5. Intensifying Screens Flashcards
device found in radiographic cassettes that contains phosphors to convert x-ray energy into light, which then exposes the radiographic film
Intensifying Screen
Intensifying Screen layers
- Protective layer
- Phosphor layer
- Reflecting or Absorbing layer
- Base
10-20 micrometer thick
Protective Coating
protects the screen from abrasion, helps eliminate static, and provides surface for cleaning
Protective Coating
active layer
Phosphor layer
50-300 micrometer
Phosphor layer
Phosphor materials
calcium tungstate, zinc sulfide, barium lead sulfate, and rare earth oxysulfides such as gadolinium, lanthanum, and yttrium
approx. 25 micrometer thick made of magnesium oxide or titanium dioxide
Reflective layer
furthest from the film
Base
approx. 1 mm thick, made of polyester
Base
serves as mechanical support for active phosphor layer
Base
intensifies x-rays reducing radiation dose
Intensifying screen
emission of visible light
Luminescence
emission of light during stimulation
Fluorescence
afterglow
Phosphorescence
emission of light after stimulation
Phosphorescence
identified by relative speed ranging from 50 (slow, detail) to 1200 (very fast)
Screen speed
Characteristic Type of Phosphor
- Color of emission
- Approximate speed
- Intensification factor
- Spatial resolution (lp/mm)
Typical Radiographic Intensifying Screens
- Calcium Tungstate
- Oxysulfides and Oxybromides of Y, La, Gd
La
Lanthanum (57)
Y
Yttrium (39)
Gd
Gadolinium (64)
Color of emission (Calcium Tungstate)
Blue
Color of emission (Oxysulfides and Oxybromides of Y, La, Gd)
Green or blue
Approximate speed (Calcium Tungstate)
50-200
Approximate speed (Oxysulfides and Oxybromides of Y, La, Gd)
80-1200
Intensification factor (Calcium Tungstate)
20-100
Intensification factor (Oxysulfides and Oxybromides of Y, La, Gd)
40-400
Spatial resolution in lp/mm (Calcium Tungstate)
8-15
Spatial resolution in lp/mm (Oxysulfides and Oxybromides of Y, La, Gd)
8-15
Intensification Factor
IF = Exposure required without screen/ Exposure required with screens
ratio of the exposure required to produce the same OD with a screen to the exposure required to produce and OD without a screen
Intensification Factor
the relationship between intensification factor and entrance skin exposure
inversely proportional
the _____ the ESE (patient dose), the ____ IF
larger ESE,
smaller IF
uncontrollable factors affecting speed
- Phosphor Composition
- Phosphor Thickness
- Reflective Layer
- Dye
- Crystal Size
- Concentration of Phosphor Crystals
controllable factors affecting speed
- Radiation Quality
- Image Processing
- Temperature
rare earth phosphors efficiently convert x-rays into usable light
Phosphor Composition
the thicker the phosphor layer, the _____ is the detective quantum effiecient
higher
have thick phosphor layers
High-speed screens
have thin phosphor layers
Fine-detailed screens
increases screen speed but also increases image blur
Reflective layer
control the spread of light
Dye
improve spatial resolution but reduce speed
Dye
the larger the phosphor (crystal), the _____ the light produced
greater
are approx. 1/2 of the crystals of high-speed screens
crystals of detail screens
results in higher screen speed
higher crystal concentration
as x-ray tube potential is increased the IF _____
increase
excessive developing time for screen-film results in ____ IF
lower
IF is ____ at high temperatures
lower
deterioration of the radiographic image; speckled background that reduces image contrast
Image noise
is a measurement of the efficiency of an image receptor in converting the x-ray exposure it receives to quality radiographic image
Detective Quantum Efficiency
refers to the ability of the screen to absorb the incident x-ray photons
Absorption Efficiency
describes how well the screen phosphor takes these x-ray photons and converts them to visible light
Conversion Efficiency
term used to evaluate the accuracy of the recorded anatomic structural lines
Spatial Resolution
refers to the smallest object that can be detected in an image
Spatial Resolution
the ability of a radiographic image to demonstrate sharp lines
Spatial Resolution
limiter by focal spot size
Spatial Resolution
measured through line-pair test pattern, expressed by the number of lp/mm
Spatial Resolution
using intensifying screens results to a _____ picture
blurry
lp/mm of Direct exposure
50 lp/mm
Increase IF, _____ Spatial resolution
decreases
term used to describe the ability of an imaging receptor to distinguish between objects having similar subject contrast
Contrast Resolution
rigid holder that contains the film and radiographic intensifying screens
Cassette
Cassette layers
- Front cover (with low atomic number)
- Front and Back Screens
- Compression Device
- Back cover (with high atomic number)
absorbs approx. half the number of x-rays
Carbon Fiber
reduces patient exposure, longer x-ray tube life
Carbon Fiber
emits light in the violet-to-blue region
Calcium Tungstate Screen (CaWO4)
principally gadolinium, lanthanum, yttrium
Rare Earth Screens
have the advantage of speed
Rare Earth Screens
increased DQE and CE (Conversion Efficiency)
Rare Earth Screens
refers to the color of light produced by a particular intensifying screen
Spectral Emission
refers to correctly matching the color sensitivity of the film to the color emission of the intensifying screen
Spectral Matching
Care of Screens
- Do not slide film into a cassette
- Do not dig the film out of the cassette
- Do not leave the cassette open
- Clean screens periodically
- Check and maintain good screen-film contact
plastic that protects the phosphor
Protective layer
absorbs radiation and converts it to light
Phosphor layer
reflects light toward the film
Reflecting layer
absorbs light directed toward it
Absorbing layer
provides support and stability for the phosphor layer
Base
The desired type of luminescence
in imaging is
Fluorescence
refers to the ability of phosphors to emit visible light
only while exposed to x-rays (with little or no afterglow)
Fluorescence
An undesired type of luminescence is
Phosphorescence
is the emission of light after x-ray exposure has terminated
Phosphorescence
causes unwanted exposure to the film
Phosphorescence or afterglow
As screen speed ______, recorded detail ______
increases (s),
decreases (r)
the capability of a screen to produce visible light is called
Screen speed
increase screen speed and decrease recorded detail
Reflective layer
decrease screen speed and increase recorded
detail
Light-absorbing layer or Light-absorbing
dyes
light-absorbing layer or light-absorbing
dyes
Isotropic emission
Any material that emits light in response to some outside stimulation is called a
Phosphor
Higher conversion efficiency results in _______
noise
increased
Image Detail =
Image Detail = Spatial Resolution + Contrast
Resolution
The unaided eye can resolve about ____ lp/mm
10
Spatial resolution improves with _____ phosphor crystals and _____ phosphor layers
smaller,
thinner
Increase phosphor thickness, ______ spatial resolution
decrease
Increase crystal size, ______ spatial resolution
decrease
Increase speed screen, ______ patient dose
decrease
The term rare earth describes those elements of group ____ in the periodic table
IIIa
These elements are transitional metals that are scarce in nature.
Rare earth
One advantage of the use of these commercial
preparations is that they often contain ______ compounds, which can be helpful
antistatic