1. Fluoroscopy: Viewing Systems
TV Monitors

2. Video CRT Monitor Consists of:
A video monitor (television tube) is a cathode ray tube (CRT)
Consists of:
TV Monitor
Cable
A vacuum tube
Electron gun
as part of cathode at back of tube
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

3. 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

4. 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

5. Video signal received by picture tube is modulated
Modulation of Signal
Video signal received by picture tube is modulated
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 ?
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

6. Monitor Image Quality 525 line monitor is capable of 1-2 lp/mm
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

7. 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).

8. 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

9. Image Quality - Brightness
Changes in brightness will affect image quality
The image will become chalky white and detail is lost
When the fluoroscope is moved from the abdomen to the chest, a sudden surge of brightness will flood the system

10. 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

11. 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”
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”
Viewer can adjust brightness and contrast of image at the monitor itself (not the imaging chain)

12. 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

13. 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

14. 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)
More lines
means
better resolution
512 scan lines on target
1024 scan lines on target

15. 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
Vertical resolution lp/mm = number of horizontal lines across object
2 x diameter of object (mm)

16. 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
Kell Factor = vertical resolution
number of scan lines
A component of vertical resolution
Imaged phantom on left side has increased Kell Factor and consequently better resolution

17. Image Quality - Lag Visible lag not caused by image intensifier
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
Image Quality - Lag
Blurring of TV image when fluoro tower is moved rapidly from one area to another
Screen lag is an undesirable yet useful property of vidicon tubes
Fluoro
Tower
Lag occurs because it takes time for the image to build up and decay on vidicon target globules

18. 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

19. 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 P&N 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 P&N holes, thus storing charges in a latent form
Computers can then manipulate the digital image

20. 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

21. CCD 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
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

22. 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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What’s Next?
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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