Table of Contents
Panoramic x-rays, CT scans and Cone Beams
Most dental professionals are familiar with the quintessential panorex machine, and know something about “CAT” scans and Cone Beams, but do not know the principles of their operation, or how they assemble their images. This page, and the two that follow it attempt to give an overview of the subject.
“Tomos” is the Greek word for “cut” or “section”, and tomography is a technique for digitally cutting a specimen open using X-rays to reveal its interior details. The Panoramic x-ray is an example of one such slice. It is the simplest form of tomogram in medicine, because it does not attempt to show any volumetric, or 3-D information. It produces a two dimensional image of a single slice along the mandibular arch.
The x-ray beam is fan shaped, with the fan “unfolded” in the vertical dimension. This beam is projected through a vertical slit close to the patient as the machine slowly revolves around his or her head. The beam and the x-ray cassette revolve around the patient’s head in the same direction (clockwise when viewed from the top), while the cassette itself translates in the opposite direction. The image produced shows the bony structures closest to the film cassette.
You might wonder how only the bony structures closest to the cassette are recorded. After all, the x-ray beam has to pass through the bony structures on the opposite (contralateral) side of the skull before encountering the structures closest to the cassette. Why doesn’t the panoramic image show those structures as well?
The panoramic x-ray machine (also known by their brand names, the two most popular being the Panorex and the Orthopantomograph [OPG])makes use of distortions, which in ordinary intraoral radiographs would produce very poor images.
- First, unlike the beam of a normal long cone dental x-ray machine, the fan shaped beam of the panorex is quite divergent (like an expanded fan). As discussed on the Shadow Casting page, a divergent beam will magnify objects that are closer to the source than those that are closer to the receptor (film or sensor).
- Since the contralateral side of the skull (which is furthest from the cassette) is closer to the source of the x-rays, any shadow cast on the receptor by structures on that side of the skull will be greatly magnified. This not only blurs the edges of contralateral shadows, but since the image is disbursed over a larger expanse, the intensity of the shadow they cast will be much dimmer than the shadow cast by objects closer to the cassette (the ipsilateral side).
- Also, since the contralateral structures are closer to the focal spot, and the fan shaped beam is highly divergent, only a small wedge of the contralateral structures are illuminated leaving fewer recognizable structures to leave disbursed shadows.
- Second, the cassette is translating in the direction opposite to the revolution of the x-ray head. The velocity of the cassette is controlled in such a fashion as to fit exactly the velocity of the projection of the anatomic features which are closest to the film. This means that the shadows cast by objects on the ipsilateral side will spend more time on the receptor than objects further away from the cassette. The longer exposure creates a darker shadow for ipsilateral objects than for those cast by objects on the contralateral side.
- Third, since the cassette is moving opposite the direction of revolution, it is moving more slowly with respect to the structures closer to the cassette than it is for the structures further away from it. Since the shadows of the contralateral structures are moving so quickly across the receptor, they will appear much more blurred than the structures which are closer and moving more slowly.
To recap: These are the reasons that the contralateral structures show either not at all, or are only faintly visible on the film.
- They appear very enlarged, and thus blurry and dim.
- They are made more dim by the fact that the shadows these objects cast spend less time falling on the receptor.
- Finally, the shadows of the contralateral objects are blurred by their greater velocity.