module 5: intensifying screens

how much of x-rays incident on film actually interact with it?

0.5%

when x-ray incident beam interacts with film, what does it contribute to?

latent image

what actually exposes film?

phosphor emission (phosphors of intensifying screens)

what does intensifying screen convert single x-ray photon into?

thousands of lower energy light photons that interact more efficiently with the IR

when visible light interacts with radiographic film it forms

latent image

an advantage of intensifying screen is it ___ efficiency of x-ray absorption and ___ patient dose

increases; decreases

what does intensifying screen increase?

x-ray tube life

what is the huge disadvantage?

less detail than direct exposure

where can screens be mounted?

top and bottom of the film cassette

order of xray photon events

x-ray photon → absorption by intensifying screen phosphors → emission of many photons of visible light → exposure of film

what is phosphor?

a luminescent material that emits light when stimulated

what is luminescence?

emitted visible light from a phosphor; fluorescence and phosphorescence

fluorescence

visible light emitted only while phosphor is stimulated

what is a desirable quality in intensifying screens

fluorescence

phosphorescence is also known as

after glow, screen lag

phosphorescence

visible light continues to be emitted after exposure has ended

what is isotropic emission

refers to radiation emitted with equal intensity in all directions

what is spectral matching

wavelength (color) of emitted light must match the light absorption characteristics of the film emulsion

what happens with mismatch of spectral matching

results in loss of/no response of film = light density

what are components of an intensifying screen?

1. protective coating

2. phosphor

3. reflective layer

4. base

what layer is closest to the film?

protective coating

what does protective coating protect?

phosphor layer form damage during handling

what does protective coating eliminate?

build up of static electricity

how much of screen does protective coating enclose?

whole screen

what is the active layer of screeN?

phosphor layer

what does phosphor do during x-rays

emits light

what is screen made of?

rare earth elements in the form of crystals

increase thickness of phosphor = __ speed

increase speed

increase thickness of phosphor = ___ patient dose

decrease dose

increase thickness of phosphor = __ spatial resolution

decrease spatial resolution

what does reflective layer enhance?

efficiency of intensifying screen from isotropic emission

where does reflective layer redirect light?

redirects light photons to film

what do some reflective layers have color enhancement for?

selectively absorb diverging light photons that have long wavelength

intensifying screen increases speed, and ___ resolution

decreases resolution

what layer is furthest from the film?

base

what is base made of?

polyester

what is the support for the screen?

base

which base is what in the cassette layer

low-z front

contact felt

base

phosphor

emulsion

base

emulsion

phosphor

base

contact felt

high-z back

First base: The Screen Base (the plastic support backing for the front intensifying screen).

Middle base: The Film Base (the polyester support right in the center of the film, sandwiched between the two emulsion layers: emulsion ⇒ base ⇒ emulsion.

Third base: The Screen Base (the plastic support backing for the back intensifying screen).

what is intensification factor (IF)

ratio of x-ray exposure needed to produce same density on a film with and without a screen

intensifying factor equation

exposure required without screen divided by exposure required with screen

what is detective quantum efficiency (DQE)

ability of phosphor to absorb x-ray energy

what does increasing DQE do?

if leaving radiation exposure constant, will improve image information

what has higher DQE, absorption, and conversion efficiency?

rare earth metals; more light emitted per x-ray absorbed by screen

what is conversion efficiency?

amount of light photons emitted per 1 x-ray photon

how does image noise appear?

as speckled background

what increases noise?

fast screens and high kVp

what does noise decrease?

decreases image contrast (more shades of gray)

what is quantum mottle?

type of noise; associated with using too few photons to create image (low mAs)

___ conversion efficiency = ___ QM

increase; increase

detective quantum vs conversion efficiency

detective: x-rays absorbed by screen

conversion: amount of light emitted for each x-ray absorbed

what is spatial resolution?

ability to image small objects that have high subject contrast

spatial resolution is minimum distance that what?

minimum distance between two objects at which they can be recognized as 2 separate objects

how is spatial resolution expressed

lp/mm ; line pairs per millimeter (line pair test pattern)

when you increase lp/mm what happens to object size?

smaller object can be imaged

different spatial resolutions

- 7 lp/mm: fast/high speed; lowest spatial resolution

- 15 lp/mm: fine detail/slow screens; higher spatial resolution

- 50 lp/mm: direct exposure film; no screen at all (just film)

___ phosphor thickness = ___ spatial resolution

increase; decrease

__ crystal size = __ spatial resolution

increase; decrease .

__ number/concentration of crystals = __ spatial resolution

increase; increase

what does speed describe?

how efficiently x-rays are converted to light

what is IR system speed determined by?

screens & film working together

what is speed measured against?

calcium tungstate screens: value of 100 (par speed)
(was original standard)

what is slow vs high speed

50-80: slow, detail

80-1200: high speed (rare earth)

what is speed influenced by?

- phosphor composition (rare earth)

- phosphor thickness

- crystal size

- concentration of crystals

- dye

what is phosphor composition?

rare earth - most efficient

- high Z number (57-71), can absorb more photons

increase phosphor thickness = __ speed, __ spatial resolution

increase speed, decrease spatial resolution

whats the difference phosphor thickness detail vs high speed have?

- detail screens: thin layer

- high speed screens: thick layer

what do larger crystals produce?

more light per interaction

what size crystals do detail screens have?

small crystals

__ size of crystal, you ___ speed

increase size, increase speed

___ concentration of crystals = __ speed

increase concentration, increase speed

what is dye added for?

to control light spread

how do you repair a damaged screen?

you can't, its irreparable

what happens to other screen when one is damaged?

screens are sold in correctly matched pairs, so both must be replaced

what should you not open cassette near?

chemicals or other liquids

why should you not leave cassette open?

exposed to contaminants

why shouldn't you store cassette near source of heat?

warped cassette compromise film-screen contact

how often should screen be cleaned?

monthly with manufacturer's cleaner (antistatic compounds)

what could objects do to screens?

scratch them

what does wire mesh test do?

checks for good screen-film contact; part of QA program

relationship for speed and resolution

inversely related

relationship for speed and patient dose

inversely related