Chapter 6Chapter 6Image ReceptorsImage Receptors

RadiologyRadiology

IntroductionIntroduction To further understand radiography, To further understand radiography,

one must know how a permanent one must know how a permanent record is produced using x-rays.record is produced using x-rays.

Radiograph is formed with light-Radiograph is formed with light-sensitive film contained in a light-sensitive film contained in a light-proof encasement. proof encasement.

Encasement in radiography is called Encasement in radiography is called a cassette. a cassette.

The CassetteThe Cassette Designed to hold a piece of x-ray film Designed to hold a piece of x-ray film

sandwiched between two fluorescent sandwiched between two fluorescent sheets of plastic called sheets of plastic called INTENSIFYING INTENSIFYING SCREENS.SCREENS.• Intensifying screens are responsible for Intensifying screens are responsible for

converting the x-ray radiation into visible light, converting the x-ray radiation into visible light, which creates a latent image on the x-ray film. which creates a latent image on the x-ray film.

• Film is then processed to convert latent image Film is then processed to convert latent image to visible image.to visible image.

• Only 5 % of exposure results from ionization of Only 5 % of exposure results from ionization of x-rays, rest is due to light emitted from x-rays, rest is due to light emitted from intensifying screens. intensifying screens.

The CassetteThe Cassette Cassette-Cassette- A rigid film holder designed to A rigid film holder designed to

hold x-ray film and intensifying screens in hold x-ray film and intensifying screens in close contact. close contact.

Constructed with light tight materials to Constructed with light tight materials to prevent unwanted exposure, but yet still prevent unwanted exposure, but yet still allow penetration of x-rays. allow penetration of x-rays.

Originally cassettes were made with Originally cassettes were made with cardboard, but could not be reused. cardboard, but could not be reused.

Different forms today include aluminum, Different forms today include aluminum, polycarbonate, magnesium, and carbon polycarbonate, magnesium, and carbon fiber. fiber.

The CassetteThe Cassette Attached to the back with hinges and Attached to the back with hinges and

catches. catches. • Forms light tight seal.Forms light tight seal.

Back of cassette is constructed with Back of cassette is constructed with heavier material than the front because is heavier material than the front because is lined with material to absorb backscatter. lined with material to absorb backscatter.

Sides are lined with felt or foam pressure Sides are lined with felt or foam pressure pads that ensure close contact of the film pads that ensure close contact of the film to the intensifying screens. to the intensifying screens.

The CassetteThe Cassette

Sizes vary.Sizes vary.• Correspond to screen and film sizes.Correspond to screen and film sizes.

Cost varies as well.Cost varies as well.

Cassette CareCassette Care Should always be handled with care.Should always be handled with care. Can be exposed to large amounts of Can be exposed to large amounts of

physical abuse.physical abuse.• Common damage includes:Common damage includes:

Dropping the cassette.Dropping the cassette. Leakage of fluid on the cassette.Leakage of fluid on the cassette.

May want to place in plastic bag when a May want to place in plastic bag when a “messy” situation occurs. “messy” situation occurs.

Cassette Care continuedCassette Care continued Should be cleaned on regular basis with Should be cleaned on regular basis with

mild soap and water. mild soap and water. Should clean exterior and interior of Should clean exterior and interior of

cassettes at least once monthly.cassettes at least once monthly. Cassettes should be numbered so that Cassettes should be numbered so that

problems can be easily identified.problems can be easily identified.• Intensifying screens are already numbered, but Intensifying screens are already numbered, but

can add number onto intensifying screen. can add number onto intensifying screen.

The CassetteThe Cassette Color coding is sometimes used to Color coding is sometimes used to

identify type of screen inside. identify type of screen inside. Front of cassette may be divided into Front of cassette may be divided into

four quadrants.four quadrants.• Shield other quadrants so that multiple Shield other quadrants so that multiple

views can be taken per film.views can be taken per film. Lead blocker (approximately 3x7 cm) Lead blocker (approximately 3x7 cm)

is found in corner of cassette. is found in corner of cassette.

Lead BlockerLead Blocker

Lead BlockerLead Blocker Present to prevent irradiation of the Present to prevent irradiation of the

part of the film necessary for part of the film necessary for identification. identification.

Careful not to have image in area of Careful not to have image in area of lead blocker. lead blocker.

Intensifying ScreensIntensifying Screens Intensifying screens-Intensifying screens- sheets of sheets of

luminescent phosphor crystals bound luminescent phosphor crystals bound together and mounted on a together and mounted on a cardboard or plastic base.cardboard or plastic base.

Two screens are normally in an x-ray Two screens are normally in an x-ray cassette and sandwich the film.cassette and sandwich the film.• Film has a coating of light sensitive Film has a coating of light sensitive

emulsion on both sides (Double emulsion on both sides (Double emulsion).emulsion).

Intensifying ScreensIntensifying Screens When phosphor crystals in screen are When phosphor crystals in screen are

hit by x-radiation, the crystals hit by x-radiation, the crystals fluoresce, and x-rays are converted fluoresce, and x-rays are converted to visible light. to visible light.

Visible light is what exposes the film. Visible light is what exposes the film. Primary purpose of screen is to Primary purpose of screen is to

reduce the amount of radiation reduce the amount of radiation exposure required to produce a exposure required to produce a diagnostic radiograph. diagnostic radiograph.

Intensifying ScreensIntensifying Screens Reduces the amount of mAs needed, Reduces the amount of mAs needed,

which in turn decreases the dose of which in turn decreases the dose of radiation to the patient and the chance of radiation to the patient and the chance of motion on the radiograph. motion on the radiograph.

Three properties determine efficiency of Three properties determine efficiency of screen materials.screen materials.• 1. They must have a high level of x-ray 1. They must have a high level of x-ray

absorption.absorption.• 2. They must have high x-ray to light 2. They must have high x-ray to light

conversion with suitable energy and color.conversion with suitable energy and color.• 3. There must be little or no “afterglow” once 3. There must be little or no “afterglow” once

radiation has ceased. radiation has ceased.

Screen ConstructionScreen Construction

Has four layersHas four layers• A base or supportA base or support• A reflective layerA reflective layer• A phosphor crystal layerA phosphor crystal layer• A protective coatA protective coat

BaseBase Serves as flexible support to attach Serves as flexible support to attach

the phosphor layer to the cassette. the phosphor layer to the cassette.

Base must have tough, moisture-Base must have tough, moisture-resistant surface and not become resistant surface and not become brittle with extended use.brittle with extended use.

The Reflective LayerThe Reflective Layer Attached to the base.Attached to the base. Made of a white substance such as Made of a white substance such as

titanium dioxide.titanium dioxide. Purpose is to reflect light emitted by Purpose is to reflect light emitted by

phosphor layer back toward the x-ray phosphor layer back toward the x-ray film.film.

Increases the efficiency of the screen Increases the efficiency of the screen so that no light is lost through the so that no light is lost through the base. base.

Phosphor Crystal LayerPhosphor Crystal Layer Consists of uniformly distributed Consists of uniformly distributed

phosphor crystals held in place with phosphor crystals held in place with binder material.binder material.

Layer can not change in thickness, Layer can not change in thickness, crack or discolor with age as any crack or discolor with age as any variance will alter amount of light variance will alter amount of light produced and alter exposure of film.produced and alter exposure of film.

Phosphor typesPhosphor types Phosphor chosen must absorb x-rays Phosphor chosen must absorb x-rays

efficiently and have minimum afterglow efficiently and have minimum afterglow and emit sufficient light of the desired and emit sufficient light of the desired color. color.

Afterglow-Afterglow- tendency of phosphor to still tendency of phosphor to still give off light after x-radiation has stopped. give off light after x-radiation has stopped.

Absorption rate of phosphor refers to the Absorption rate of phosphor refers to the extraction of x-ray photons from the extraction of x-ray photons from the beam. The more x-ray quanta absorbed, beam. The more x-ray quanta absorbed, the greater the amount of light produced. the greater the amount of light produced.

Protective CoatProtective Coat Is a clear coating placed on the outer Is a clear coating placed on the outer

surface of the screen.surface of the screen. Provides protection to the phosphor Provides protection to the phosphor

layer.layer. Must be strong enough to resist Must be strong enough to resist

marks and abrasions and must be marks and abrasions and must be easy to clean.easy to clean.

Screen SpeedScreen Speed General Rules:General Rules:

• Large crystal: Faster screens-less detail-Large crystal: Faster screens-less detail-high grain. high grain.

• Small crystals: Slower screens-more Small crystals: Slower screens-more detail- low grain.detail- low grain.

Screen SpeedsScreen Speeds Calcium tungstate-Calcium tungstate- high x-ray absorption high x-ray absorption

ability and physically strong but lacks in ability and physically strong but lacks in light conversion ability.light conversion ability.

Rare-earth elements-Rare-earth elements- higher x-ray to light higher x-ray to light conversion ability.conversion ability.

Other factors can affect screen speed Other factors can affect screen speed aside from phosphor type.aside from phosphor type.• Governed by crystal size, phosphor layer Governed by crystal size, phosphor layer

thickness, reflective layer efficiency, and dyes thickness, reflective layer efficiency, and dyes in the phosphor layer. in the phosphor layer.

Crystal SizeCrystal Size Larger the crystal, greater the light Larger the crystal, greater the light

emission.emission.

X-ray strikes crystal, entire crystal X-ray strikes crystal, entire crystal starts to fluoresce.starts to fluoresce.

Thus need less x-radiation.Thus need less x-radiation.• However detail of image decreases as However detail of image decreases as

crystal size increases. crystal size increases.

Phosphor Layer ThicknessPhosphor Layer Thickness When thickness is increased, x-ray When thickness is increased, x-ray

absorption and light emission are absorption and light emission are increased.increased.

Increase in thickness results in Increase in thickness results in decrease of image detail.decrease of image detail.

Reflective Layer EfficiencyReflective Layer Efficiency Is positioned between base and Is positioned between base and

phosphor layer.phosphor layer. Purpose is to reflect all light emission Purpose is to reflect all light emission

from phosphor layer toward x-ray from phosphor layer toward x-ray film. film.

Needs to be high in reflective ability Needs to be high in reflective ability in order to decrease x-radiation in order to decrease x-radiation needed. needed.

Dyes in the Phosphor LayerDyes in the Phosphor Layer Light absorbing dye may be Light absorbing dye may be

incorporated into the binder material incorporated into the binder material of the phosphor layer in some of the phosphor layer in some screens.screens.

Dye is to decrease lateral spreading Dye is to decrease lateral spreading of the light emitted from the of the light emitted from the phosphor crystals.phosphor crystals.

May reduce light emitted and speed May reduce light emitted and speed of screen.of screen.

Screen Speed RatingsScreen Speed Ratings Many screens to choose from.Many screens to choose from. Screens divided into three Screens divided into three

categories:categories:• SlowSlow• MediumMedium• FastFast

Slow ScreensSlow Screens Also called high definition, ultra-Also called high definition, ultra-

detail, or fine grain.detail, or fine grain.

Specifically designed from Specifically designed from radiographic examinations that radiographic examinations that require optimal detail and in which require optimal detail and in which exposure time is not critical.exposure time is not critical.

Medium ScreensMedium Screens Also referred to as Regular, Also referred to as Regular,

midspeed, normal or par speed.midspeed, normal or par speed.

Most common in private veterinary Most common in private veterinary practice. practice.

Provide good resolution with low Provide good resolution with low exposures.exposures.

Fast ScreensFast Screens Also referred to as High Speed.Also referred to as High Speed.

Reduces exposure time to patient.Reduces exposure time to patient.

Able to penetrate thick tissue where Able to penetrate thick tissue where more exposure is necessary.more exposure is necessary.

Less detail.Less detail.

Screen SpeedsScreen Speeds May depend on manufacturerMay depend on manufacturer Quantum Mottle-Quantum Mottle- an artifact of faster an artifact of faster

screens that results in density screens that results in density variations due to random spatial variations due to random spatial distribution of the phosphor crystals distribution of the phosphor crystals within the screen.within the screen.• Gives radiographs a spotted or mottled Gives radiographs a spotted or mottled

appearance. appearance.

Intensifying Screens ContinuedIntensifying Screens Continued Usually mounted in pairs in the Usually mounted in pairs in the

cassette.cassette.

Labeled front and back and should Labeled front and back and should be placed accordingly.be placed accordingly.

Should be firmly mounted in Should be firmly mounted in cassette.cassette.

Screen CareScreen Care Should be inspected and cleaned on Should be inspected and cleaned on

regular basis.regular basis.

Try to keep free from dust and Try to keep free from dust and animal hair.animal hair.

After cleaning, should be left open to After cleaning, should be left open to completely dry before reloading. completely dry before reloading.

X-ray FilmX-ray Film Purpose is to provide a permanent record Purpose is to provide a permanent record

containing essential diagnostic informationcontaining essential diagnostic information Consists of polyester base coated on both Consists of polyester base coated on both

sides with light-sensitive emulsion sides with light-sensitive emulsion containing silver halide crystals.containing silver halide crystals.

When visible light or x-rays interact with When visible light or x-rays interact with silver halide crystals, latent image is silver halide crystals, latent image is formed. Through processing, this is formed. Through processing, this is converted to a visible image.converted to a visible image.

Film CompositionFilm Composition Layers include:Layers include:

• Base-Base- flexible support with adhesive flexible support with adhesive coating on each side.coating on each side.

• Emulsion- Emulsion- next to base. Composed of next to base. Composed of gelatin containing silver halide crystals.gelatin containing silver halide crystals.

• Supercoat-Supercoat- protective material to protective material to decrease the possibility of damage to decrease the possibility of damage to emulsion.emulsion.

Latent imageLatent image An invisible image on the x-ray film An invisible image on the x-ray film

after it is exposed to ionizing after it is exposed to ionizing radiation or visible light before radiation or visible light before processing. processing.

After processing is converted to a After processing is converted to a visible image.visible image.

Silver hallide crystals converted to Silver hallide crystals converted to metallic silver that are exposed. metallic silver that are exposed.

If not exposed, will be clear.If not exposed, will be clear.

Film TypesFilm Types Screen FilmScreen Film

• Manufactured with silver crystals that Manufactured with silver crystals that are sensitive to fluorescent light emitted are sensitive to fluorescent light emitted from intensifying screens and less from intensifying screens and less sensitive to ionizing radiation. sensitive to ionizing radiation.

• Requires less exposure to x-rays to Requires less exposure to x-rays to produce quality image.produce quality image.

Film Type ContinuedFilm Type Continued Nonscreen FilmNonscreen Film

• Exposed by the direct action of x-Exposed by the direct action of x-radiation.radiation.

• More sensitive to ionizing radiation.More sensitive to ionizing radiation.• Requires greater exposure because Requires greater exposure because

there is no intensifying screens.there is no intensifying screens.• No real protective layer.No real protective layer.

Film SpeedFilm Speed Speeds are determined from Speeds are determined from

exposures required to produce an exposures required to produce an image with adequate density.image with adequate density.• Film latitude- The exposure range of a Film latitude- The exposure range of a

film that will produce acceptable film that will produce acceptable densities.densities.

Film Speed TypesFilm Speed Types Fast Film (Ultraspeed)Fast Film (Ultraspeed)

• Larger silver halide crystalsLarger silver halide crystals• Requires less exposure to x-rays or Requires less exposure to x-rays or

fluorescent light from intensifying fluorescent light from intensifying screens.screens.

• Produces grainier image that lacks Produces grainier image that lacks definition.definition.

• Less latitude in exposure factors and Less latitude in exposure factors and processing. processing.

Film SpeedsFilm Speeds Medium Film (Standard or Pat speed)Medium Film (Standard or Pat speed)

• Most widely used.Most widely used.• Compromises between fine grain and Compromises between fine grain and

speed.speed.• Has medium latitude.Has medium latitude.• Suitable for a wide range of Suitable for a wide range of

examinations.examinations.• Standard by which manufacturers rate Standard by which manufacturers rate

other films. other films.

Film SpeedsFilm Speeds Slow FilmSlow Film

• Smaller halide crystalsSmaller halide crystals• Requires greater exposure by x-rays or Requires greater exposure by x-rays or

fluorescent light from intensifying fluorescent light from intensifying screens.screens.

• Less grainy and more defined image.Less grainy and more defined image.• Greater latitude.Greater latitude.

Film CareFilm Care Film boxes should be stored on end so film Film boxes should be stored on end so film

is vertical.is vertical.• If stored horizontally too long, emulsions will If stored horizontally too long, emulsions will

blend together between the films.blend together between the films. Temp should be cool with low humidity. Temp should be cool with low humidity. Should not be stored close to ionizing Should not be stored close to ionizing

radiation, or where vapors can reach it.radiation, or where vapors can reach it. Should be checked regularly for expiration.Should be checked regularly for expiration.