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.
- mA measures current, which controls the quantity of x-ray production
- 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.
- 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)
- When the x-rays enter the body they will either be completely absorbed, partially absorbed, or scattered.
- Photoelectric interaction is when those x-rays are completely absorbed by the body structure after they knock out the inner shell electron. (patient dose).
- 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
- Those x-rays that exit the patient (the remnant beam) will now interact with the intensifying screens of your cassette.
- Keep in mind that the grid will absorb a good percentage of the scatter created by the patient.
- When the x-rays interact with the intensifying screens they are absorbed by the phosphor crystals in the screen.
- This absorption of x-rays in the phosphor material is what results in the emission of visible light.
- 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
- The visible light photons interact with the silver halide crystals of the emulsion of the film.
- Silver halide crystals contain silver and bromine ions (halogens).
- 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.
- This free electron will travel to the sensitivity speck (the imperfection on the surface of the crystal).
- An abundance of electrons at the speck will give the speck a negative charge.
- 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.
- 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
- 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.
- The two reducing agents will enter the crystal and reduce the silver atoms to metallic silver.
- It is this reduction process that gives us our blacks and grays and makes the image visible.
- The fixer tank will remove the unexposed silver halide crystals and prepare the film for good archival quality.
- The wash tank removes any residual chemicals, and the dryer will dry the film in preparation for interpretation.