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CHARACTERISTICS AND MEASUREMENT OF RADIATION l Oral radiology MCQs

ام سي كيوو راديو , ام سي كيو اكس راي




CHARACTERISTICS AND MEASUREMENT OF RADIATION l Dental radiology MCQs




Basics and background

  • X rays were accidentally discovered when Roentgen in 1895 noticed fluorescent glow of crystals on a table near him when he was working with high voltage [ evacuated light bulb with +ve and -ve electrodes]

  1.                     Radiation = the transmission of energy through space , could be in particulate or electromagnetic form.
  2.                     Atoms have protons [+ve] and neutrons [no charge] in the nucleus and electrons [ -ve] orbiting around the nucleus
  3.                     An atom is electrically neutral because the number of electrons = the number of protons

 

Two types of xrays are produced:


                   Majority = Bremsstrahlung ( breaking) radiation : 

  • happens when high speed electrons are slowed down as they pass close to or strike the nucleus

                  Minority = characteristic radiation : 

  • If a K-shell electron is ejected and an L-shell electron drops into the space, the energy of the x-ray will be equal to the difference in binding energies between the K- and L-shells

 

How xrays are generated:

  1. Electrical energy goes toa tungsten filament [ cathode] filament heats and releases electrons that pass and hit a target [ anode] generating x rays
  2. Low energy soft xrays don’t contribute to the formation of an xray image – they expose the body to radiation but without reaching the film – they are useless and must be eliminated

  •         Filtration = The process of removing soft x rays .

  • provided by inherent filtration in the x ray machine + added filtration by aluminum disks placed in the path of the x-ray beam [ aluminum disks remove the x-rays that had enough energy to get through the inherent filtration but are still not energetic enough to contribute to image formation]
  • x-ray machine operating at 70 kVp or higher must have total filtration of 2.5 mm aluminum
  • X-ray machines operating below 70 kVp need to have a total filtration of 1.5 mm aluminum
  •                     Collimation = to restrict the area of the head that is exposed to radiation + reduce scatter radiation
  •                     Collimator = lead disk with a hole in the middle - located in the end of the PID - The size / shape of the hole determines the size/ shape of the x-ray beam


X-ray films and processing

  1. Images are stored as an unseen (latent) image that will be changed to a seen image by processing the film.
  2. Analogue Xray films :

  •                         A. Intraoral [ non screen films]
  •                         B. Extraoral [ screen films]

 

Intra oral films:

  • Consist of a film base with gelatin emulsion containing silver bromide crystals on both sides of the film base .
  • Gelatin is used as an emulsion medium
  • because it is chemically inert
  • Outer protective packet has 2 sides:

  1.          Front: faces the x-ray tube, single color (white)
  2.         Back: opposite side to x-ray tube, two colors (white and green) + has the opening tab

  •                    Black paper wrapper: protects the film base from light, saliva and damage during opening.
  •                    Lead foil: thin sheet of lead present at the back of the film to:

  • prevents x –rays from going to deeper tissues after passing through the film + prevent scattered radiation from deeper tissues to come back to the film
  • Raised dot:

  1.        Convex: indicates front of the film (facing x-ray)
  2.        Concave: indicates back of film (facing lead foil)

 

Latent image formation:


 Radiolucent objects [ soft tissues ]

  • Many x-rays penetrate and expose many silver halide crystals maximum silver precipitate

 Thick radiolucent objects [ bone ]

  • Fewer x-rays penetrate and not as many silver halide crystals are exposed fewer silver precipitate

 Radio opaque objects [ amalgam ]

  • very few x-rays penetrate silver halide crystals not exposed no silver precipitate 


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