Transcript Analog Imaging II Intensifying Screens

Analog Imaging II
Intensifying Screens
By Professor Stelmark
SCREEN CONSTRUCTION
Use of film to detect x-rays and to image anatomical structures is inefficient.
In fact, less than 1% of the x-rays incident on radiographic film interact with
the film and contribute to the latent image
The radiographic intensifying screen amplifies the effect of image-forming
x-rays that reach the screen-film cassette.
The intensifying action of screens can be described by a formula called
the intensification factor (IF). This factor accurately represents the degree to
which exposure factors (and patient dose) are reduced when intensifying screens
are used.
On the one hand, use of a radiographic intensifying screen lowers patient
dose considerably; on the other hand, the image is slightly blurred. With
modern screens, however, such image blur is not serious.
Radiographic intensifying screens resemble flexible sheets of plastic or
cardboard. They come in sizes that correspond to film sizes.
Usually, the radiographic film is sandwiched between two screens. The film
used is called double-emulsion film because it has an emulsion coating on
both sides of the base.
Protective Coating
The layer of the radiographic intensifying screen closest to the radiographic
film is the protective coating. It is 10 to 20 μm thick and is applied to the face
of the screen to make the screen resistant to the abrasion and damage caused
by handling. This layer also helps to eliminate the buildup of static electricity
and provides a surface for routine cleaning without disturbing the active
phosphor. The protective layer is transparent to light.
Phosphor
The active layer of the radiographic intensifying screen is the phosphor. The
phosphor emits light during stimulation by x-rays. Phosphor layers vary in
thickness from 50 to 300 μm, depending on the type of screen. The active
substance of most phosphors before about 1980 was crystalline calcium
tungstate embedded in a polymer matrix. The rare Earth elements gadolinium,
lanthanum, and yttrium are the phosphor material in newer, faster screens
The phosphor converts the x-ray beam into light
Favorable Properties of a Radiographic Intensifying Screen Phosphor
• The phosphor should have a high atomic number so that x-ray absorption is
high. This is called detective quantum efficiency (DQE).
• The phosphor should emit a large amount of light per x-ray absorption. This is
called the x-ray conversion efficiency (CE).
• The light emitted must be of proper wavelength (color) to match the sensitivity
of the x-ray film. This is called spectral matching.
• Phosphor afterglow, the continuing emission of light after exposure of the
phosphor to x-rays, should be minimal.
• The phosphor should not be affected by heat, humidity, or other environmental
conditions.
The American inventor Thomas A. Edison developed calcium tungstate. Although
Edison demonstrated the use of radiographic intensifying screens before the
beginning of the 20th century, screen-film combinations did not come into general
use until about the time of World War I.
With improved manufacturing techniques and quality control procedures, calcium
tungstate proved superior for nearly all radiographic techniques and, until the
1970s, was used almost exclusively as the phosphor.
Since then, rare Earth screens have been used in diagnostic radiology. These
screens are faster than those made of calcium tungstate, rendering them more
useful for most types of radiographic imaging. Use of rare Earth screens results
in lower patient dose, less thermal stress on the x-ray tube, and reduced
shielding for x-ray rooms.
Reflective Layer
Between the phosphor and the base is a reflective layer. approximately 25 μm
thick, that is made of a shiny substance such as magnesium oxide or titanium
dioxide. When x-rays interact with the phosphor, light is emitted isotropically.
Less than half of this light is emitted in the direction of the film. The reflective layer
intercepts light headed in other directions and redirects it to the film. The reflective
layer enhances the efficiency of the radiographic intensifying screen, nearly
doubling the number of light photons that reach the film.
Some radiographic intensifying screens incorporate special dyes in the
phosphor layer to selectively absorb those light photons emitted at a large
angle to the film. These light photons increase image blur. Because they must
travel a longer distance in the phosphor than those emitted perpendicular to
the film, these photons are more easily absorbed by the dye. Unfortunately, this
addition reduces screen speed somewhat.
Base
The layer farthest from the film is the base. The base is approximately 1 mm
thick and serves principally as a mechanical support for the active phosphor
layer. Polyester is the popular base material in radiographic intensifying
screens, just as it is for radiographic film.
Favorable Properties of Radiographic Intensifying Screen Base
• Rugged and moisture resistant
• Resistant to radiation damage and discoloration with age
• Chemically inert and not prone to interact with the phosphor layer
• Flexible
• Lacking impurities that would be imaged by x-rays
Any material that emits light in response to some outside stimulation is called
a luminescent material, or a phosphor, and the emitted visible light is
called luminescence. A number of stimuli, including electric current (the
fluorescent light), biochemical reactions (the lightning bug), visible light (a
watch dial), and x-rays (a radiographic intensifying screen), cause
luminescence in materials.
Two types of luminescence have been identified. If visible light is emitted
only while the phosphor is stimulated, the process is
called fluorescence. If, on the other hand, the phosphor continues to emit
light after stimulation, the process is called phosphorescence.
SCREEN CHARACTERISTICS
The radiologic technologist is concerned with three primary characteristics of
radiographic intensifying screens: screen speed, image noise, and spatial
resolution.
Because screens are used to reduce patient dose, one characteristic is the
magnitude of dose reduction. This property is called the intensification
factor and is a measure of the speed of the screen.
With some exceptions, an increase in screen speed can result in
increased image noise. Image noise, the speckled appearance on some
images degrades image quality.
Screen Speed
Many types of radiographic intensifying screens are available, and each
manufacturer uses different names to identify them. Collectively, however,
screens usually are identified by their relative speed expressed numerically.
Screen speeds range from 100 (slow, detail) to 1200 (very fast).
Screen speed is a relative number that describes how efficiently x-rays are
converted into light. Par-speed calcium tungstate screens are assigned a
value of 100 and serve as the basis for comparison of all other screens.
Calcium tungstate screens seldom are used anymore. High-speed rare Earth
screens have speeds up to 1200; detail screens have speeds of
approximately 50 to 80.
Several factors influence radiographic intensifying screen speed; some of
these are controlled by the radiologic technologist.
Ultimately, the screen speed is determined by the relative number of x-rays
that interact with the phosphor, and how efficiently x-ray energy is converted
into the visible light that interacts with the film.
Properties of Radiographic Intensifying Screens That Are Not Controlled by
the Radiologic Technologist
• Phosphor composition. Rare earth phosphors efficiently convert x-rays into usable light.
• Phosphor thickness. The thicker the phosphor layer, the higher is the DQE. High-speed
screens have thick phosphor layers; fine-detailed screens have thin phosphor layers.
• Reflective layer. The presence of a reflective layer increases screen speed but also
increases image blur.
• Dye. Light-absorbing dyes are added to some phosphors to control the spread of light.
These dyes improve spatial resolution but reduce speed.
• Crystal size. Larger individual phosphor crystals produce more light per x-ray interaction.
The crystals of detail screens are approximately half the size of the crystals of high-speed
screens.
• Concentration of phosphor crystals. Higher crystal concentration results in higher screen
speed.
Properties of Radiographic Intensifying Screens That Are Controlled
by the Radiologic Technologist
• kVp As the kVp increases so is the speed of the screen.
• Temperature As the temperature of the screen increases its speed will
decrease
• Age As the age of the screen increases its speed will decrease.
Screen Factor
Screen Speed
Recorded Detail
Patient Dose
Thicker phosphor
layer
↑
↓
↓
Larger phosphor
crystal size
↑
↓
↓
Reflective layer
↑
↓
↓
Absorbing layer
↓
↑
↑
Dye in phosphor
layer
↓
↑
↑
Quantum Mottle
Quantum mottle, commonly called image noise, can be defined as the
statistical fluctuation in the quantity of x-ray photons that contribute to image
formation per square millimeter. When a very low number of photons are
needed by the intensifying screens to produce appropriate image density, the
image appears mottled or splotchy. This appearance can also be described as a
“salt and pepper look,” versus a consistent, homogeneous density. This mottled
appearance is often a direct result of using very fast speed film-screen systems
that require very small amounts of exposure.
Quantum mottle decreases recorded detail, which results in a radiographic
image that is grainy, or noisy, in appearance. An optimal image displays more
recorded detail. The only strategy for reducing quantum mottle is the use of
more mAs (more photons). This can be accomplished by using a slower speed
system (requiring more mAs) or increasing the mAs while decreasing the kVp.
Screen speed and density are directly proportional. The mAs conversion
formula for screens is a formula for the radiographer to use in determining
how to compensate or adjust mAs when changing intensifying screen
system speeds.
Use of the mAs Conversion Formula for Screens
If 10 mAs were used with a 400 speed screen system to produce an optimal
radiograph, what mAs would be necessary to produce a radiograph with the
same density using a 100 speed screen system?
When changing from a 400 speed system to a 100 speed system, it takes 4
times the mAs to maintain density. This also means that the patient receives
4 times the radiation dose.
SCREEN-FILM COMBINATIONS
Screens and films are manufactured for compatibility; this helps to ensure
good results.