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
How will a thicker screen improve the image?
Increased thickness of film - increased absorption of x-rays
Decreased detail due to light diffusion
What is the difference in absorption between calcium tungstate and a rare earth metal?
A calcium tungstate screen has an absorption efficiency of about 20% (high par)
Where as a rare earth metal has an efficiency of 40-60%
Why do rare earth metals have a higher absorption efficiency?
All phosphors will have a lot of photoelectric effect.
However, the range of energies of diagnostic x-ray beams range from 30-50keV
Tungsten kshell 69.5
gd = 50
lanthanum - 40
More PE absorption will occur with the rare earty metals
How does yttrium screens compare to calcium tungstate?
Which is better and why?
Yttrium (Z = 39, K-edge = 17)
They have very similar absorption efficiencies from 17 (kedge of yttrium) to 69 (kedge of calcium)
Yttrium is preferred, because it has a higher absorption efficiency
What is quantum mottle?
Statistical fluctuation in # of photons per unit area tha tcontribute to image formation
What is the primary form of quantum mottle?
fluctuation in x-ray emission
How do you decrease quantum mottle?
Increase # of x-rays
What is the purpose of a grid?
Absorb scatter radiation
What is the bad thing about grids?
why does it do that?
In order to have the same quality image, will have to increase the exposure factors
Grids are meant to absorb scatter radiation, but no primary.. unfortunately will absorb some primary radiation
What is a grid ratio?
Ratio between the height of the lead strips, and the spaces between them.
Ratio = h/D
In general, what can you say about higher grid ratios?
Generally, the are better grids - absorb more scatter
What is the make-up of a grid?
Series of lead foil strips, separated by a space that is filled with support material such as carbon fiber or aluminum
What are the different patterns of a grid?
linear or crossed
Focused or parallel
any combination thereof
What is a convergent line? convergent point?
The point in space above the grid where the beam ideally originates
Line - would be for a linear focused grid
Point - crossed focused grid
What is a potter-bucky?
device that moves grid back and forth so no lines are present on the image.
What is the purpose of using an air-gap?
Has similar results to a grid, with less exposure than a grid
When a patient is up against the detector - scatter is very close to grid and doesn’t have space to travel outwards and avoid the grid
By placing distance betwen patient and the detector - more scatter is likely to miss the detector
An air-gap works best under what conditions?
Low energies - larger scattle angles (billiard ball from Compton scatter)
Small FOV - small imate receptor will miss more scatter
What is draw back of using an air-gap?
how can that be reduced?
more likely to have magnification
Increasing source to detector distance, (would not effect the patient exposure)
What are the two major ways to decrease magnification?
Increase source to image distance
keep object as close to detector as posible
What are the components of a fluoroscopic image intensifier?
Vacuum housing Input phosphor Electron optics Accelerating anode Output phosphor
What occurs in a fluoroscopic image intensifier?
X-ray beam passes through the patient –> enters the image intensifier tube –> input fluorescent screen absorbs x-rays and converts energy to light photons
Light photons strike photocathode –> emits photoelectrons –> high potential difference between photocathode and anode that causes acceleration towards the anode
Electrostatic lens focuses the electrons –> guides towards fluorescent screen without distorting image, also increases the electrons KE
Electrons strike output screen –> emit light photons that carry fluoroscopic image to eye of the observer
X-ray –> light photons –> electrons –> light photons
What phosphor is used at the input phosphor of a fluoroscopic image intensifier? the output phosphor?
Input - CsI
Output - ZnCd
Why is CsI preferred to ZnCd at the input phosphor of a fluorscopic image intensifier?
1) Larger amount of CsI can be packed into an area
2) Attenuation coefficient of CsI better matches the spectrum of x-rays emerging from the patient
3) More favorable atomic number of CsI (Cs = 55, I=53), and ZnCd (Zn = 30, Cd = 48) - slightly closer to the keV produced
4) Light produced by CsI more closely matches the sensitivity of the photocathode
For a single x-ray photon - approximately how many light photons are produced?
3000 light photons
What is conversion efficiency of the photocathode in a fluoroscopic image intensifier?
Per xray photon - approximately how many electrons are produced by the photocathode?
10-20%
Per x-ray photon - 3000 light photons = 200-400 electrons produced by photocathode
What is the photocathode made of?
Antimony
What is electronic gain?
Increase in KE of the electrons
What is outpu tphosphor made of?
ZnCd
At the output phosphor - how many light photons are generated?
Per electron - 2000 light photons are generated
What occurs at the output phosphor?
Electrons accelerated across tube are converted to light photons - approximately 2000 light photons / electron
Overall - per light photon produced at the input phosphor - how many light photons are produced at the output phosphor?
1 xray –> 3000 light photons –> 10-20% conversion efficiency at photocathode –> 300-400 electrons –> 2000 light photons
Per light photon ~ 50 light photons
How do you calculate the brightness gain of a fluorscopic unit?
Brightness gain = electronic gain x minification gain
How is electronic gain calculated?
Has to do with increased KE across the tube, and how many light photons are produced per electron
Generally - 50 is the standard number
How do you calculate the minification gain?
(input phosphor diameter / output phosphor diameter) ^2
Describe the minification gain
If you reduce the image diameter it’s like a magnifying glass and the sun - you are focusing the electrons into a smaller area which will increase the brightness of the output image (start a fire on the sidewalk)
If the input phosphor area is changed (FOV decreases) what will happen to the minification gain?
What happens to the brightness gain?
Minification gain will decrease (input/output)^2
Brightness gain will decrease
How much does the minification improve the quality of the image?
It doesn’t. :)
Just increases the overall brightness - has nothing to do with spatial resolution
Describe the magnification mode used in fluoroscopy?
Voltage is changed applied to the electrodes - will only focus the electrons from the central portion of the photocathode –> onto the entire output phosphor.
(just because looking at a smaller area on the input screen, doesnt mean that the entire output phosphor isn’t used as well)
basically will be zooming in on an area
What does magnification provide?
Increases the spatial resolution
What are the drawbacks of using magnification modes in fluoro?
Double the magnification - quadruple the exposure to the patient
How does magnification affect the brightness gain? Why?
As magnification increases - brightness gain decreases due to the change in the minification gain (input/output)^2
What are recommendations for collimator error?
Federa regulation - light field and x-ray field must be aligned so that sum of misalignments between light field and x-ray field at the corners must not be >2cm
What is the point of using collimators?
Focused FOV - better images
Decrease volume of patient irradiated
What is inherent filtration within a tube?
housing oil and field light mirror in collimator
What is added filtration?
Sheets of metal intentionally placed in the beam to change its effective energy
What is the most common filter used?
How much is added?
Aluminum
Usually 1-2mm of Aluminum
What is the purpose of filtration?
Filters out the low energy x-rays via photoelectric effect
Will strengthen the overall energy of the beam
What does filtration do to the HVL?
Increases the HVL
What will filtration do to the intensity of the x-ray beam?
Decrease the intensity - will need to increase mAs to avoid quantum mottle
What is NOT important in the efficiency of an intensifying screen?
The number of x-ray photons does NOT influence the speed of a screen
It DOES have to do with the ENERGY of the x-rays
What is percentage of X-ray absorption by CaWO4 vs a rare earth metal screen?
How about, conversion efficiency?
X-ray absorption
CaWO4 - 20%
Rare earth - 40-60%
Conversion efficiency
CaWO4 - 5%
Rare earth - 20%
What is conversion efficiency of a rare earth screen vs CaWO4?
20% for rare earth
5% for CaWO4
What is intrinsic conversion of a phosphor in a film screen?
Fraction of absorbed energy converted to light
What is quantum detection efficiency?
Fraction fo # xrays photons absorbed by screen
What is total conversion energy of a screen?
Ability of screen to convert deposited energy by absorbed x-ray photons into film darkening
How is overall efficiency of a screen calculated?
QDE x CE
What does the total conversion efficiency of a screen depend on?
Intrinsic conversion efficiency of the phosphor
Efficiency of light propagation through screen to the emulsion
Efficiency of film emulsion in absorbing the emitted light
What are the ways that you can increase speed of a screen, and how do the rare earth screens accomplish that? (or not accomplish)
Thicker phosphor layer - doesn’t really need it
Conversion efficiency - Rare earth - 20%, CaWO4 - 5%
Higher x-ray absorption efficiency - Rare earth 40-60%, CaWO4 - 20%
Emission spectrum - both CaWO4 and rare earth are aimed to match the spectrum of light sensitivity of the emulsion
How do Gadolinium screens and yttrium compare to CaWO4 for absorption efficiency?
Gadolinium (Z = 64)
Tungsten (Z = 74)
Yttrium (Z = 39)
energies of x-rays used in diagnostic imaging are typically in the realm of 30-50kVp - so a rare earth metal (such as gadolinium) will have a k-edge that is much closer to the spectrum of x-rays
yttrium - has the exact same absorption pattern as CaWO4 - however, yttrium has an improved conversion efficiency (18 vs 5%)
