CHAIN OF EVENTS IN RADIOGRAPHIC IMAGE FORMATION

 

Creation of x-radiation

Hi-speed electrons originate at the filament of the cathode when we set mA on the control panel.

  1. mA measures current, which controls the quantity of x-ray production
  2. When we hit the exposure button, these high-speed electrons travel from the cathode end of the tube to the anode end of the tube and strike the tungsten anode.
  3. The electrons suddenly decelerate on impact and with this deceleration, create two forms of electromagnetic energy: heat (99%) and x-radiation (1%).

X-radiation interaction with matter (the patient)

  1. When the x-rays enter the body they will either be completely absorbed, partially absorbed, or scattered.
  2. Photoelectric  interaction is when those x-rays are completely absorbed by the body structure after they knock out the inner shell electron. (patient dose).
  3. Compton interaction is when the x-ray knocks out the outer shell electron and then the x-ray travels in a different direction (occupational dose).

X-radiation interaction with the image receptor

  1. Those  x-rays that exit the patient (the remnant beam) will now interact with the intensifying screens of your cassette.
  2. Keep in mind that the grid will absorb a good percentage of the scatter created by  the patient.
  3. When the x-rays interact with the intensifying screens they are absorbed by the phosphor crystals in the screen.
  4. This absorption of x-rays in the phosphor material is what results in the  emission of visible light.
  5. The visible light will expose the area of film adjacent to the emissions.  These visible light photons are what is carrying the pattern of the x-rays that were absorbed and partially absorbed. This pattern is in the form of energy levels.

Latent image formation

  1. The visible light photons interact with the silver halide crystals of the emulsion of the film.
  2. Silver halide crystals contain silver and bromine ions (halogens).
  3. When the visible light photons interact with these halogens, ionization occurs; an electron is knocked out and is free to travel within the crystal lattice.
  4. This free electron will travel to the sensitivity speck (the imperfection on the surface of the crystal).
  5. An abundance of electrons at the speck will give the speck a negative charge.
  6. This negative charged speck is where the latent image is formed. The positive silver ions floating around the crystal lattice will be attracted to the sensitivity speck.
  7. When the positively charged silver ions combine with the negative electrons at the sensitivity speck, this results in a cluster of silver atoms at the speck. THIS IS THE LATENT IMAGE.

Processing the latent image

  1. When the film enters the developer tank, the reducing agents (phenidone and hydroquinone) will enter only those silver halide crystals that have a latent image center.
  2. The two reducing agents will enter the crystal and reduce the silver atoms to metallic silver.
  3. It is this reduction process that gives us our blacks and grays and makes the image visible.
  4. The fixer tank will remove the unexposed silver halide crystals and prepare the film for good archival quality.
  5. The wash tank removes any residual chemicals, and the dryer will dry the film in preparation for interpretation.