Accessories: film screen, grids - 11.0 Flashcards
What is the film emulsion?
Gelatin base with silver halide crystals (AgBr or AgI0 bound within the gel
What is the difference between a single an double film emulsion?
Single: Only one side of the polyester base is covered by the emulsion
Double emulsion: emulsion on both sides
What is the benefit of using single emulsion over double?
Better resolution
Less crossover
What is the benefit of using double emulsion film?
Increased speed
Same exposure used for a single emulsion film will produce double the density
What is crossover?
How does it occur?
(b) Crossover is the exposure of emulsion to light from the opposite intensifying screen
Cause is due to incomplete absorption of the light from the emulsion → crosses over the film base → light exposes the emulsion on the other side
What is the problem with emulsion?
Decreases image quality: light diffusion, scatter, reflection
How could you resolve crossover?
Adding a colored dye to emulsion
Matching screen emission to the silver halide (using screen that emits light in similar wavelength to the phosphor - increase absorption efficiency)
use of tabular grain - increases the surface area of the emulsion - increased chance of interactions
What is the problem with adding a colored dye to the emulsion layer?
Will decrease conversion efficiency of the screen
What is efficiency of plain film vs film-screen at absorbing x-rays?
Plain film = 0.65%
Film screen - up to 30%
What is wavelength spread of visible light-UV spectrum ?
340-750nm
What is UV wavelength?
Blue light?
Green light?
Red light?
UV 350-450nm
Blue 450-500nm
Green 500-580nm
Red 620-750nm
When you use a screen - what percentage of film is made up of interaction with light from screen, vs direct xray interaction?
95% from screen
5% from x-ray
How is speed of screen related to the detail?
Inversely related
High speed screens = less detail.
What factors determine the speed of a screen?
Thickness of htethephosphor Higher absorption phosphors Size of the phosphor crystals Presence of light absorbing dye in the phosphor Phosphor conversion efficiency
How does thickness of the phosphor affect the speed? how?
Thicker screen = higher speed
More efficient at absorbing x-rays
What is the disadvantage of using a thicker phosphor layer?
Increased light diffusion = less detail
What are higher absorption phosphors?
Any of the rare earth metals
CaWOR - 20%
Rare earth metals - 40-60%
What is the function of a cassette?
Prevent ambient light from interaction with the film
What is present within the cassette?
Carbon fiber outer layer (allows passage of x-rays)
Foam - add pressure
Intensifying screens
What are 4 layers of the intensifying screen?
Base
reflecting layer
phosphor layer
plastic protective coat
What determines the # of light photons produced in the intensifying screen?
The x-ray energy absorbed by the screen.
The number of -xrays absorbed is less important
What is the purpose of the intensifying screen
decrease radiation to patient
decrease workload on x-ray machine
decrease exposure time and image blur
What does the intensifying screen do?
Made up of scintillating material (phosphor): commonly of Calcium Tungstate (CaWO4), or rare earth metals
X-rays undergo indirect interaction with the phosphor. The phosphor absorbs the x-ray photon and converts them to UV or visible light (350-750nm)
What is the intrinsic conversion efficiency of the phosphor?
Efficiency with which the phosphor converts x-rays to light
How do you calculate the intrinsic conversion efficiency of the phosphor?
Ratio of light created / xray energy (keV or eV) absorbed
How would you calculate the # of light photons produced by a screen given the energy and conversion efficiency?
E = 1.24 / wavelength of screen = Energy (eV)
At 100% efficiency a 50 keV x-ray will produce 50 / Energy (eV) = light photons (if conversion efficiency was 100%)
Then multiply by conversion efficiency
What is the quantum detection efficiency otherwise known as?
Absorption efficiency
What is the QDE?
Fraction of incident x-ray photons that interact with a screen
What is the best way to improve the QDE?
Increase the thickness of hte phosphor (increaesd diffusion, less resolution - MTF)
What is the total conversion efficiency of a screen film?
Ability of a screen to convert deposited energy by absorbed x-ray photons into film darkening
This is different than the intrinsic conversion efficiency (fraction of absorbed energy emitted as light)
What does the total conversion efficiency of a screen film rely on?
Intrinsic conversion efficiency of the phosphor
Efficiency of light propagation through the screen
Efficiency of absorbing emitted light by the film emulsion
What can be done to improve the total conversion efficiency of a screen film?
Light absorbing dye within the emulsion
Create a phosphor that emits lights within UV spectrum as these are more readily absorbed
Add a reflective layer between screen and its support
What are hte benefits of using a film screen?
Decreased radiation exposure to patient and personnel
Decreased work on x-ray tube and generator
Increased efficiency - film is not as great as absorbing x-rays
Increased resolution
What is noise?
Variations in the film density that do not have anything to do with variations in the attenuation of th epatient
What noise can occur in association with a screen?
Random variations in x-ray photons interacting with screen
Random variations in fraction of light ommitted by screen that is absorbed by emulsion
Random distributions of grains within the emulsion
How do you decrease noise in a screen film?
Increase the # of xray photons
What happens to noise if you increase conversion efficiency of the screen?
You will increase noise
By increasing the efficiency of the screen - will have to use less x-rays
Less x-rays = more noise
What happens to noise if you increase absorption efficiency of the screen?
No change in noise
If you incrase the absorption efficiency by 10%, have to decrease x-ray beam by 10%
However, you are not decreasing the # of xrays interacting with the screen - so no change in noise
How do you incrase the speed of a screen?
Increased phosphor thickness
Increased phosphor efficiency
Higher absorption phosphors