The Long and Short of it All

The Long and Short of it All
  • Sherry Ortiz
  • Jul 23, 2012

 

When I start talking about electron beams, focal spots, and the how angle of the target can affect projection geometry on tooth image I often get glassy-eyed stares from my students.  I decided to put this blog in language we all can easily understand without having a background in physics or geometry.

 

There are several factors which can prevent your radiographic images from being crisp and sharp and not “fuzzy.” Also, images can become magnified. Considering the small proportions of dental structures it is important to know how to minimize the fuzziness and magnification of the radiographic image.

 

One factor which affects the definition of the image is the focal spot. Because I promised no physics, simply put the focal spot is an area on the anode target where the electrons strike to generate x-rays. It is sometimes referred as the anode or source of x-rays and is housed in the x-ray tube itself. We never see the focal spot or the x-ray tube for that matter. The smaller the focal spot, the sharper your image will be. The focal spot in dental x-rays are generally small somewhere between 0.5 to 1.2 mm. However, the focal spot cannot be adjusted so this is something to consider when purchasing your equipment.

 

An example of parallelling.

 

The first thing you can do to decrease fuzziness and increase the sharpness is to remember to keep the film, PSP or sensor as close to the object (teeth) you are imaging as possible. Reducing the object-film distance will result in a sharper image than images taken with the film further away from the teeth. However due to the curve of the arch or the thickness of a sensor, sometimes it is difficult to get close contact with the teeth. Having the teeth away from the film may result in a fuzzy and magnified image.

 

What can be done to minimize both fuzziness and magnification is to increase the focal-object distance or source-object distance (same thing-different terms).  Some x-ray tube heads come with cones which can be interchanged. You can use a short or long cone. By using the long cone instead of the short cone you will increase your source-object distance and increase the sharpness on your radiographic image. Some manufactures now house the x-ray tube farther away from the end of the cone so what might look like a short cone to you is really a long source-object distance. You can check with the manufacture to see what your equipment specs are.

 

When you increase the source-object distance you also decrease magnification. Magnification is the equal enlargement of the actual size of the tooth that is projected on the radiograph. Magnification should not to be confused with shape distortion such as elongation or foreshortening of the image. When using a short source-object distance or short cone when taking radiographs you may not know that the image of the tooth will be magnified. Increasing the source-object distance by using a long cone will help project the actual size of the tooth. There is a geometric formula that can calculate this. But all you need to do is just take a flashlight and shine it on an object and then move the light back. You can see that the object gets smaller and sharper the further you move the light source from the object.

 

So remember, to get the sharpest, true-to-size tooth images keep the imaging plate as close to the tooth structure as possible and use a long cone to keep your source-object at a distance which minimizes magnification.

 

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