Transcript L6 Optimizing the Image Ch. 7

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OPTIMIZING THE IMAGE
Lavin: Chapter 7
CTVT: pp. 539-541
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Blue vs Green Technology
• Blue light technology
• Only type until 1981
• Screens emit ultraviolet blue light
• Require blue receiving film
• Being phased out
• Green light technology
• Since 1981
• Superior imaging system
• Emits green or blue-green light
• Require green receiving film
If a problem with an image, check the film/screen combination…
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Radiographic Record-keeping
A radiographic log should always be completed following a radiograph.
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Learning Objectives: Chapter 7
• Define motion, distortion, magnification,
and blur and understand the ways they
can impact a radiographic image.
• Understand the purpose of filters
• Define collimation, and understand its
impact on secondary/scatter radiation
• Know how and when to use grids
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Factors Impacting Radiographic Quality
• Distortion - Misrepresentation of the anatomical part
by size or shape
• Magnification
– A form of distortion that magnifies size
• Motion – A lack of image sharpness caused by overall
patient movement
• Blur - Movement of a particular part that reduces the
visibility of details such as small objects and structures
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Distortion
• Misrepresentation of the anatomical part by size or shape
• Occurs when the x-ray beam is not perpendicular to the
cassette
• Can be shape, size, elongation, or foreshortening
• Not desired
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Distortion (Cont.)
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Magnification
• A form of distortion
• Magnifies size with area of
interest closer than normal to
the source
• Position of the structure in the
body influences
• Minimizing magnification is
“normally” desired.
• Farther away from receptor =
the more magnified
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Magnification
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Magnification
SID is…?
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Motion
• Not a form of distortion but an
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enemy of resolution
Overall patient movement =
lack of image sharpness
Voluntary or involuntary
Controlled with short exposure
times
Immobilization devices
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Blur
• Movement of a part that reduces the visibility of details
• Can be used to advantage by timing a radiograph for a
point where something is moving and blurred, allowing a
clear picture of a stationary structure behind it
• Types:
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Involuntary - cardiac, peristalsis, spasm, tremor
Voluntary - patient movement
Equipment – rotor activation, x-ray tube lock failure,
cassette movement
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Motion vs Blur
• Motion:
• Blur:
• Any unwanted movement
• May be intentional, as in
or restlessness of the
patient during an x-ray
exposure
• May be prevented by
immobilization devices,
sedation, or effective
restraint
computerized tomography
• Uses patient movement
(like breathing) to blur
body parts outside the
level of interest so the
intended anatomy images
clearly
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Filtration
• Some emitted radiation is not useful
• Aluminum filtration - Placed in the
beam to absorb lower-energy
“extra” x-ray photons before they
reach the target
• Produces a clearer image
• Can be:
• Inherent - Filters the beam before it
leaves the tube
• Added – Inserted outside the x-ray tube
but within the beam to even out
discrepancies between body part sizes
Filtration
Inherent filtration
• Part of the tube
• Includes the envelope, the oil in the tube,
and the collimation mirror
Added filtration:
• Put into place by the installer to meet
governmental regulations
Special filters:
• Added to the primary beam to enhance
imaging
• Can use saline or rice bags to absorb
scatter
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Wedge Filter
• A type of special filter
• Also called a
compensating filter
• Evens out the difference
in tissue measurement
• Attached to the collimator
with the thickest end at
the thinnest part of the
patient
Collimation
• Focuses the x-ray beam
to reduce scattered and
extrafocal radiation
• Produces a light beam
that shows the area to be
irradiated during the
exposure
• Decreased collimation =
field size increases
Brown: Lavin’s Radiography for Veterinary Technicians
Copyright © 2014, 2007, 2003, 1999, 1994 by Saunders, an imprint of Elsevier Inc.
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Collimation
• Final filtration of the x-ray
beam
• Defines the area of interest
• Knobs elongate or close the
shutters
• Collimate before placing the
animal on the table
• Reduces scattered radiation
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Grids & Grid Trays
• Previously called Potter-Bucky diaphragm
• Controls scatter radiation before it reaches the cassette
• Allows only the primary x-ray beam to pass through
• Made of lead strips interfaced with radiolucent spacers
• Placed directly under the table between the animal and
cassette
• Grid tray holds cassette in place
• Can deteriorate over time
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Grids (cont.)
• Identified by “grid ratio”
• Too low = don’t filter
• Too high = eliminates some of useful beam
• Most useful is 8:1 or 10:1
• Absorbs some primary radiation, so greater exposure times
needed
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Parallel vs Focused Grid
Parallel
Focused
Focused grids allow more primary radiation to reach the film.
Grid Deterioration
• Over time, projects an image of itself on every radiograph
• Can appear as gridlines
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Decreasing Scatter Radiation
• Decreasing exposure to
scattered radiation:
• Careful collimation
• Setting accurate technical
factors
• Reducing retakes
• Decreasing scatter
radiation impacts on the
radiograph:
• Beam-limiting devices
• Correct kVp settings
• Filtering
• Grids
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Illumination of the Radiograph
• Illuminators:
• Used to transmit light evenly through
a specially produced glass in order
to identify and interpret radiographic
images
• Must be maintained at optimal
brightness
• Glass should be cleaned regularly
• Should be positioned in a room that
can be dimmed