Image/film quality - 11 and 13

Question 1

What is the difference between using a double vs a single emulsion film?

Answer 1

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

Question 2

What is crossover?

Answer 2

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

Question 3

How is crossover reduced?

Answer 3

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

Question 4

What does H&D stand for?

Answer 4

Hurter and Driffield

Question 5

Briefly - what is an H&D curve?

Answer 5

Predicts response of film to an exposure

Question 6

What is on the y and x axis of an H&D curve?

Answer 6

X-axis: log relative exposure

Y-axis: density

Question 7

What is transmittance?

Answer 7

T = I / Io
I = intensity of light that passes through a film 
I = intensity measured without a film

Question 8

What is the optical density? How is it measured?

Answer 8

Logarithmic function of the inverse of transmittance:

Log Io / I

Question 9

What is the lowest OD possible?

Answer 9

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

Question 10

What should base + fog NOT exceed?

Answer 10

0.2

Question 11

What are the difference sections of an H&D curve?

Answer 11

Toe - areas of low exposure
Shoulder - areas of high exposure
Slope of the curve = contrast

Question 12

What is the gamma of the H&D curve?

Why is this important?

Answer 12

The maximum slope

It’s not really important

Question 13

What is the average gradient of an H&D curve?

Answer 13

This is the overall contrast of a given type of radiographic film

Question 14

How is average gradient calculated?

Answer 14

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)

Question 15

What is the average gradient of most films?

Answer 15

2.5-3.5

Question 16

What does a steeper H&D curve represent?

Answer 16

A steeper H&D curve: higher contrast

Question 17

What does a shallow H&D curve represent?

Answer 17

A film with lower contrast

Question 18

How are transmittance and opacity related?

Answer 18

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

Question 19

How is optical density related to opacity?

Answer 19

Optical density is equal to the log of the opacity

Question 20

How is optical density related to transmittance?

Answer 20

Optical density is equal to the inverse of the log of transmittance

Question 21

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?

Answer 21

Same curve, but the faster screen would be shifted towards the left - requires less exposure to achieve the same optical density

Question 22

If you had two screens, one had higher contrast than the other, how would you draw these?

Answer 22

Curve with a steeper slope = higher contrast

Curve with shallower slope = lower contrast

Question 23

What is dynamic range?

Answer 23

Ratio of the maximum and minimum doses that can be imaged

Question 24

How does dynamic range between screen film and digital relate? on an H&D curve?

Answer 24

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

Question 25

How good is film on its own at detecting x-rays vs screen film?

Answer 25

X-rays - 0.65% of x-rays are detected
Screen film (with rare earth) - up to 30%

Question 26

Using a film-screen - approximately what % of the interactions with light are due to intensification from the screen?

Answer 26

95%

Question 27

What are the benefits of using a double emulsion film?

Answer 27

Double the density

Faster speed

Question 28

What is speed?

Give an example.

Answer 28

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

Question 29

What is technical, mathetmatical way to describe speed?

Answer 29

Reciprocal of exposure in roentgens requried to produce a density of 1.0 above base+fog

Question 30

What are the two ways that a screen-film are measured?

Answer 30

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

Question 31

You have two films of varying contrast… what will their H&D curves look like when they are superimposed?

Answer 31

They will cross

Question 32

If you have two films with different contrast, will one always be faster than the other?

Answer 32

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

Question 33

What are the factors that affect the speed of a screen?

Answer 33

Thickness of phosphor layer
Size of crystals
Light absorbing dye
Phosphor conversion efficiency