of a point in the object moves as the beam passes the point. This movement is in the same direction as the beam movement but at a lower speed.. Motion unsharpness!

If the film moves at a speed that follows the moving projection of a particular object point, this point will always be projected on the same spot on the film, and will not appear unsharpness in the resulting image. Fortunately, the film speed required to accomplish this goal is the same speed needed to equalize the horizontal and vertical magnification factors...

The radiographic shadows of object points lying outside the plane of equal horizontal and vertical magnification have a projected speed at the film plane different from the speed of the film. This occurs for points located either toward the rotation center or toward the film. As a result, the projection of these points move relative to the film and therefore are portrayed as lines on the film rather than as discrete points.

Since the discrepancy between the speed of the film and the speed of the projection of the points in the object increases with the distance from the distortion-free object plane, the motion unsharpness increases in both directions from this plane. At some distance the unsharpness reaches a level at which an object point is no longer perceptible in the image. Unsharpness increases

In this way a zone in the object may be defined that contains those object points that are depicted with sufficient resolution so that they may be distinguished. This zone is called the image layer or focal trough, and the distortion-free plane that lies in the central region of the zone is called the central plane of the image layer. Unsharpness increases

A constant film speed in relation to the beam places the central plane of the image layer at a defined distance from the rotation center of the beam..

If the speed of the film is increased, the position of the layer shifts away from the rotation center. If the speed of the film is decreased, the position of the layer shifts toward the rotation center. For large V For smaller V

If the speed of the film increase or decrease during the exposure rather than remaining constant, a continuous shift of the position of the layer occurs. –Acceleration shifts the position of the layer successively away from the rotation center of the beam. –Deceleration shifts the position of the layer successively toward the rotation center.

The distance from the rotation center of the beam to the central plane of the image layer is called the effective projection radius. The thickness of the layer depends on the length of the effective projection radius. –The longer the radius, the thicker the radius. Thus, an altered film speed relative to the beam not only changes the position of the layer but, indirectly, also the thickness of the layer. For large V For smaller V

The layer thickness depends on several parameters besides the effective projection radius. Among these, the width of the beam is most important. The layer thickness is inversely proportional to the width(=focal spot size 임 ) of the beam. - The layer thickness doubles if the beam width is reduced by one half. The layer formation is a side effect resulting from the use of a beam of finite width to obtain an amount of radiation sufficient to exposure the film.