1. Fluoroscopy – Viewing Systems TV Monitors
Based on:
Principles of Radiographic Imaging, 3rd 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
PPT created by Jed Miles, BSRS, RT(R), CRT-CA

2. Video CRT Monitor
A video monitor (television tube) is a cathode ray tube (CRT)
Consists of:
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 the front screen
Video signal is amplified and transmitted by cable to the television monitor, where 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
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. 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)
Unlike camera tube, electron beam of CRT monitor varies in amplitude or intensity in accordance with modulation of video signal

6. Scanned Raster Pattern
Commercial and closed circuit television is a series of projected scans that create frames (like a movie)
Electron gun produces electron beam focused on the target to produce a TV picture (frame)
1 frame = 525 scanned lines, each scan line consists of thousands of dots. (looks like: ……………....)
To avoid flicker, each frame divided into two halves or fields
1st half scans even-numbered lines
2nd half scans odd-numbered lines

7. Interlaced Scans
Electron beam scans diagonal lines as active traces while horizontal lines are inactive retraces to position for next active trace
262.5 lines scanned every 1/60 second, entire 525-line raster pattern scanned every 1/30 second
Equals 30 frames per second =15,700 scanned lines per second
Thus, 2 interlaced scans or fields produce 1 complete frame

8. Progressive Scan
Progressive raster pattern scans sequentially from top to bottom (no interlacing)
Provides increased resolution
Analog commercial television uses a 525 horizontal raster line pattern per frame (2 x 262½ fields)
High resolution video systems now offer over 1,000 lines per frame

9. Monitor Image Quality
Monitor quality is affected by the number of scan lines and the bandpass of the TV camera system
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

10. 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

11. 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 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

12. 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
Brightness levels controlled by automatic brightness control (ABC)
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 and brightness is adjusted for satisfactory luminance

13. Viewing Conditions Change with viewing distance
This allows the raster pattern to blend from the viewers perspective
525 line pattern on 9” monitor has minimum viewing distance of 37”
525 line pattern on 17” monitor has minimum 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”

14. 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
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
Example using dots: Versus …………
Increased bandwidth = increased horizontal resolution

15. 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 = better resolution
512 scan lines on target
1024 scan lines on target

16. Image Quality – Vertical Resolution cont.
Varies with size of object, as well as diameter of input phosphor
It is essentially, the vertical reproduction of the image as seen from the output phosphor by the pick up tube
Measured by the following formula
Vertical resolution (lp/mm) = number of horizontal lines across object
2 x diameter of object in millimeters

17. Vertical Resolution - Kell Factor
Component of vertical resolution
Defined as ratio between actual vertical resolution of TV monitor (as specified in TV lines) and number of horizontal scan lines
Kell factor = vertical resolution
number of scan lines
Imaged phantom on left side has increased Kell Factor and consequently better resolution

18. Image Quality - Lag
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
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

19. Charge Coupled Device (CCD)
CCD is a semiconducting device capable of storing a charge from light photons striking a photosensitive surface
Sensitive component of CCD is a layer of crystalline silicone
Mounted at output phosphor of image intensifier tube and coupled by fiber optics or lens system
Early 1980’s – first CCD replaced the TV camera in a video system

20. 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, an electrical charge is generated
Semiconductors store this charge in P & N holes, thus storing charges in a latent form
Video signal 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)
Computers can then manipulate the digital image

21. CCD Spatial Resolution
Spatial resolution of CCD determined by its physical size and pixel count
Systems incorporating a 1024 matrix can produce images with 10 lp/mm

22. CCD Resolution
Higher sensitivity to light (detective quantum efficiency or DQE) and lower level of electronic noise than TV camera
Results in higher signal-to-noise (SNR) ratio and better contrast resolution
Able to use a lower patient dose per image
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

23. CCD Advantages High: No:
Spatial resolution
Signal-to-noise ratio (SNR)
Detective quantum efficiency (DQF)
No:
Warm up time required
Spatial distortion
Maintenance
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

24. What’s Next?
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