I.s

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

9. Gadolinium
Lanthanum
Yttrium
Found in low
abundance in
nature

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

14. Active layer – x-ray photons
converted to light photons
*Photoelectric Effect

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

17.  FASTER SPEED – REDUCES PATIENT
EXPOSURE
 FASTER SPEED - REDUCES IMAGE DETAIL
INCREASES NOISE (LIGHT BLURING
AROUND IMAGE)

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.

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

35.  Warped cassette
–poor contact

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