Transcript Film Optical Density

Film Optical Density
Spectral Matching &
sensitometry
Film screen
•
•
•
•
Radiographic Density
Overall blackening of the radiographic
image as a result of the energy deposited
inthe film emulsion.
Film screen
Radiographic Contrast / Grayscale
• The difference between two adjacent
• radiographic densities in the image.
•
•
The overall number of density shades
present in the image.
Densitometer
Density
• .25 TO -2.5
• The straight line of
the H&D curve
To see changes in
optical density
• In order to see changes in optical density
on a radiograph you must increase you
mAs by at least 20 – 30%
• To double the density on a radiograph you
must double your density
Optical Density
• A numerical value indicating the degree of blackening on
the film.
(average OD seen on a radiograph = 1.2 Range is 0.21 –
2.5)
# of photons coming through film =
# of photons hitting film
OD=
1 =0
1
1
100
1 = 1 101
10
OD #
2
3
1 = 2 102 1 = 3 103
100
1000
• Carltons – CH 21 (pg 307) – increments of optical density
•
•
Density log = incident light intensity
•
transmitted light intensity
•
•
OD #
% of light photons transmitted
•
0
100 %
•
1
10 %
•
2
1 %
•
3
0.1 %
• If the quantity of black metallic silver on a radiograph
• is such that it allow
•
1 % of the light to pass through – what is the OD?
•
10%
•
100%
•
1.
2.
3.
4.
An unexposed and processed film will
have a density of about?
zero
0.1
1.0
2.5
If the quantity of black on the film allows 1% of
light to pass through, that film has a density of
1. 0.01
2. 0.1
3. 1.0
4. 2.0
If the quantity of black on the film allows 10% of
light to pass through, that film has a density of
1. 0.01
2. 0.1
3. 1.0
4. 2.0
•
1.
2.
3.
4.
The relationship between the intensity of
light striking a film and the intensity of
light transmitted through the film is an
expression of which of the following?
Radiographic contrast
Radiographic density
Recorded detail
Radiographic filtration
Image-forming x-rays
• Two kinds of photons are responsible for
the OD and contrast on an image: Photons
that pass through without interacting and
those that are scattered though Compton.
• X-rays that exit the patient are remnant
and those that interact with the IR are
image-forming.
Optical Density
• Controlling factor: mAs
• mAs determines the quantity of xrays
• What is the formula to determine
quantity of x-rays?
How is OD measured on
a monitor
Optical Denisty H & D curve
RT 244
Week 13/14
Rev 11/09
Log denisty
H & D curve
• If the exposure of the film
results in optical densities
(OD) that lie in the toe or
shoulder regions, where
the slope of the curve is
less, contrast is reduced.
• Slope of the straightline portion of the
characteristic curve is
greater for image
receptor A than for
image receptor B.
Image receptor A has
higher contrast.
• How does
the slope
affect
• Latitude?
• Contrast?
Sensitometry
• What is the purpose?
• What does it measure?
• Why is it important?
More on this in RT 255 QA/QC
Sensitometric curve
•
•
•
•
• What is the ave
temp? for a 90 sec
processor?
DENSITY
Developer temp
Can effect
Slope of curve
Less exposure is
needed for higher
• Dev temps
100º
90º
Log of relative exposure