2. Cassettes serve 3 important functions:
1. Protect film from exposure to light
2. Protect film from bending and
scratching during use.
3. Contain intensifying screens, keeps film
in close contact to screen during
exposure.
3. The CASSETTE is
used to hold the
film during
examinations. It
consist of front and
back intensifying
screens, and has a
lead (Pb) backing.
The cassette is light
tight
4. Exposure side of
cassette is the
“front”.
Made of radiolucent
material – easily
penetrated by x-rays,
lightweight
metal alloy or plastic
material made of
resin.
Intensifying screen
mounted to inside of
front.
5. Back made of metal or plastic
Inside back is a layer of lead foil –
prevents backscatter that could fog the
film
Inside foil layer is a layer of padding –
maintains good film/screen contact
Back intensifying screen mounted on
padding
Has the ID blocker (patient identification)
6. 1% of xray photons that leave patient
Interact with phosphors of intensifying
screens
100’s of light photons created to make
image on film
Light photons expose silver halide crystals
in the film emulsion
Turn black metallic silver after procession
7. Flat surface coated
with fluorescent
crystals called
phosphors
that glow, giving off
light when exposed to
x-rays.
8. RARE EARTH – (emits green light)
Developed in 1980’s
Most efficient – most common in use
today
CALCIUM TUNGSTATE (blue light)
Not as efficient
10. Direct x-ray exposure to film required
25 to 400 times more radiation to create an
image on the film
BETTER DETAIL THAN FILM SCREEN (NO
BLURRING OF IMAGE FROM LIGHT)
ALL EXPOSURE MADE FROM X-RAY PHOTONS
BIG DOSE TO THE PATEINT
11. DISADVANTAGES:
less detail than direct exposure
(detail better with rare earth than
calcuim tungstate screens)
ADVANTAGES:
1. Reduce patient exposure
2. Increase x-ray tube life
12. Polyester plastic base – support layer
Phosphor layer – active layer
Reflective layer – increases screen
efficiency by redirecting light headed in
other directions
Protective coating
15. A relative number that describes how
efficiently x-rays are converted into
usable light
Ranges from 100 (slow) to 1200 (fast)
16. Greater efficiency = less exposure =
faster
-Standard screen speed class of 100
-200 screen speed is twice as fast
Speeds for routine work: 200 – 800
Speeds for high detail: 50 – 100
Increasing speed also increases image
noise
18. Relative Speed = Film speed & Screen
speed
mAs 1 = RS 2
mAs 2 RS 1
They are inversely related
19. Speckled background on the image
Caused when fast screens and high kVp
techniques are used. Noise reduces
image contrast
The percentage of x-rays absorbed by
the screen is the detective quantum
efficiency (DQE)
The amount of light emitted for each x-ray
absorbed is the conversion efficiency
(CE)
20. Quantum Mottle causing a grainy,
mottled or splotch image
Often results of using very fast-speed
screen-film systems
21. The light photons generated in the
intensifying screen are emitted by
phosphor crystals.
These crystals are significantly larger
than the silver halide crystals in the
film
use of a screen reduces image
sharpness somewhat
Some examinations requiring
extremely fine detail use screens with
small crystals.
22.
23.
24.
25.
26.
27.
28.
29. Have higher DQE (detective quantum
efficiency). Higher x-ray absorption
abilities.
Have higher CE (conversion efficiency).
More light emitted per x-ray absorbed by
the screen.
30. The use of intensifying screens lowers
spatial resolution compared with direct-exposure
radiographs.
Spatial resolution
expressed by the
number of line pairs
per millimeter
(lp/mm)
31. Very fast screens = 7 lp/mm
Fine-detail screens= 15 lp/mm
Direct-exposure screens = 50 lp/mm
37. Image artifacts can appear if screens
are modified
Small scratches can leave artifacts
Dirty screens can leave artifacts
Screens should be cleaned once each
month with manufacturer’s cleaner with
antistatic compounds