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INTERACTION OF X-RAY WITH MATTER:-
INTERACTION OF X-RAY 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.
Two focal spot
Ideal GridIdeal Grid
• block all scattered radiation› Reality: lead strips permit some scatter to get
through to film
LeadInterspace
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1.The Screen1.The Screen-Film-Film Cassette Cassette Cassette Cassette
Light-tight and ensures screen Light-tight and ensures screen contact with film contact with film
Front surface - carbon fiber Front surface - carbon fiber ID flash card area on back ID flash card area on back Back lined with lead to reduce Back lined with lead to reduce
back-scatter. back-scatter. 1 or 2 Intensifying Screens 1 or 2 Intensifying Screens
Convert x-rays to visible light Convert x-rays to visible light Mounted on layers of Mounted on layers of
compressed foam (produces compressed foam (produces force) force)
Ease to open & closed, under Ease to open & closed, under low light conditions. Lightweight, low light conditions. Lightweight, no sharp edges or corners no sharp edges or corners which may injure patients or which may injure patients or staff. staff.
Vinyl covered front providing Vinyl covered front providing warmth.warmth.
Availability in range of film sizes Availability in range of film sizes
CassettesCassettes
• Cassettes are rigid, light-tight devices that hold x-ray film in contact with the intensifying screen
CassettesCassettes
• Configuration varies
Structure of the film, intensifying screen and cassette
Screen PhosphorsScreen 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.
X-ray film and Intensifying screensX-ray film and Intensifying screens
Poor film screen contact produce image unsharp (poor detail image)
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.
X-ray film and Intensifying screens
A
B
The Fundamentals of Radiography. Kodak
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Noise Effects of Changing CE vs. AE (2)Noise Effects of Changing CE vs. AE (2)
What happens to noise in image when the AE is increased (thicker What happens to noise in image when the AE is increased (thicker
screen)? screen)? If AE is increased, 10% more x-ray photons detected, then If AE is increased, 10% more x-ray photons detected, then
reduction of 10% in incident x-ray beam is required to deliver same reduction of 10% in incident x-ray beam is required to deliver same amount of film darkening (as before increasing AE) amount of film darkening (as before increasing AE)
Since the fraction of increase in x-ray photon detection and Since the fraction of increase in x-ray photon detection and reduction in incident x-ray intensity is same, the total number of reduction in incident x-ray intensity is same, the total number of detected x-ray photons is the same. No change in noise detected x-ray photons is the same. No change in noise
However, spatial resolution will get worse with thicker screensHowever, spatial resolution will get worse with thicker screens
ScreensScreens
• 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).
ScreensScreens
• The thicker the phosphor layer the less detail of the image
• Note the spreading of the light in the thicker screen layers
Screens - ResolutionScreens - 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
Spectral MatchingSpectral 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
Spectral MatchingSpectral 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.
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Intensifying Screen Function and Geometry Intensifying Screen Function and Geometry
Modulation Transfer Function Modulation Transfer Function (MTF)(MTF) describes the resolution describes the resolution properties of an imaging system properties of an imaging system
The MTF illustrates the fraction The MTF illustrates the fraction (or %) of an object’s contrast that (or %) of an object’s contrast that is recorded by the imaging is recorded by the imaging system as a function of object system as a function of object size (spatial frequency) size (spatial frequency)
Frequency (linepairs or Frequency (linepairs or cycles/mm) cycles/mm) F=1/2∆, ∆ = object sizeF=1/2∆, ∆ = object size
As screen thickness ↑ MTF ↓As screen thickness ↑ MTF ↓
A general relationship between exposure requirements (sensitivity) and A general relationship between exposure requirements (sensitivity) and speed values: speed values: sensitivity (mR) =128/ speedsensitivity (mR) =128/ speed..
For example, a screen with true speed value of 100 requires an exposure of For example, a screen with true speed value of 100 requires an exposure of 1.28 mR to produce a 1 unit of density.1.28 mR to produce a 1 unit of density.
100-speed – detail work (thinner screens, slower, better spatial resolution) 100-speed – detail work (thinner screens, slower, better spatial resolution) 600-speed – angiography (thicker screens, decreased spatial resolution)600-speed – angiography (thicker screens, decreased spatial resolution) The range of system sensitivity and speed values used in radiography is The range of system sensitivity and speed values used in radiography is
shown below:shown below:
Sensitivity or Speed (3)Sensitivity or Speed (3)
Speed Sensitivity (mR)
12000.1
8000.16
4000.32
2000.64
1001.28
502.56
255.0
2020
Sensitivity or Speed 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 Most speed numbers are referenced to a so called par referenced to a so called par speed system that is assigned speed system that is assigned a speed value of 100.a speed value of 100.
Comparison of typical exposure factors in an AP projection of the knee(12:1 moving grid)
Comparison of typical exposure factors in an 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)
Comparison screen speed to image detailsComparison screen speed to image details
Using of fast speed more intensifying screen increase image unsharpess by diffusion of light
X-Ray Film ConstructionX-Ray Film Construction• Film base• Adhesive layer
– attaches emulsion to base
• Emulsion layer
• Supercoating
FilmBase
AdhesiveLayers Emulsion
Layers
Supercoating
.007”
.0005”
Film BaseFilm Base• structural support for fragile emulsion• low light absorption• no visible pattern• flexible, thick, & strong
– processing– handling– viewbox insertion / removal abuse
• dimensional stability– in processing– For archival
» varying humidity
FilmBase
Film Base MaterialsFilm 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
FilmBase
.007”
EmulsionEmulsion
• 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
EmulsionLayers
SupercoatingSupercoating
• Thin supercoating covers emulsion
• protects from mechanical damage
• makes film smooth & slick for use with processors
Supercoating
Silver HalideSilver Halide
• light sensitive
• 90 - 99% silver bromide
• 1 - 10% silver iodide– increases sensitivity
• small crystals (grains) precipitated and emulsified in gelatin
– crystal has lattice structure» Ag+ / Br- / I-
– silver nitrate added
Silver HalideSilver 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 specksensitivity speck
» traps electrons to begin formation of latent image centers
Light Image on FilmLight 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
Latent Image CentersLatent 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
LATENT IMAGE FORMATION
LATENT IMAGE FORMATION
SensitizationSensitization
Sensitization (Con’t)Sensitization (Con’t)
Latent Image Formation: Gurney-Mott
Latent Image Formation: Gurney-Mott
Light photon absorbed Light photon absorbed by/ejects Br electronby/ejects Br electron
Electron trapped at Electron trapped at sensitivity specksensitivity speck
Neg electron attracts Neg electron attracts interstitial Aginterstitial Ag++ ion ion
Ag+ and eAg+ and e-- combine to combine to form neutral (black) Agform neutral (black) Ag
If >6-10 AgIf >6-10 Ag00 accumulate accumulate at speck, it becomes a at speck, it becomes a
latent image center: ie, latent image center: ie, it is it is developabledevelopable..
ProcessingProcessing
• 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 processingAg+ + electron Ag
• processing initiated at latent image speck
– grain either develops entirely or not at all
ProcessingProcessing
• 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
Processing Developing Solution
Processing Developing 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)
Developing TimeDeveloping 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
DevelopingDeveloping
• Temperaturecontrolled to ~0.5 degrees90 - 95 degrees for 90 second processor100+ 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
FixingFixing
• Function– removes remaining silver halide / silver ions
without damaging metallic silver
– hardens gelatin
• composition– cyanides (poisonous & not usually used)
– thiosulfates» sodium or ammonium salt
• hypohypo
– buffers to maintain pH
WashingWashing
• Removes fixer chemicals
• Fixing leaves milky appearance on film
– unwashed film turns brown with age
(3)Production Permanent Form
(3)Production 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.
A Film Processor
The Two Steps in the Formation of a Film Image
Latent Image
Many x-rays penetrate and
expose many silver halide crystals
Fewer x-rays penetrate and not as
many silver halide crystals are exposed
Few, if any, x-rays penetrate; silver
halide crystals not exposed
Air/soft tissue Bone Amalgam/gold
=Exposure centers
Air/soft tissue Bone Metal
Developing
Development centers in crystals struck by x-rays are converted into black metallic
silver
Air/soft tissue Bone Metal
Developing (continued)
Entire crystal converted to black metallic silver
Air/soft tissue Bone Metal
Fixing
In the fixing solution, the unexposed silver halide crystals are removed from the film by the fixing
solution .
Sequence of Events That Convert a Transparent Film Grain into Black Metallic Silver
A NegativeRadiograph
A Positive Radiograph
Fog and ScatterFog and Scatter• Fog and Scatter reduce contrast
Fog and ScatterFog 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
FogFog• Development of film grains not
exposed to light or x-rays
• produces unwanted density
• lowers radiographic contrast
Determining FogDetermining Fog• run half sheet of film through
developer, fixer, wash, dryer
• run other half through all but developer
• compare densities› difference is fog
Exposure FogExposure Fog
• also called “fog” but different from development fog
• refers to accidental exposure to radiation
Wall DarkroomWall
Development FogDevelopment 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
Fog and ScatterFog and Scatter
• Alter characteristic curve
• Reduce contrast at clinical densities
• less effect at higher densities
Some References:
Questions pleaseQuestions please