Chapter 6 Image Receptors

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Chapter 6Image Receptors

• Radiology

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Introduction

• To further understand radiography, one must know
  how a permanent record is produced using x-rays.
• Radiograph is formed with light-sensitive film
  contained in a light-proof encasement.
• Encasement in radiography is called a cassette.

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The Cassette

• Designed to hold a piece of x-ray film
  sandwiched between two fluorescent sheets of
  plastic called INTENSIFYING SCREENS.
• Intensifying screens are responsible for
  converting the x-ray radiation into visible
  light, which creates a latent image on the x-ray
  film.
• Film is then processed to convert latent image to
  visible image.
• Only 5 of exposure results from ionization of
  x-rays, rest is due to light emitted from
  intensifying screens.

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The Cassette

• Cassette- A rigid film holder designed to hold
  x-ray film and intensifying screens in close
  contact.
• Constructed with light tight materials to prevent
  unwanted exposure, but yet still allow
  penetration of x-rays.
• Originally cassettes were made with cardboard,
  but could not be reused.
• Different forms today include aluminum,
  polycarbonate, magnesium, and carbon fiber.

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The Cassette

• Attached to the back with hinges and catches.
• Forms light tight seal.
• Back of cassette is constructed with heavier
  material than the front because is lined with
  material to absorb backscatter.
• Sides are lined with felt or foam pressure pads
  that ensure close contact of the film to the
  intensifying screens.

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The Cassette

• Sizes vary.
• Correspond to screen and film sizes.
• Cost varies as well.

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Cassette Care

• Should always be handled with care.
• Can be exposed to large amounts of physical
  abuse.
• Common damage includes
• Dropping the cassette.
• Leakage of fluid on the cassette.
• May want to place in plastic bag when a messy
  situation occurs.

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Cassette Care continued

• Should be cleaned on regular basis with mild soap
  and water.
• Should clean exterior and interior of cassettes
  at least once monthly.
• Cassettes should be numbered so that problems can
  be easily identified.
• Intensifying screens are already numbered, but
  can add number onto intensifying screen.

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The Cassette

• Color coding is sometimes used to identify type
  of screen inside.
• Front of cassette may be divided into four
  quadrants.
• Shield other quadrants so that multiple views can
  be taken per film.
• Lead blocker (approximately 3x7 cm) is found in
  corner of cassette.

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Lead Blocker
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Lead Blocker

• Present to prevent irradiation of the part of the
  film necessary for identification.
• Careful not to have image in area of lead
  blocker.

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Intensifying Screens

• Intensifying screens- sheets of luminescent
  phosphor crystals bound together and mounted on a
  cardboard or plastic base.
• Two screens are normally in an x-ray cassette and
  sandwich the film.
• Film has a coating of light sensitive emulsion on
  both sides (Double emulsion).

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Intensifying Screens

• When phosphor crystals in screen are hit by
  x-radiation, the crystals fluoresce, and x-rays
  are converted to visible light.
• Visible light is what exposes the film.
• Primary purpose of screen is to reduce the amount
  of radiation exposure required to produce a
  diagnostic radiograph.

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Intensifying Screens

• Reduces the amount of mAs needed, which in turn
  decreases the dose of radiation to the patient
  and the chance of motion on the radiograph.
• Three properties determine efficiency of screen
  materials.
• 1. They must have a high level of x-ray
  absorption.
• 2. They must have high x-ray to light conversion
  with suitable energy and color.
• 3. There must be little or no afterglow once
  radiation has ceased.

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Screen Construction

• Has four layers
• A base or support
• A reflective layer
• A phosphor crystal layer
• A protective coat

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Base

• Serves as flexible support to attach the phosphor
  layer to the cassette.
• Base must have tough, moisture-resistant surface
  and not become brittle with extended use.

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The Reflective Layer

• Attached to the base.
• Made of a white substance such as titanium
  dioxide.
• Purpose is to reflect light emitted by phosphor
  layer back toward the x-ray film.
• Increases the efficiency of the screen so that no
  light is lost through the base.

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Phosphor Crystal Layer

• Consists of uniformly distributed phosphor
  crystals held in place with binder material.
• Layer can not change in thickness, crack or
  discolor with age as any variance will alter
  amount of light produced and alter exposure of
  film.

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Phosphor types

• Phosphor chosen must absorb x-rays efficiently
  and have minimum afterglow and emit sufficient
  light of the desired color.
• Afterglow- tendency of phosphor to still give off
  light after x-radiation has stopped.
• Absorption rate of phosphor refers to the
  extraction of x-ray photons from the beam. The
  more x-ray quanta absorbed, the greater the
  amount of light produced.

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Protective Coat

• Is a clear coating placed on the outer surface of
  the screen.
• Provides protection to the phosphor layer.
• Must be strong enough to resist marks and
  abrasions and must be easy to clean.

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Screen Speed

• General Rules
• Large crystal Faster screens-less detail-high
  grain.
• Small crystals Slower screens-more detail- low
  grain.

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Screen Speeds

• Calcium tungstate- high x-ray absorption ability
  and physically strong but lacks in light
  conversion ability.
• Rare-earth elements- higher x-ray to light
  conversion ability.
• Other factors can affect screen speed aside from
  phosphor type.
• Governed by crystal size, phosphor layer
  thickness, reflective layer efficiency, and dyes
  in the phosphor layer.

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Crystal Size

• Larger the crystal, greater the light emission.
• X-ray strikes crystal, entire crystal starts to
  fluoresce.
• Thus need less x-radiation.
• However detail of image decreases as crystal size
  increases.

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Phosphor Layer Thickness

• When thickness is increased, x-ray absorption and
  light emission are increased.
• Increase in thickness results in decrease of
  image detail.

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Reflective Layer Efficiency

• Is positioned between base and phosphor layer.
• Purpose is to reflect all light emission from
  phosphor layer toward x-ray film.
• Needs to be high in reflective ability in order
  to decrease x-radiation needed.

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Dyes in the Phosphor Layer

• Light absorbing dye may be incorporated into the
  binder material of the phosphor layer in some
  screens.
• Dye is to decrease lateral spreading of the light
  emitted from the phosphor crystals.
• May reduce light emitted and speed of screen.

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Screen Speed Ratings

• Many screens to choose from.
• Screens divided into three categories
• Slow
• Medium
• Fast

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Slow Screens

• Also called high definition, ultra-detail, or
  fine grain.
• Specifically designed from radiographic
  examinations that require optimal detail and in
  which exposure time is not critical.

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Medium Screens

• Also referred to as Regular, midspeed, normal or
  par speed.
• Most common in private veterinary practice.
• Provide good resolution with low exposures.

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Fast Screens

• Also referred to as High Speed.
• Reduces exposure time to patient.
• Able to penetrate thick tissue where more
  exposure is necessary.
• Less detail.

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Screen Speeds

• May depend on manufacturer
• Quantum Mottle- an artifact of faster screens
  that results in density variations due to random
  spatial distribution of the phosphor crystals
  within the screen.
• Gives radiographs a spotted or mottled
  appearance.

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Intensifying Screens Continued

• Usually mounted in pairs in the cassette.
• Labeled front and back and should be placed
  accordingly.
• Should be firmly mounted in cassette.

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Screen Care

• Should be inspected and cleaned on regular basis.
• Try to keep free from dust and animal hair.
• After cleaning, should be left open to completely
  dry before reloading.

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X-ray Film

• Purpose is to provide a permanent record
  containing essential diagnostic information
• Consists of polyester base coated on both sides
  with light-sensitive emulsion containing silver
  halide crystals.
• When visible light or x-rays interact with silver
  halide crystals, latent image is formed. Through
  processing, this is converted to a visible image.

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Film Composition

• Layers include
• Base- flexible support with adhesive coating on
  each side.
• Emulsion- next to base. Composed of gelatin
  containing silver halide crystals.
• Supercoat- protective material to decrease the
  possibility of damage to emulsion.

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Latent image

• An invisible image on the x-ray film after it is
  exposed to ionizing radiation or visible light
  before processing.
• After processing is converted to a visible image.
• Silver hallide crystals converted to metallic
  silver that are exposed.
• If not exposed, will be clear.

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Film Types

• Screen Film
• Manufactured with silver crystals that are
  sensitive to fluorescent light emitted from
  intensifying screens and less sensitive to
  ionizing radiation.
• Requires less exposure to x-rays to produce
  quality image.

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Film Type Continued

• Nonscreen Film
• Exposed by the direct action of x-radiation.
• More sensitive to ionizing radiation.
• Requires greater exposure because there is no
  intensifying screens.
• No real protective layer.

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Film Speed

• Speeds are determined from exposures required to
  produce an image with adequate density.
• Film latitude- The exposure range of a film that
  will produce acceptable densities.

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Film Speed Types

• Fast Film (Ultraspeed)
• Larger silver halide crystals
• Requires less exposure to x-rays or fluorescent
  light from intensifying screens.
• Produces grainier image that lacks definition.
• Less latitude in exposure factors and processing.
  

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Film Speeds

• Medium Film (Standard or Pat speed)
• Most widely used.
• Compromises between fine grain and speed.
• Has medium latitude.
• Suitable for a wide range of examinations.
• Standard by which manufacturers rate other films.

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Film Speeds

• Slow Film
• Smaller halide crystals
• Requires greater exposure by x-rays or
  fluorescent light from intensifying screens.
• Less grainy and more defined image.
• Greater latitude.

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Film Care

• Film boxes should be stored on end so film is
  vertical.
• If stored horizontally too long, emulsions will
  blend together between the films.
• Temp should be cool with low humidity.
• Should not be stored close to ionizing radiation,
  or where vapors can reach it.
• Should be checked regularly for expiration.