Fluoroscopy: Viewing Systems
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Transcript of Fluoroscopy: Viewing Systems
Fluoroscopy: Viewing SystemsTV Monitors
Video CRT Monitor
A video monitor (television tube) is a cathode ray tube (CRT)
Consists of:
External coilsfor focusing and steering
electron beam
Fluorescent phosphor coating
inside front screen
Anode plated onto front
screen
A vacuum tubeElectron gun
as part of cathode at back of tube
TV Monitor
Video signal
Video signal is amplified and transmitted by cable to the television
monitor
It is transformed back into a visible
image
Cable
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Electron Beam
• Image created as electron gun produces stream or beam of electrons from camera’s video signal onto TV screen’s 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
• 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
Phosphor crystals
Phosphor crystals emit light when struck by electrons and transmit it as visual image through glass of screen to viewer
Modulation of Signal
Video signal received by picture tube is modulated
Magnitude of video signal received from the camera tube is directly proportional to light
intensity received by the camera tube(vidicon, plumbicon, or orthicon)
Video signal magnitude
Light intensity received by camera tube
Unlike camera tube, electron beam of CRT monitor varies in amplitude or intensity in
accordance with modulation of video signal
Video signal modultion
Electron beam of CRT monitor
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 inthe fluoro imaging chain resolution
525 line monitor is capable of 1-2 lp/mm
525 line monitor is capable of 1-2 lp/mm 1000-line system doubles
the spatial resolution
1000-line system doubles the spatial resolution
In television-camera tube:
As electron beam reads optical signal, signal is
erased.
TV Camera & TV Monitor Image Quality
Overall image quality is affected by:
Horizontal resolution
Vertical resolution
Contrast Brightness Lag
In television-picture tube:
As electron beam creates television optical signal, it immediately fades (hence
the term fluorescent screen).
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
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
The image will become chalky
white and detail is lost
Image Quality - Brightness
Brightness levels controlled by:
Usually ABC will stabilize image brightness and x-ray exposure factors
• Brightness level can be manually increased but will not improve image quality
• Usually brightness and contrast are adjusted in combination:
Contrast is brought to near maximum
Brightness is adjusted for satisfactory luminance
automatic brightness
control (ABC)
Viewing Conditions
Viewing conditions change with viewing distance allowing the raster pattern to blend from the viewer’s perspective
525-line pattern on 9” monitor has
min. viewing distance
of 37”
525-line pattern on 17” monitor has min.
viewing distance of 70”
High resolution monitors (over 1,000 lines) of the
same sizes may be viewed at closer distances
Viewer can adjust brightness and contrast of image at the monitor itself (not the imaging chain)
Image Quality – Horizontal Resolution
• This number will set and limit the resolving power (capability) of the TV camera• Product of scan lines, frame rate, and frequency rate
Bandwidth or bandpass refers to total number of cycles per second available for display by television camera and monitor
electronics
Image Quality – Horizontal Resolution
• Frequency bandwidth is maximum number of samples per line per unit time
• Increasing bandwidth will allow the camera to sample more often per second
Horizontal resolution is ability to resolve image dots on each scan line
Increased bandwidth = increased horizontal resolution
versus
Image Quality – Vertical Resolution
512 scan lines on target
1024 scan lines on target
Vertical resolving power is ability of a TV system to resolve objects spaced apart in the vertical direction (to resolve horizontal lines)
More lines meansbetter
resolution
Vertical resolution is, essentially, the vertical reproduction of the image as seen from the output phosphor by the pick up tube
Image Quality – Vertical Resolution
Vertical resolution lp/mm = number of horizontal lines across object
2 x diameter of object (mm)
Size of objectSize of object
Diameter of input phosphorDiameter of input phosphor
Vertical resolution varies with:
Vertical Resolution - Kell Factor
Imaged phantom on left side has increased
Kell Factor and consequently better
resolution
Kell Factor = vertical resolution
number of scan lines
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
Image Quality - Lag
Screen lag is an undesirable yet Screen lag is an undesirable yet useful property of vidicon tubesuseful property of vidicon tubes
Fluoro
Tower
Blurring of TV image when fluoro tower is moved rapidly Blurring of TV image when fluoro tower is moved rapidly from one area to anotherfrom one area to another
Lag occurs because it Lag occurs because it takes time for the image takes time for the image to build up and decay on to build up and decay on vidicon target globulesvidicon target globules
Charge Coupled Device (CCD)
• 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
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
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
CCD can then be sampled pixel by
pixel (raster format)
Video signal is emitted in raster scanning pattern by moving stored charges along P&N holes to edge of CCD
where they are discharged into a conductor
Semiconductors store this charge in P&N holes, thus storing
charges in a latent form
Computers can then manipulate the digital image
CCD Spatial Resolution
Systems incorporating a 1024 matrix can produce
images with 10 lp/mm
Spatial resolution of CCD determined by its:
Physical SizePhysical Size
Pixel CountPixel Count
CCD Resolution
Has linear response curve as compared to other image receptors which have
sigmoid shaped curves
Enables imaging with low light levels (less dose) possible with retained
contrast resolution
• Results in higher signal-to-noise (SNR) ratio and better contrast resolution
• Able to use a lower patient dose per image
Higher sensitivity to light (detective quantum efficiency or DQE) and lower
level of electronic noise than TV camera
CCD Advantages
HIGH: Spatial resolution Signal-to-noise ratio (SNR) Detective quantum efficiency (DQE)
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
NO: Warm up time required Spatial distortion Maintenance
What’s 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