Xray Film Holders Flashcards
Film holder
Functions to shield the film from white light until it can be processed
Non-screen, direct exposure, or cardboard cassette film holder
Designed to be used with non-screen film delivers excellent detail of small parts such as extremities
Require very high radiation dose
Screen cassette film holder
Most common type of holder, contains intensifying screens or scintillators
Creates 95%-98% of image
Frame
Either metal, plastic, or carbon fiber
Front
Must be radiolucent and homogenous
Back
Most cassettes have a varying amount of lead to absorb backscatter that would fog the image
Pressure pad
A felt of foam pad located between the back screen and the back of the cassette
Meant to maintain good film screen contact
Film/screen contact
Close contact between the screen and the film must be maintained in order to guarantee image sharpness
If there is poor contact there will be localized blurring in the image
Causes of poor film/screen contact
1.Bent or warped cassette
2. Warped screens
3. Foreign objects in the cassette
4. Worn out pressure pad
Intensifying screens
They contain crystals that will emit light when exposed to x-rays.
They are called scintillators in DR
Intensifying function
Reduce the exposure to the patient
Film only absorbs 1% of incident photons while screen absorbs 50x mores
Base of screen
Made up of either cardboard or polyester plastic, about 1mm thick, and act as a support for the rest of the screen
Reflective layer
this is a layer of titanium dioxide that is about 25 μm thick.
Reflects light back toward film emulsion, increasing speed of screen
Phosphor or active layer
Layer of microscopic crystals that will emit light when exposed to X-rays
Thicker in back to maintain speed as the front
Phosphors must have which characteristics?
1.High atomic number - required to absorb X-rays.Higher atomic number, the greater the absorption
2.Emit large amount of light- The ability of screens to convert X-rays to lights, called conversion efficiency
3. Spectral emission - Light must be of proper color
4. No afterglow or lag - Continued luminescence of the screen after the exposure to xrays has stopped
Absorbtion efficiency
The ability to absorb X-rays
Spectral matching
Correctly matching the color sensitivity of the film to the color emission of the intensifying screen
Protective layer
Layer of protective material to keep active layer from being damaged during the handling and cleaning
Luminescence
The emission of light by a crystal following exposure to an outside stimulus, such as exposure to xrays or electrons
Occurs because xrays excite electrons, and when electrons return, they emit visible light
Luminescence can occur by what 2 processes?
- Fluorescence - Light is emitted within 10^-8 seconds after being exposed to outside stimulus, immediate exposure
- Phosphorescence - Light is emitted sometime after 10^-8 after exposure to an outside stimulus, there is lag time, visually see it
When does the greatest absorption of xrays occur?
when the x-ray photon energy and the binding energy of the K-shell electron are almost the same.
• This is called the K-edge effect
Screen speed
Amount of light that is emitted for a given amount of x-ray exposure
Intensification factor
Numerical value calculated
I.F.=exposure without screens/exposure with screens
I.F. Values for X-ray screens tend to range between 30 and 60
Relative speed
Numerical value that has also carried over into digital image receptors
New mAs=old relative speed x old mAs/new relative speed
Specific name
Older method used for non-rare earth screens
Intrinsic factors
Those inherent or built in to the screen by manufacturer
Type of phosphor material
Different phosphor materials will emit different amounts of light for the same x-ray exposure due to differences in atomic number and K-edge values.
Phosphor materials are categorized into what 2 categories?
- Non-rare earth - older materials that emit blue-violet light when exposed to x-rays.
- Rare earth - made up of materials from the rare earth or lanthanide series of elements from the periodic table.
Main rare earth phosphors
- Gadolinium oxysulfide - Emits in the green portion of color spectrum
- Lanthanum oxybromide. Emits light in blue-violet spectrum
- Yttrium oxysulfide - emits both blue-violet and green
- Lutetium tantalite - emits light in the blue portion
Thickness of phosphor layer
The thicker the layer of crystals, the faster the speed of the screen
Rare earth screens can be made thinner because material is faster
Results in poorer image resolution
Size and shape of phosphor crystals
Larger crystals, faster speed of intensifying screen
Packing density
Number of phosphor crystals per unit area
Faster speed screens will have more crystals per unit area
Decreases image resolution
Dye in phosphor layer
Light absorbing dyes may be added to phosphor layer of slower speed screens to control reflected light
Increases resolution but decreases screen speed
Extrinsic factors
Factors that are outside of the screen itself
Ambient temperature
The greater the temperature of the screen, the slower it will behave in speed
Higher temp give crystals more energy which causes them to emit light
KVp
Energy of the X-ray beam will determine how much light is emitted from the screen
Spectral response
The sensitivity of the film to a specific color of light
