cassettes and screens used in radiography

1. RADIOGRAPHIC CASSETTES AND
INTENSIFYING SCREENS
DR.ROHAN JOHN JACOB

2. CASSETTE
• A cassette is a light proof rigid holder that
contains screens and film .
• Cassettes are usually hinged (latches) on one side
and can be opened from the other side.
• The side which is having the latches is the back
side and other side facing the patient is called
front side.
• The front side is made of material of low atomic
number, like plastic or carbon fiber. This is to
maximize the transmission with low attenuation.

3. CASSETTE[contd.]
• Carbon fiber (Z = 6) absorbs only 50% of X-rays
compared to aluminum.
• Cassette with carbon fiber can be operated
with low radiographic techniques, resulting in
lesser patient dose.
• The back side is usually made of heavy metals
(lead), having high atomic number, to
minimize back scatter.

4. CASSETTE[contd.]
• There is a small window at the back side, to
provide patient ID.
• The patient‘s information can be given in the
form of flash card; later on, it is exposed to ID
camera. The ID camera optically exposes the
window and records the image of flash card.

5. FIG. 3.3 • A screen-film cassette whose configuration is featured by a double-
emulsion sheet in between two phosphor screens (A). Compared to the use of
single-emulsion film and/or single screen, this configuration has high
sensitivity to incoming X-rays at the expense of compromised spatial
resolution (cross-sectional view shown in B). Reprinted with permission from
Smith WL. Radiology 101. 4th ed. Philadelphia, PA: Wolters Kluwer
Health/Lippincott Williams & Wilkins; 2014.
by a photostimulable phosphor (PSP) plate. Aray energy is absorbed by the phosphor, electrons

6. CASSETTE[contd.]
• Inside the cassette, there are two permanently
mounted intensifying screens, called front and
back screens.
• The X-ray film is loaded between the two screens.
• The screens may have different thickness or equal
thickness.
• Compressive materials, such as radiolucent
plastic foam, is kept in between the back screen
and the cassette cover.

7. CASSETTE[contd.]
• The compressive material maintains good screen-
film contact, when the film is loaded.
• Good physical contact between film and screen is
necessary, to avoid artifacts and to have good
image quality.
• X-rays, by passing through the back screen get
back scattered and reaches the film, which results
in image fog.
• The film loading and unloading into the cassette
is done in the darkroom.
• Cassettes are available in different sizes.

8. Cross-section view of X-ray film

10. INTENSIFYING SCREEN
• Film is insensitive to X-rays and requires more
amount of X-rays to produce an image, which
will increase patient dose.
• To avoid this, intensifying screens are used in
cassettes in medical imaging.
• They absorb X-ray photons and emit more
visible light or ultraviolet, for which the X-ray
film is more sensitive.

11. INTENSIFYING SCREEN [contd.]
• The light or UV exposes the film and gives the
final image, which will improve the efficiency
of radiographic imaging, with lesser patient
dose.
• Thus, the intensifying screens amplify the
effect of image formation.
• Generally, intensifying screen consists of four
layers, namely, (i) base, (ii) reflecting layer, (iii)
phosphor, and (iv) protective coating.

13. LAYERS OF SCREEN
Base :
• The base is made of polyester with 1 mm
thickness.
• The base serves as a mechanical support on
which the reflecting layer, phosphor and
protective layers are mounted.
• The base material should be moisture free,
resistant to radiation damage and
discoloration, chemically inert, and flexible.

14. Reflecting Layer
• The reflecting layer is made of a white substance,
such as titanium dioxide (Ti O2) or magnesium
oxide.
• It is a shiny material of thickness 25 micrometer,
which reflects light towards the phosphor and
makes the light emission isotropic.
• Thus, the reflective layer increases the efficiency
of the intensifying screen, by doubling the
number of light photons.

15. PHOSPHOR
• The phosphor is a crystal of inorganic salts,
which emits light when exposed to X-rays.
• The thickness of the phosphor ranges from
50–300 micrometer with individual crystal size
of 5–15 micrometer.

16. PHOSPHOR[contd.]
• The commonly used phosphors are calcium
tungstate (CaWO4), zinc cadmium sulfide,
cesium iodide and barium strontium sulfate.
• The phosphor should have high atomic
number, high conversion efficiency, spectral
matching and lesser after glow.
• It should not be affected by heat, humidity
and environmental factors.

17. PHOSPHOR[contd.]
• Rare earth phosphors (Z = 57–71) such as
Gadolinium oxysulfide (Gd2O2S), lanthanum
oxysulfide (La2O2S), lanthanum oxybromide
(LaOBr) and yttrium tantalate (YTaO4) are
used as screen phosphors.
• They can be manufactured with speed range
of 200–1200.

18. PHOSPHOR[contd.]
• The spectral emission of rare earth phosphors
is discrete and is centered at 540 nm.
• Therefore, green sensitive film must be used
with these phosphors.
• These phosphors reduce patient dose, due to
lower radiographic techniques.

19. PHOSPHOR[contd.]
• They also have lesser thermal stress and
require lesser room shielding, as radiation
levels are low.
• Though cesium iodide (CsI) is used in
fluoroscopy and digital radiography, it is
moisture sensitive and fragile. Hence, it is not
used in screen-film radiography.

20. PROTECTIVE COATING
• The protective coating (10–20 micrometer) is
transparent to light and faces the X-ray film.
• It is resistant to abrasion and damage caused
by handling.
• It also prevents formation of static electricity,
and provides a surface for cleaning.

21. INTENSIFYING SCREEN [contd.]
• The total thickness of the intensifying screen is
about 1.15–1.3 mm.
• When X-ray passes through the front screen,
the phosphor absorbs the X-rays and emits
light in all directions.

22. INTENSIFYING SCREEN [contd.]
• The reflecting layer reflects the light towards
the film, so that no photon is lost.
• Some portion of the X-rays that are bypassing
the X-ray film is absorbed and converted into
light by the back screen.

23. INTENSIFYING SCREEN [contd.]
• Thus, the intensifying screen converts large
amount of X-ray photons (95%) into light
photons of blue or green wavelength.
• Since the X-ray film is sensitive to blue or
green light, it absorbs the entire light and
gives the image.

24. SCREEN CHARACTERISTICS
• Quantum Detection Efficiency :
The quantum detection efficiency (QDE) or
absorption efficiency of a screen is the ratio
between the amount of X-rays absorbed and
amount of X-rays incident.

25. Quantum Detection Efficiency[contd.]
• Though thicker screens give higher QDE, they
suffer with lateral light diffusion, resulting in
blurred images.
• Thicker screen also reduces spatial resolution.
This is the reason why two thin screens are used
in radiography, which reduces the light diffusion
path without loss of spatial resolution.
• The QDE of Gd2O2S is highest for photon energy
greater than 50 keV.

26. Conversion Efficiency
• The conversion efficiency of a phosphor is the
ratio between the amount of light emitted
and the X-ray absorbed.
• The overall efficiency of screen film system is
product of absorption efficiency and
conversion efficiency.

27. Conversion Efficiency [contd.]
• It depends upon the intrinsic conversion
efficiency of the phosphor. It is 5% for calcium
tungstate and 15% for Gd2O2S with green
light emission of wavelength 545 nm.
• Presence of light absorbing dye reduces the
conversion efficiency.

28. Speed
• The speed of a screen is inversely related to
the exposure (1/R) required to produce a
given density.
• As the speed increases, the exposure required
decreases.
• Screens are generally classified as fast,
medium and slow speed screens.

29. Speed[contd.]
• High speed screens (1200) are thicker and
provide less spatial resolution with less patient
exposure.
• Slow speed screens (100) are thinner, but have
better spatial resolution. The rare earth screens
are faster, because they have a higher absorption
efficiency and higher conversion efficiency, at the
mean X-ray energy used.
• Patient exposure is decreased greatly when
intensifying screens are used.

30. Intensification Factor
• The reduction in patient exposure is measured by
a term called intensification factor.
• The intensification factor is the ratio of the X-ray
exposure needed to produce a given density
(optical density of 1) on a film without screen and
with screen. It is a measure of the speed of a
screen.
• The usual intensification factors are 30–50.
• The Gd2O2S gives an intensification factor of 50,
over the diagnostic X-ray range.

31. Noise
• Noise appears on the radiograph as speckled
background. It occurs when fast screens and
high kVp techniques are used.
• Higher the conversion efficiency, more will be
the noise.
• However, increase in absorption efficiency
will not affect noise.
• Increase in conversion efficiency enhances the
quantum mottle, resulting in higher noise.

32. Noise[contd.]
• This may occur in very fast screens, with
grainy and mottled image.
• Rare earth screens are 2 times faster than
calcium tungstate, which do not increase
noise significantly.

33. Spatial Resolution
• Spatial resolution refers to how an object can
be imaged and it is expressed in line pairs (lp)
per mm.
• If this number is higher, better the spatial
resolution and smaller the object that can be
imaged.
• Screens have lower spatial resolution
compared to direct exposure film.

34. Spatial Resolution[contd.]
• The spatial resolution is 7 lp/mm for fast
screens, 15 lp/mm for fine-detail screen, and
50 lp/mm for direct exposure film.
• Human eye can resolve upto 10 lp/mm.
• When the film is used with screens, the light
interacts with film by larger area, which is the
cause for reduction in spatial resolution.
• Smaller crystals and thinner phosphor layer
improves spatial resolution.

35. Handling of Screen
• Screen must be handled with care.
• Any foreign material on the screen, such as
paper, blood, scratches, hair, dust, and stains
will block light photons and produce area of
under exposure, leading to artifact and image
degradation.

36. Handling of Screen [contd.]
• The film should not be made to slide into the
cassette, while loading.
• Its sharp edge may scratch the screen.
• The film should be removed by tilting the
cassette, so that the film falls down on the
technologist hand.

37. Handling of Screen [contd.]
• Finger nails should not be used to take the
film from the cassette.
• The cassette should not be kept open in the
darkroom.
• The screen may be cleaned periodically
(monthly) with a solution containing anti-
static compounds or soap and water.

38. Handling of Screen [contd.]
• It should be rinsed and dried after every
cleaning.
• Film-screens should have good contact and
this must be checked periodically with a help
of a wire mesh.

39. References
• Christensen’s physics of diagnostic
radiology.4th ed.
• The physics of radiology and imaging.
(Thayalan)
• Christman R. Foot and Ankle Radiology. 2nd
ed.
• Smith WL. Radiology 101. 4th ed.