Species: Feline | Classification: Miscellaneous
- A radiographic image may be recorded in a number of ways:
- X-ray film.
- Cine or video film.
- The radiographic image is formed when x-rays passing through a patient and are selectively absorbed by structures within the patient.
- The remaining x-rays fall on the film.
- The image can be considered a shadowgram .
- The film is usually held in a cassette which contains, in addition, a pair of intensifying screens.
- Standard sizes:
- 18 x 24 cm.
- 24 x 30 cm.
- 30 x 40 cm.
- 35 x 35 cm.
- 35 x 43 cm.
Cassettes can be easily damaged by being dropped or mishandled.
- Conventional cassette is a light-proof box, hinged at one side.
- Usually steel frame.
- Fastenings secure but easy to use in the dark room.
- The front is radiolucent made of:
- Carbon fiber.
- Corner window where a piece of lead prevents radiographic exposure of film to allow for marking Radiography: x-ray film .
- The back is radio-opaque to prevent scattered x-rays Radiography: scattered radiation and grid , reflected from the table-top, through and affecting the film:
- Steel with a lead foil lining.
- A soft springy pad.
- Keeps screen in close contact with film once loaded into cassette.
- Film screen contact can be checked by placing chickenwire on cassette and exposing - if image is blurred in places, then contact is poor.
If cassette is bent or dented close contact between the screens and the film is lost resulting in image unsharpness Radiography: image quality .
- Pressure pad keeps screen sprung against film.
- If double-sided x-ray film is being used two screens will be present and the film is sandwiched between them.
- Screens convert incident x-ray photons to light which is mainly responsible for exposing the film.
- Vastly reduces the amount of x-rays necessary to produce a visible image compared to non-screen film → reduced x-ray doses and exposure times.
- Usually polyester
- Chemically stable.
- White pigment, spread evenly over screen, which reflects light from phosphor towards film.
- Only present in high-speed screens.
- A phosphor converts x-rays to light ( fluorescence).
- ie absorbs electromagnetic radiation of a short wavelength and instantly re-emits it at a longer wavelength.
- Wavelength (color) of light produced depends on the particular phosphor.
- Calcium tungstate.
- Barium lead sulfate.
- More modernrare-earthscreens are four times more efficient at producing light from x-rays and contain
- Gadolinium oxysulfide.
- Lanthanum oxybromide.
- Yttrium tantalate.
- Rare earth screens
- Permit reduction in exposure factors by up to 90% (which reduces dose to patient and can cut tube current or exposure times).
- Improve image quality.
Rare earth phosphors are less likely to produce light from scattered radiation so contrast is improved.
- A protective layer of cellulose acetate.
- Screen "speed" depends on type of phosphor used and size of phosphor crystals.
- Afasterscreen will give more exposure of the film for a given x-ray exposure.
- For a particular phosphor, screen speed increases and resolution diminishes with the size of crystals.
- Intensification factoris the ratio of exposure without screens to exposure with screens to produce the same density on the radiograph.
- Different phosphors emit light of different wavelengths (colors) and it is important to match film's spectral sensitivity to emission spectrum of the screens .
Failure to do this makes system very inefficient → under-exposure of films.
Care of Intensifying Screens
- Screens are very susceptible to damage from
- Denting to the cassette.
- Water splashes.
- Chemical splashes.
- Screen damage results in film artifacts - usually white marks Radiography: film faults .
- Artifacts can also result from foreign bodies eg hairs or paper fragments in the cassette at time of exposure .
- Regularly cleaning (weekly) is necessary using proprietary screen cleaners.
- In addition these solutions inhibit the build up of static electricity on screens → static artifacts on films .
- The cleaning solution is used to dampen a soft cloth which is wiped over the surface of the screen.
- A dry swab is used to dry the screen.
Leave open in clean environment to air dry before reloading the cassette with film.
- Screens which are regularly cleaned and carefully handled should last about 10 years.