Image/film quality - 11 and 13 Flashcards
What is the difference between using a double vs a single emulsion film?
Single - better resolution, slower
Double emulsion - two emulsions on either side of the film. Will have 2 images superimposed with each other –> decreased resolution, more crossover, faster speed, double the density
What is crossover?
A double emulsion film would ideally only be exposed to light from the intensifying screen adjacent to the emulsion.
The emulsion is not super efficient at absorbing light photons, so some photons will pass through to the emulsion on the other side of the base
This leads to light diffusion since it is traveling a farther distance
How is crossover reduced?
Adding a light absorbing dye to the emulsion that helps absorb x-ray exposure
Matching screen emission to the silver halide light sensitivity (most common way)
Tabular grains that increase surface area
What does H&D stand for?
Hurter and Driffield
Briefly - what is an H&D curve?
Predicts response of film to an exposure
What is on the y and x axis of an H&D curve?
X-axis: log relative exposure
Y-axis: density
What is transmittance?
T = I / Io I = intensity of light that passes through a film I = intensity measured without a film
What is the optical density? How is it measured?
Logarithmic function of the inverse of transmittance:
Log Io / I
What is the lowest OD possible?
Base + fog
There is always a background fog due to the presence of dye within the emulsion, and due to exposure of the film to natural background radiation
Usually around 0.13-0.18
What should base + fog NOT exceed?
0.2
What are the difference sections of an H&D curve?
Toe - areas of low exposure
Shoulder - areas of high exposure
Slope of the curve = contrast
What is the gamma of the H&D curve?
Why is this important?
The maximum slope
It’s not really important
What is the average gradient of an H&D curve?
This is the overall contrast of a given type of radiographic film
How is average gradient calculated?
It’s a slope between two points
y-axis:
Lower point: OD = 0.25 + base fog (0.13)
Higher point = 2 + base fog (0.13)
X-axis: log relative exposures at the optical density levels
(ODhigh - ODlow) / (log Ehigh - log Elow)
What is the average gradient of most films?
2.5-3.5
What does a steeper H&D curve represent?
A steeper H&D curve: higher contrast
What does a shallow H&D curve represent?
A film with lower contrast
How are transmittance and opacity related?
Opacity and transmittance are reciprocals of each other
T = I / Io (intensity of light with a film / intensity of light without a film)
Opacity = Io / I
How is optical density related to opacity?
Optical density is equal to the log of the opacity
How is optical density related to transmittance?
Optical density is equal to the inverse of the log of transmittance
If you were to draw and H&D curve for two different screens, with the same contrast, but different speeds… how would you do that?
Same curve, but the faster screen would be shifted towards the left - requires less exposure to achieve the same optical density
If you had two screens, one had higher contrast than the other, how would you draw these?
Curve with a steeper slope = higher contrast
Curve with shallower slope = lower contrast
What is dynamic range?
Ratio of the maximum and minimum doses that can be imaged
How does dynamic range between screen film and digital relate? on an H&D curve?
Digital has a much larger dynamic range - a large variety of exposures can still result in a perfect image.
Digital H&D curve is a straight line (linear)
Film-screen - H&D curve is curvilinear and exponential
How good is film on its own at detecting x-rays vs screen film?
X-rays - 0.65% of x-rays are detected Screen film (with rare earth) - up to 30%
Using a film-screen - approximately what % of the interactions with light are due to intensification from the screen?
95%
What are the benefits of using a double emulsion film?
Double the density
Faster speed
What is speed?
Give an example.
Amount of x-ray exposure required to achieve a specific optical density.
A higher speed screen or film will require less exposure to achieve the same density as a slower screen
What is technical, mathetmatical way to describe speed?
Reciprocal of exposure in roentgens requried to produce a density of 1.0 above base+fog
What are the two ways that a screen-film are measured?
Absolute and relative
Absolute: inverse of exposure required to achieve a density of 1 + base fog
Relative: par speed. CaWO4 is generally a 100 par. If a screen is 4x as fast - it will be a 400par
You have two films of varying contrast… what will their H&D curves look like when they are superimposed?
They will cross
If you have two films with different contrast, will one always be faster than the other?
No - generally films with different contrast - the relative speed may reverse with a change in density.
Example: Helps if you draw the curves :)
At 0.5 optical density - Screen A may be faster
At 1.5 optical density - Screen A &B may be the same speed (crossing point of the curve)
At 2.0 optical density - Screen B may be faster
What are the factors that affect the speed of a screen?
Thickness of phosphor layer
Size of crystals
Light absorbing dye
Phosphor conversion efficiency
