Dental x-rays – Some of the reasons for taking radiographs of oral tissues and structures can be outlined as follows:
Determine the presence of hidden decay in areas of the teeth that can not be seen by a mirror and explorer examination. Determine the presence of cysts or tumors. See the development of the permanent dentition under the deciduous teeth.
Show the condition of roots, periodontium and bone levels. Show deep restorations. Show impacted teeth. Indicate the presence of poor dental restorations. Show fractures of the jaw or teeth. Show hidden calculus. Reveal the presence of abnormal structures.
With the information obtained from the dental x-rays, the dentist is able to restore the oral tissues to health, improve appearance, and take steps to prevent future problems. Some less obvious conditions, such as growths or tumors, can be seen only with a radiographic survey. These conditions could be life threatening if left untreated. The benefits of radio graphs far outnumber the hazards of exposure to dental x-rays.
Some of the important facts of dental x-rays can be summarized as follows:
• Dental x-rays are high-energy waves; it takes a lot of electrical energy to form them.
• They have no mass.
• Dental x-rays are neutral (they have no electrical charge of their own).
• They travel at a speed of 186,000 miles per second (the speed of light).
• Dental x-rays are invisible.
• They travel in straight lines.
• They can not be focused on a point.
• Dental x-rays harm living tissue.
• They affect photographic film.
• Show periapical abscess.
• They cause fluorescence.
Dental x-rays must penetrate tissue to produce a radiograph. The penetrating ability depends on three factors:
1. The wavelength. The shorter the wavelength, the greater the penetrating power. Because you can not see them, it is hard to imagine waves passing through an object. Light waves reach your eyes as a glow, not waves.
2. The distance from the source of the X-ray to the object to be X-ray. The source of the Dental x-rays is the target in the vacuum tube. As the waves exit the tube and travel through the collimator and tissue, they become weaker (less penetrating). The further they travel from the source, the weaker the waves become.
3. The density of the object or tissue to be penetrated. The amount of radiation that penetrates tissues and strikes the film is what creates the radiographic image.
Each type of tissue allows different amounts of penetration.
The pulp of the tooth and the gingival tissue are made up of blood vessels, nerves, and tissue fluid. These substances are called radiolucent tissues because they are easily penetrated by radiation. When Dental x-rays strike a dense substance (like enamel), not much radiation goes through to strike the film. The X-rays won’t affect the film as much, and the object will appear light on the developed radiograph.
When Dental x-rays go though dentin, slightly more radiation can go through to the film because dentin is less dense. The object will appear slightly darker in the developed film. Some rays will go through space (between the teeth or where there are no teeth). All radiation reaches the film and these areas appear black. Silver filling material contains lead, so it is not penetrable by Dental x-rays. This area will appear white on the developed film.
The penetrating ability of the Dental x-rays is important for two reasons:
1. The diagnostic quality of the processed radiograph. If too much radiation reaches the film, it will be too dark to read. If too little radiation reaches the film, it will be too light to tell the difference between tissues.
2. Safety to the tissue. If the Dental x-rays does not penetrate the tissue, that means the tissue has absorbed it and cell damage will result.
Generation of Dental x-rays
Three things are necessary to convert electric current into X-rays:
1. A source of electrons.
2. High voltage (units of electrical force), which causes the electrons to move.
3. A target to stop the electrons.
TYPES OF RADIATION from dental x-rays
The dental machine produces several types of radiation from dental x-rays: primary, secondary, scattered, stray, and Bremsstrahlung. Primary radiation is the useful beam, which passes through the tissue and affects the film. Matter exposed to radiation gives off secondary radiation. Scattered radiation is the portion of the ray, which continues through the patient and is deflected onto other parts of the body or objects in the room.
It is the result of the interaction between primary radiation and the object it comes in contact with. Stray radiation is formed by electrons, which strike areas other than the target inside the vacuum tube.
There is one other form of radiation inside the vacuum tube. It is made up of those electrons which glance off the edges of the target, creating what is called Bremsstrahlung, which is German for “breaking radiation.”
CONTROLS OF X-RAY PRODUCTION
Milliamperage control. Transformers are devices that can increase or decrease the flow of electricity. The milliamperage control is a step-down transformer, which decreases the current coming into the vacuum tube. As the current is slowed, resistance is created that builds up heat in the filament wire of the cathode.
The higher the heat, the greater the number of electrons produced and the stronger the Dental x-rays beam will be. This is referred to as the quantity of the dental x-rays. By regulating the step-down transformer with the milliamperage control knob or button, the staff working on X-ray machine can adjust the current and the amount of radiation produced.
Kilovoltage control. A second transformer, the step-up transformer, increases the positive charge at the anode so the electrons move faster from the cathode to the anode. Dental x-rays (and heat) are produced when these electrons hit the anode’s tungsten target with a force great enough to disturb the tungsten atoms.
In other words, by increasing the voltage, you are increasing the speed at which the electrons flow. As they are stopped by the target, a shorter wavelength ray is created. The shorter the wavelength, the greater the penetration power (quality) of the dental x-rays beam. So, by regulating the step-up transformer with the kilovolt control knob or button, quality of the radiation produced can be controlled.
Timing control in dental x-rays. In addition to the miffiamperage and kilovoltage controls, there is a timing control. The timing control is used to adjust the length of time the radiation produced will come out of the collimator Time is marked in seconds or fractions of seconds because only a short exposure time is needed for a radiograph. Too little exposure time results in films those are too light to read. Too much time exposure will produce films that are too dark.