Revision lecture 1

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