1. INTERACTION OF X-RAY
INTERACTION OF X-RAY
WITH MATTER:WITH MATTER:• X-rays entering a patient can be absorbed,
scattered, or transmitted.
• When an X-ray is absorbed in a patient, all of the
X-rays energy is transferred into the patient's
tissue.
• Scattering changes the X-ray's direction and
reduces its energy.
• Transmitted x-rays pass through the patient
without interaction.
• Most diagnostic x-rays are absorbed or
scattered.
4. Ideal Grid
Ideal Grid
• block all scattered radiation
›
Reality: lead strips permit some scatter to get
through to film
Lead
Interspace
5. 1.The Screen-Film Cassette
Cassette
Light-tight and ensures screen
contact with film
Front surface - carbon fiber
ID flash card area on back
Back lined with lead to reduce
back-scatter.
1 or 2 Intensifying Screens
Convert x-rays to visible light
Mounted on layers of
compressed foam (produces
force)
Ease to open & closed, under
low light conditions. Lightweight,
no sharp edges or corners
which may injure patients or
staff.
Vinyl covered front providing
warmth.
Availability in range of film sizes
5
9. Screen Phosphors
Screen Phosphors
• Screen color differs based on the type of phosphor and dyes
imbedded in screen.
• Emits blue, ultraviolet or green light that exposes the film.
• Decreases exposure by 100 fold compared to non-screen film
technology.
10. X-ray film and Intensifying screens
X-ray film and Intensifying screens
Poor film screen contact
produce image unsharp (poor
detail image)
11. X-ray film and Intensifying screens
Close contact must be maintained between the
film and intensifying screens (in light tight
cassettes) to minimize the loss of detail and to
protect the film from extraneous light.
A
B
The Fundamentals of Radiography. Kodak
12. Noise Effects of Changing CE vs. AE (2)
What happens to noise in image when the AE is increased (thicker
screen)?
If AE is increased, 10% more x-ray photons detected, then
reduction of 10% in incident x-ray beam is required to deliver same
amount of film darkening (as before increasing AE)
Since the fraction of increase in x-ray photon detection and
reduction in incident x-ray intensity is same, the total number of
detected x-ray photons is the same. No change in noise
However, spatial resolution will get worse with thicker screens
12
13. Screens
Screens
• The x-ray photon will strike the phosphor crystal in the
screen.
• The excited phosphor will emit a specific wavelength of
light that exposed the film.
• The efficiency of the screen is based on:
› Thickness of phosphor layer
› The type of phosphor
› The size of the phosphor crystal
• Trade-off: increased efficiency, decreased spatial
resolution (thicker layer, larger crystal size).
14. Screens
Screens
• The thicker the
phosphor layer
the less detail of
the image
• Note the
spreading of the
light in the thicker
screen layers
15. Screens -- Resolution
Screens Resolution
• Resolution (detail) can be measured
using a line resolution phantom.
• Resolution is defined and the
smallest number of line pairs than
can be seen
› High detail screen (50 speed)
may resolve 16 LP/mm
› Rapid screen (400 speed) may
resolve 5 LP/mm but require 1/8th
the radiation
16. Spectral Matching
Spectral Matching
• Intensifying screen phosphors contain
different elements
• The color of light emitted will also differ
› Yttrium tantalate - Violet
› Barium lead sulfate – Blue
› Lanthanum oxybromide – Blue
› Gadolinium oxysulfide – Green
› Calcium Tungsten - Blue
• Activators
› Thulium activated - Blue
› Terbium activated - Green
17. Spectral Matching
Spectral Matching
• It is important that the wavelength of light
emitted by the screen matches the sensitivity
of the film.
• Specific filters required for safety light for green
versus blue sensitive film.
18. Intensifying Screen Function and Geometry
Modulation Transfer Function
(MTF) describes the resolution
properties of an imaging system
The MTF illustrates the fraction
(or %) of an object’s contrast that
is recorded by the imaging
system as a function of object
size (spatial frequency)
Frequency (linepairs or
cycles/mm)
F=1/2∆, ∆ = object size
As screen thickness ↑ MTF ↓
18
19. Sensitivity or Speed (3)
A general relationship between exposure requirements (sensitivity) and
speed values: sensitivity (mR) =128/ speed.
For example, a screen with true speed value of 100 requires an exposure of
1.28 mR to produce a 1 unit of density.
100-speed – detail work (thinner screens, slower, better spatial resolution)
600-speed – angiography (thicker screens, decreased spatial resolution)
The range of system sensitivity and speed values used in radiography is
shown below:
Speed
(Sensitivity (mR
1200
0.1
800
0.16
400
0.32
200
0.64
100
1.28
50
2.56
25
5.0
20. Sensitivity or Speed
Sensitivity is expressed in
terms of exposure required to
produce a film density of 1 unit
above the base plus fog level.
Speed values such as 100,
200, 400, ect, compare the
relative exposure requirement
of different screen.
Most speed numbers are
referenced to a so called par
speed system that is assigned
a speed value of 100.
20
21. Comparison of typical exposure factors in an
Comparison of typical exposure factors in an
AP projection of the knee(12:1 moving grid(
AP projection of the knee(12:1 moving grid(
›
Type of radiography
exposure factors
---------------------------------------------------------------
›
Ultrahigh speed screens
20mAs(100mAx0.2 sec at 60kvp)
›
High speed screens
30mAs(100mAx0.3 sec at 60kvp)
›
Par speed screens
60mAs(100mAx0.6 sec at 60kvp)
›
Slow speed screen
120mAs(100mAx1.2 sec at 60kvp)
›
Direct exposure
›
4800mAs(100mAx12 sec at 60kvp)
22. Comparison screen speed to image details
Comparison screen speed to image details
Using of fast speed more intensifying screen increase image unsharpess by diffusion
of light
23.
24.
25. X-Ray Film Construction
X-Ray Film Construction
• Film base
• Adhesive layer
– attaches emulsion to base
• Emulsion layer
• Supercoating
Adhesive
Layers
”007.
”0005.
Supercoating
Film
Base
Emulsion
Layers
26. Film Base
Film Base
•
•
•
•
structural support for fragile emulsion
low light absorption
Film
no visible pattern
Base
flexible, thick, & strong
– processing
– handling
– viewbox insertion / removal abuse
• dimensional stability
– in processing
– For archival
» varying humidity
27. Film Base Materials
Film Base Materials
• early films used cellulose
nitrate
– Flammable
• “safety” base
– cellulose triacetate used until 1960’s
– polyester
» .007 inches thick
– Base color
» Clear
» Blue dye added
» Requested by radiologists
• reduces eye strain
Film
Base
”007.
28. Emulsion
Emulsion
• Most films use two emulsions
– each emulsion <=.5 mil thick
» thicker emulsion = less light penetration
• Gelatin
– keeps silver halide grains dispersed / prevents
clumping
– allows penetration of processing solutions
without compromising strength or permanence
– made from cattle bones
Emulsion
Layers
31. Silver Halide
Silver Halide
• precipitation determines crystal size
& concentration
– typical size: 1 - 1.5 microns
– 1 grain averages 1 - 10 million silver ions
• chemical sensitization of crystal
– sulfur-containing compound added to emulsion
– silver sulfide formed
» usually located on crystal surface
» called sensitivity speck
» traps electrons to begin formation of latent image centers
32. Light Image on Film
Light Image on Film
• light photon allows escape of electron in
bromine ion (Br -)
• neutral bromine atoms leave crystal, go into
emulsion gelatin
• electron travels to, fixed in sensitivity speck
• negative sensitivity speck attracts mobile silver
(Ag+) ion forming silver atom
Ag+ + electron
Ag
• repeated trapping of electrons results in
growth of silver
33. Latent Image Centers
Latent Image Centers
• one light photon produces one silver atom
• silver atoms collect at sensitivity speck
– no visible change in grain
• visible amounts of silver deposited at latent
image centers during processing
• one or more latent image centers per grain
– 3 - 6 centers required for grain to be developable
– centers may contain 100’s of silver atoms
37. Latent Image Formation:
Latent Image Formation:
Gurney-Mott
Gurney-Mott
Light photon absorbed
by/ejects Br electron
Electron trapped at
sensitivity speck
Neg electron attracts
interstitial Ag+ ion
Ag+ and e- combine to
form neutral (black) Ag
If >6-10 Ag0 accumulate
at speck, it becomes a
latent image center: ie,
.it is developable
38. Processing
Processing
• Amplifies latent image by
100,000,000!
• forms visible silver
• reduces silver ions into neutral black
metallic silver atoms which remain
on the film after processing
Ag+ + electron
Ag
• processing initiated at latent image
speck
– grain either develops entirely or not at all
39. Processing
Processing
• Silver atoms at latent image
center act as catalyst
• Grains with no latent image also
develop much more slowly
– Developer time is fundamental in development
– processing should stop when maximum
difference between exposed & unexposed
crystals
40. Processing Developing
Processing Developing
Solution
Solution
• developing agent
– hydroquinone
– phenidone or metol
– combination yields development rate greater than sum of
each
• alkali
– adjusts pH
• preservative (and oxidation preventative)
– sodium sulfite
• restrainers
– antifoggants (reduces development of unexposed grains)
41. Developing Time
Developing Time
• Controlled by
– Speed of transport
– Film path in develop rack
» System of rollers & chains which direct film through
developer tank
• “90 second” processor
– 90 seconds from film in to film out
– Time in developer ~ 20 second
– Time in fixer & wash tanks controlled by size
of fixer & wash racks
» Transport speed does not change because at any
time films may be in any or all tanks
42. Developing
Developing
• Temperature
controlled to ~0.5 degrees
90 - 95 degrees for 90 second processor
100+ degrees for 60 second processor
• Replenishment
– automatic addition of fresh chemistry to
replace chemistry depleted in development
– even with replenishment, chemistry must be
completely replaced periodically
43. Fixing
Fixing
• Function
– removes remaining silver halide / silver ions
without damaging metallic silver
– hardens gelatin
• composition
– cyanides (poisonous & not usually used)
– thiosulfates
» sodium or ammonium salt
• hypo
– buffers to maintain pH
45. (3)Production
(3)Production
Permanent Form
Permanent Form
• This involves the action of a
chemical agent to make the
hidden image visible. This stage
is known as development, and it
is followed by further chemical
processes which fix the image
and make it into permanent
record.
48. Latent Image
Air/soft tissue
Many x-rays
penetrate and
expose many silver
halide crystals
Bone
Amalgam/gold
Fewer x-rays
penetrate and not as
many silver halide
crystals are exposed
Few, if any, x-rays
penetrate; silver
halide crystals not
exposed
Exposure centers=
56. Fog and Scatter
Fog and Scatter
• Fog and Scatter reduce contrast
• Scatter
›
›
›
produces unwanted density
mostly a result of Compton interactions
increases with
» kVp
» part thickness
» field size
• collimation reduces scatter
57. Fog
Fog
• Development of film grains not
exposed to light or x-rays
• produces unwanted density
• lowers radiographic contrast
58. Determining Fog
Determining Fog
• run half sheet of film through
developer, fixer, wash, dryer
• run other half through all but
developer
• compare densities
›
difference is fog
59. Exposure Fog
Exposure Fog
• also called “fog” but different
from development fog
• refers to accidental exposure to
radiation
Wall
Darkroom
Wall
60. Development Fog
Development Fog
• Development of unexposed grains
›
“true” fog
• Sources of optical density increase
›
Storage
» high temperature
» high humidity
›
›
›
chemistry contamination
excessive developer time
excessive developer
temperature
61. Fog and Scatter
Fog and Scatter
• Alter characteristic curve
• Reduce contrast at clinical
densities
• less effect at higher densities
