Luminescent screens
Presenter: Dr. Raghavendar
03/05/2024
• Xray converted into radiation in the visible light spectrum and this
light may be used to expose x ray film.
(radiography/photofluroscopy)
• or light may be viewed directly – (fluoroscopy)
• Direct exposure will require more patient dose.
• All radiographic film examinations require that the radiographic
image be converted into light.
LUMINESCENCE & FLURORESCENCE
LUMINESCENCE – REFERS TO THE EMISSION OF LIGHT BY A
SUBSTANCE.
SOURCE – LIGHT, CHEMICAL RXS, IONOIZING RADIATION
FLURORESCENCE: form of luminescence produced when light is
emitted instantaneously.(within 10-8 sec of the stimulation)
Examples of fluorescence include intensifying screens in screen-film radiography.
PHOSPHORESCENCE: If the emission of light is delayed beyond 10-8
sec of stimulation
Examples of phosphorescence include
A. photostimulated luminescence (PSL) in computed radiography, and
B.thermoluminescent dosimetry (TLD) in radiation dose monitoring.
In radiology
• FLURORESCENCE –is the ability of crystals of
certain inorganic salts (called phosphors) to emit light
when excited by x rays.
Intensifying screens
• Calcium tungstate (the phosphor in intensifying screens) and zinc cadmium sulfide
(the phosphor in fluorescent and photofluorographic screens).
NEW PHOSPHORS
• cesium iodide in image intensifier tubes.
NEW INTENSIFYING SCREEN PHOSPHORS
• barium strontium sulfate,
• Yttrium
• rare earths gadolinium and lanthanum
• rare earth tantalates.
• mammography, a single screen on the back side and a single emulsion film is used.
An intensifying screen has four layers:
1. a base, or support, made of plastic or cardboard
The total thickness of a typical intensifying screen is about 15
2. a reflecting layer (Ti02)
or 16 mils (1 mil= 0.001 in. = 0.0254 mm). 3. a phosphor layer
4. a plastic protective coat
•Base.
The screen support, or base, may be made of high-grade cardboard or of a polyester
plastic. ( 10 mils = 2 54 f.Lm).
•Reflecting Coat.
(The reflecting coat is made of a white substance, such as titanium dioxide (Ti02),
and is spread over the base in a thin layer, about 1 mil thick).
The light produced by the interaction of x-ray photons and phosphor crystals is
emitted in all directions.
Much of the light is emitted from the screen in the direction of the film.
The reflecting layer acts to reflect light back toward the front of the screen.
Protective Layer.
The protective layer applied over the phosphor is made of a plastic,
largely composed of a cellulose compound that is mixed with other
polymers. It forms a layer about mils thick.
This layer serves three functions: 0.7 to 0.8
1.it helps to prevent static electricity.
2. it gives physical protection to the delicate phosphor layer.
3. it provides a surface that can be cleaned without damaging the
phosphor layer.
Phosphor
• The original phosphor used in x-ray intensifying screens was crystalline
calcium tungstate (CaW04).
NEWER increase screen speed over.
• Natural calcium tungstate (scheelite) is no longer used because
• Synthetic calcium tungstate better quality
(fusing sodium tungstate and calcium chloride under carefully
controlled conditions.)
• The calcium tungstate crystal must be absolutely free of any
contaminant if it is to fluoresce properly.
• Calcium tungstate produces light primarily in the blue
region of the visible spectrum wavelength range of
3500 to 5800 A (1 angstrom = 0.0001 f.L = 0.00000001 em)
and a peak wavelength of about 4300 A (430 nm), which is
seen by the eye as a violet color.
• eye is not sensitive to light of this wavelength
x-ray film emulsion exhibits maximum sensitivity to light
from calcium tungstate screens.
WHY RED LIGHT used in DARK ROOM??
Film does not exhibit
photosensitivity to red
light, so red light can be
used in the darkroom
without producing any
photographic effect on
the film.
Why Intensifying Screens used ???
• BECAUSE Convert a few absorbed x-ray photons into many
light photons.
Intrinsic conversion efficiency of the phosphor:??
(efficiency with which the phosphor converts x rays to light )
Defined as the ratio of the light energy liberated by the crystal to the x-
ray energy absorbed.
• The intrinsic conversion efficiency of calcium tungstate is about 5%.
Calculate number of light photons?
- Energy of the absorbed x-ray photon,
-wavelength of the emitted light, and
- the conversion efficiency of the phosphor
= the number of light photons generated is easily calculated.
• The energy of a 4300 A ( 430 nm) blue light photon is about 3 eV. At
100% efficiency.
• New intensifying screen phosphors are faster than calcium tungstate
is that the new phosphor has a higher conversion efficiency (up to
20%).
Screen efficiency
• The ability of light emitted by the phosphor to escape from the screen
and expose the film is termed the "screen efficiency.“
Latent image center
• Def: a latent image center as the end product of the photographic
effect of light or x rays on the film emulsion.
• It is possible to measure the magnitude of the photographic effect of
an exposure by counting the number of latent image centers formed as
a result of that exposure.
A measure of this decrease in exposure is termed the
"intensification factor" of the screen.
• Image centers resulting from the use of intensifying screens.
• In our example, the ratio of the photographic effect of screen versus
nonscreen, or direct, exposure is 34: 1.
• If kVp remains constant, direct film exposure will require 34 times
as many mAs as a film-screen exposure.
• The intensification factor of a screen is the ratio of the x-ray
exposure needed to produce the same density on a film with and
without the screen (intensification factor is commonly determined at a
film density of 1.0).
Speed of Calcium Tungstate Intensifying Screens.
• determine how "fast" or "slow" a calcium tungstate screen will
be. ????
1.thickness of the phosphor layer,
2. size of the phosphor crystals ,
3.presence or absence of light-absorbing dye in the phosphor layer,
and
4.phosphor conversion efficiency.
• faster screen will allow a lower x-ray exposure to the patient,
• reciprocal relationship; that is, high speed means less detail.
• This statement is also true for new screen phosphors, but the subject is
more complex because higher speed does not always require a thicker
screen.
• These screens are classified as fast, medium (par speed), and slow (detail),
with intensification factors in the range of 100, 50, and 25, respectively.
• A thicker phosphor layer will result in a faster screen because the thick
layer will absorb more x-ray photons than a thin layer.
• Thick screens will be faster but will cause a decrease in the clarity of the
image recorded on the film. This decrease in image clarity is primarily
caused by diffusion of light in the phosphor layer.
• If a thick phosphor layer is employed, an x-ray photon may be absorbed in
the phosphor at some distance from the film.
• light photons generated by this absorbed x-ray photon are emitted in all
directions.
• Not all the light will reach the film, but that which does will expose an area of
the film that is much larger than the size of the calcium tungstate crystal that
emitted the light
• In addition, some light scattering takes place in the screen, and further
increases the area of illumination. The resultant light diffusion obviously
causes images to have less sharp borders.
• If an x-ray beam is directed onto a film-screen combination that has a thick
lead block covering half the ilm, one would expect half the film to be exposed
and half to be entirely unexposed, with the line between the two areas being
very sharp.
• If the x-ray beam were to expose film alone (without screens), the line
between exposed and unexposed areas would be very sharp.
• If the film is sandwiched between intensifying screens the border is
less sharp because some light will diffuse into the area under the edge
of the lead block no x-ray exposure.
• This light, which diffused under the edge of the lead block, is not
available to expose the portion of the film that is not covered by the
block. A thin screen causes less light diffusion than a thick one because
light photons are produced closer to the film.
• Another way to decrease light diffusion is to incorporate a
substance that absorbs light in the screen. The substance is
commonly a yellow dye!
• The light photons that emerge from the crystal immediately adjacent to
the film will obviously travel the shortest distance before leaving the
phosphor layer. Scattered photons must travel longer distances in the
phosphor layer, and thus they have a better chance of being absorbed
by the dye. The dye will decrease the speed of the screen because it
decreases the amount of light emitted.
• Light-absorbing dyes are included in screens designed to produce
greater detail.
Resolving Power:
• The maximum number of line pairs per millimeter that can be
resolved by the screen-film system is called the resolving power. A
line pair means a line and a space.
• For example,
two lines (or two line pairs) per mm means that there are two lines and
two spaces per mm. Each line is mm wide, and each space is mm
wide, thus making each line pair mm wide.
• X-ray film is able to record up to 100 line pairs per mm, but the
slowest screens can record only a little over 10.
Screen-Film Contact
• The cassette in which the intensifying screens are mounted provides a
light-tight container for the film.
• It also serves to hold the film in tight contact with the screens over
its entire surface.
• With good filmscreen contact a dot of light produced in the screen
will be recorded as a comparable dot on the film. If contact is poor,
this dot of light will diffuse before it reaches the film, so that its
radiographic image is unsharp.
Cleaning
• Intensifying screens must be kept clean.
• Any foreign material on the screen, such as paper or blood, will block
light photons and produce an area of underexposure on the film
corresponding to the size and shape of the soiled area.
• The cause of screen failure is mechanical attrition.
• Screens are best cleaned with a solution containing an antistatic
compound and a detergent; the solution should be applied gently
(never rub vigorously) with a soft lint-free cloth.
• The cassette should never be closed after cleaning until it is absolutely
dry.
• TLD has three parts – 1. plastic holder. 2. nickel-coated aluminum
card with TLD discs
• Discs are made of a thermoluminescent material, commonly
calcium sulphate doped with dysprosium (CaSO4:Dy) or lithium
fluoride (LiF). The discs are 0.8 mm thick and have a 1.35 cm
diameter .
three filters against each disc Top: aluminum and copper
• Middle : OPEN RECTANGULAR WINDOW : Perspex Lower:
open window
• Lithium fluoride in TLD trap and stores energy in from of ionizing
radiation when exposed to it. Energy is absorbed by the electrons in the
material. This temporarily moves electrons from their ground state to an
excited state. The TLD is usually in low-temperature environment to
prevent the electrons from returning to their ground state. This prevents
the material from releasing the stored energy as visible light at this stage.
• When the TLD material is heated, either during analysis the trapped
electrons return to their ground state, releasing the stored energy in the form
of visible light. The emitted light is then detected using a photomultiplier
tube.
• The intensity of the emitted light is directly proportional to the amount of
radiation absorbed by the TLD material.
THANK YOU
