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INTENSIFYING SCREENS

H F INTENSIFYING SCREENS

QuestionAnswer
What % of the x-rays incident on the radiographic film interact with the film Less than 1%
Use of screens results in a _____ radiation dose to the patient but there may be a slight _______ to the film lower/ blur
Visible light interacts with the film to form latent image
Any material that emits light in response to some outside stimulus luminescent material or a phosphor
2 types of luminescence 1. Fluorescence 2. Phosphorescence
Radiographic intensifying screens convert x-ray energy to visible light
visible light is emitted only when stimulated Fluorescence
the phosphor continues to emit light after stimulated Phosphorescence
the phosphor continues to emit light after stimulated screen lag or after glow
relative number used to identify the efficiency of conversion of x-rays to visible light Screen speed
emits light during stimulation phosphor
phosphor used before 1980 calcium tungstate
rare earth phosphors gadolinium, lanthanum, and yttrium
Can produce more light with less exposure Rare earth screens
faster than calcium tungstate screens Rare earth screens
reflects the light that is going in the opposite direction and directs it to the film reflective layer
This is the thickest layer base
This layer is primarily for mechanical support base
Light photons are produced isotropically
amaount of photons are emitted in the direction of the film Less than 1/2
the speed of the screens are influenced by The thickness of the phosphor layer and the size of the crystals
This layer helps to resist abrasion and damage Protective coating
This layer also helps to decrease static electricity on the screen Protective coating
layer that is closest to the film Protective coating
what are the layers of the screen 1. Protective coating 2. Phosphor 3. Reflective layer 4. Base
Two reasons to limit the size of the x-ray field: to decrease unnecessary exposure and Decrease scatter radiation so that image contrast improves
X-rays that enter the image receptor image forming x-rays
X-rays that exit the patient remnant x-rays
The simplest beam restricting device Aperture diaphragms
positive beam limiting devices Automatic collimators
the most common beam limiting devices for radiography Variable aperture collimators
Has a white light and mirror to show the field and the central ray Collimators
These are metal structures that extend from the x-ray tube and restrict the useful beam to the required size Cones and cylinders
Consists of 2 sets of adjustable lead shutters that provided a rectangular or square field Collimators
used to decrease the patient dose Screens
the way to determine how efficient the screens are at reducing the dose The intensification factor
IF = Exposure without a screen / Exposure with a screen
refers to how small an object can be to be identified on an image Spatial resolution
refers to the ability to distinguish between and image tissues of similar density Contrast resolution
Speckled background and is seen with fast screens Image Noise
Quantum mottle Image Noise
Screen speed categories Par, High speed, Fine detail
rare earth – 50 to 80 speed Fine detail
High spatial resolution Fine detail
rare earth – up to 1200 speed High speed
Most use from 400-600 High speed
Calcium tungstate – 100 speed Par
% of the x-rays striking the radiographic screens interact with the screen 30
involves the outer shell electrons Luminescence
Created by: hakrrins on 2008-08-17



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