IMAGE QUALITY REVIEW

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IMAGE QUALITY REVIEW
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Intended learning outcome

• The student should learn at the end of this
  lecture how to produce an optimal radiological
  image for diagnosis.

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What affects DENSITYon the radiographic image?
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Factors Affecting mAs Density

• Patient factors
• size of pt., density / pathology of tissue
• kVp
• Collimation
• Distance
• Grids
• Film/Screen Combinations
• Processing

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Influences technique density on image
Patients Body Habitus (size)
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Creating the Image

• Scatter
• Creates fog
• Lowers contrast (more grays)
• Increases as
• kV increases
• Field size increases
• Thickness of part increases

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Effects of collimation on scatter
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• Collimate to area of interest -reduces scatter
  and radiation dose to the patient

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Grids

• A device with lead strips that is placed between
  the patient and the cassette
• Used on larger body parts to reduce the number of
  scattering photons from reaching the image

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Grid is placedbetween patient (behind table or
upright bucky) cassetteIf placed BACWARDS CAN
CAUSE GRID ERRORS
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GRIDS CAN LEAVE LINES ON THE IMAGE
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Recorded Detail

• The degree of sharpness in an objects borders
  and structural details.
• Other names
• -sharpness of detail
• -definition
• -resolution
• -degree of noise

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2 principal characteristics of any image are
Spatial Contrast Resolution

• Spatial resolution
• Resolution is the ability to image two separate
  objects and visually distinguish one from the
  other
• Spatial resolution is the ability to image small
  objects that have high subject contrast (eg.
  bone-soft tissue interface, calcified lung
  nodules)

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2 principal characteristics of any image are
Spatial Contrast Resolution

• Spatial resolution
• Determined by focal-spot size and other factors
  that contribute to blur
• Diagnostic x-ray has excellent spatial
  resolution. It is measured in line pairs per mm.
  (CT measured in cm)

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Factors that affect the detail of an image
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Main Factors Affecting Recorded Detail

• kVp mAs
• Motion
• Object Unsharpness
• Focal Spot Size
• SID (Source to Image Distance)
• OID (Object to Image Distance)
• Material Unsharpness

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GEOMETRIC QUALITIES

• DETAIL
• DISTORTION
• MAGNIFICATION

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• POOR
  
• DETAIL
• GOOD DETAIL

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Motion

• Can be voluntary or involuntary
• Best controlled by short exposure times
• Use of careful instructions to the pt.
• Suspension of pt. respiration
• Immobilization devices

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Decrease Motion Unsharpness

• Instruct patient not to move or breath
• Use Immobilization devices
• Use Short exposure times
• Lock equipment in place

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Blurring of image due to patient movement during
exposure.
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SID Source to Image Distance

• The greater the distance between the source of
  the x-ray (tube) and the image receptor
  (cassette), the greater the image sharpness.
• Standard distance 40 in. most exams
• Exception Chest radiography 72 in.

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OIDObject to Image Distance

• The closer the object to the film, the sharper
  the detail.
• OID ?, penumbra ?, sharpness ?
• OID ?, penumbra ?, sharpness ?
• Structures located deep in the body, radiographer
  must know how to position to get the object
  closest to the film.

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Focal spot size determined by filament in
cathode surface area used at anode
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Distortion

• Misrepresentation of the true size or shape of an
  object
• -MAGNIFICATION (size distortion)
• -TRUE DISTORTION (shape distortion)

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MAGNIFICATION

• TUBE CLOSE TO THE PART (SID)
• PART FAR FROM THE CASSETTE (OID)

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40 SID VS 72 SID
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Size Distortion OID

• If source is kept constant, OID will affect
  magnification
• As OID ?, magnification ?
• The farther the object is from the film, the more
  magnification

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A goodB C shape distortion
(elongation of part)
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D E shape distortion (foreshortening of
part)
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Shape Distortion

• Misrepresentation of the shape of an object
• Controlled by alignment of the beam, part
  (object), image receptor
• Influences Central ray angulation body part
  rotation