Fluoroscopy: Viewing Systems

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Fluoroscopy Viewing Systems
TV Monitors
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Video CRT Monitor
A video monitor (television tube) is a cathode
ray tube (CRT)
Consists of
TV Monitor
A vacuum tube
Electron gun as part of cathode at back of tube
Cable
External coils for focusing and steering electron
beam
Fluorescent phosphor coating inside front screen
Anode plated onto front screen
Video signal
Video signal is amplified and transmitted by
cable to the television monitor
It is transformed back into a visible image
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Electron Beam

• Image created as electron gun produces stream or
  beam of electrons from cameras video signal onto
  TV screens phosphor
• Intensity of electron beam modulated by control
  grid attached to electron gun
• Electron beam focused onto output fluorescent
  screen by external electrostatic coils
• Scans across the output screen using the exact
  same raster pattern as the camera tube

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Phosphor Crystals
Phosphor crystals emit light when struck by
electrons and transmit it as visual image
through glass of screen to viewer
Phosphor crystals

• Composed of linear crystals aligned perpendicular
  to glass envelope to reduce lateral light
  dispersion
• Phosphor layer usually backed by thin layer of
  aluminum, which transmits electron beam but
  reflects light

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Modulation of Signal
Video signal received by picture tube is modulated
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Monitor Image Quality
Monitor quality is affected by the number of scan
lines and the bandpass of the TV camera system.
Raster pattern from camera is presented on the
monitor.
Video monitor is the most restrictive element
in the fluoro imaging chain resolution
525 line monitor is capable of 1-2 lp/mm
1000-line system doubles the spatial resolution
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TV Camera TV Monitor Image Quality
Overall image quality is affected by
Horizontal resolution
Vertical resolution
Contrast
Brightness
Lag
In television-camera tube As electron beam
reads optical signal, signal is erased.
In television-picture tube As electron beam
creates television optical signal, it immediately
fades (hence the term fluorescent screen).
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Image Quality - Contrast
Contrast levels of a TV monitor can be adjusted
on the monitor itself
Contrast should be set as follows
Darkest object in the scene just below black
level on the monitor
Brightest objects of interest do not completely
saturate or white out details of the image
It is appropriate to adjust contrast and
brightness control to maximize the visibility of
object even at the expense of increased noise
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Image Quality - Brightness
Changes in brightness will affect image quality
When the fluoroscope is moved from the abdomen to
the chest, a sudden surge of brightness will
flood the system
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Image Quality - Brightness

• Brightness level can be manually increased
• but will not improve image quality
• Usually brightness and contrast are adjusted
• in combination

Brightness levels controlled by
automatic brightness control (ABC)
Contrast is brought to near maximum
Usually ABC will stabilize image brightness and
x-ray exposure factors
Brightness is adjusted for satisfactory luminance
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Viewing Conditions
Viewing conditions change with viewing distance
allowing the raster pattern to blend from the
viewers perspective
525-line pattern on 9 monitor has min. viewing
distance of 37
High resolution monitors (over 1,000 lines) of
the same sizes may be viewed at closer distances
525-line pattern on 17 monitor has min. viewing
distance of 70
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Image Quality Horizontal Resolution
Bandwidth or bandpass refers to total number of
cycles per second available for display by
television camera and monitor electronics

• This number will set and limit the resolving
  power (capability) of the TV camera
• Product of scan lines, frame rate, and frequency
  rate

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Image Quality Horizontal Resolution
Horizontal resolution is ability to resolve image
dots on each scan line

• Frequency bandwidth is maximum number of samples
  per line per unit time
• Increasing bandwidth will allow the camera to
  sample more often per second

versus
Increased bandwidth increased horizontal
resolution
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Image Quality Vertical Resolution
Vertical resolving power is ability of a TV
system to resolve objects spaced apart in the
vertical direction (to resolve horizontal lines)
512 scan lines on target
1024 scan lines on target
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Image Quality Vertical Resolution
Vertical resolution varies with
Size of object
Diameter of input phosphor
Vertical resolution is, essentially, the vertical
reproduction of the image as seen from the
output phosphor by the pick up tube
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Vertical Resolution - Kell Factor
The Kell Factor is the ratio between actual
vertical resolution of TV monitor (as specified
in TV lines) and the number of horizontal scan
lines
A component of vertical resolution
Imaged phantom on left side has increased Kell
Factor and consequently better resolution
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Image Quality - Lag

• Occurs because it takes certain amount of time
  for the image to build up and decay on vidicon
  target globules
• Visible lag not caused by image intensifier

Screen lag is an undesirable yet useful property
of vidicon tubes
Blurring of TV image when fluoro tower is moved
rapidly from one area to another
Fluoro Tower
Lag occurs because it takes time for the image to
build up and decay on vidicon target globules
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Charge Coupled Device (CCD)
CCD is a semiconducting device capable of storing
a charge from light photons striking a
photosensitive surface
Sensitive component is a layer of crystalline
silicone

• Mounted at output phosphor of image intensifier
  tube and coupled by fiber optics or lens system
• Early 1980s first CCD replaced the TV camera in
  a video system

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CCD How it works
?Light strikes the crystalline silicone
(photoelectric cathode) of the CCD
?Electrons are released proportionally to the
intensity of the incident light
?Silicon is illuminated and an electrical charge
is generated
?Video signal is emitted in raster scanning
pattern by moving stored charges along PN holes
to edge of CCD where they are discharged into a
conductor
?CCD can then be sampled pixel by pixel (raster
format)
?Semiconductors store this charge in PN holes,
thus storing charges in a latent form
?Computers can then manipulate the digital image
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CCD Spatial Resolution
Spatial resolution of CCD determined by its
Physical Size
Pixel Count
Systems incorporating a 1024 matrix can produce
images with 10 lp/mm
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CCD Resolution
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CCD Advantages

• Extremely fast discharge time (no image lag or
  blooming)
• Useful for high speed imaging applications
• Unlimited life span
• Unaffected by magnetic fields
• Linear response
• Lower dose rates needed

• HIGH
• Spatial resolution
• Signal-to-noise ratio (SNR)
• Detective quantum efficiency (DQE)

• NO
• Warm up time required
• Spatial distortion
• Maintenance

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Whats Next?

• Please close this PowerPoint presentation, and
  continue the lesson.

Presented by
Based on Principles of Radiographic Imaging, 4th
Ed. By R. Carlton A. Adler Radiologic
Science for Technologists, 8th Ed. By S. Bushong
Syllabus on Fluoroscopy Radiation Protection,
6th Rev. By Radiologic Health Branch
Certification Unit