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 an electron beam
Fluorescent phosphor coating on the inside of the front screen
Anode is 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. Video CRT Monitor – How it Works
An image is created as the electron gun produces a stream or beam of electrons from the camera’s video signal onto the TV screens phosphor
The electron beam follows the same raster pattern used by the video camera
The video signal received by the picture tube is modulated
Modulation means the magnitude of the video signal received from the camera tube is directly proportional to the light intensity received by the camera tube (vidicon, plumbicon, or orthicon)
Thus, unlike the camera tube, the electron beam of the CRT monitor varies in amplitude or intensity in accordance to the modulation of the video signal

4. Video CRT Monitor – How it Works
The intensity of the electron beam is modulated by the control grid, which is attached to the electron gun
The electron beam is focused onto the output fluorescent screen by external electrostatic coils
Electron beam scans across the output screen using the exact same raster pattern as the camera tube
Phosphor crystals emit light when struck by electrons and transmit it as a visual image through the glass of the screen to the viewer
Phosphor crystals are composed of linear crystals aligned perpendicular to the glass envelope to reduce lateral light dispersion
Phosphor layer is usually backed by a thin layer of aluminum, which transmits the electron beam but reflects the light

5. Black and White - Video CRT Monitor

6. Analog TV Monitor

7. Color - Video CRT Monitor
Close up of color CRT Tube

8. Video CRT Monitor – Image Quality
Video CRT Monitors (television) quality is affected by the number of scan lines and the bandpass of the TV camera system
The 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

9. TV Camera & TV Monitor – Image Quality
Overall Image quality is affected by
Horizontal resolution
Vertical resolution
Contrast
Brightness
Lag
In the television-camera tube, as the electron beam reads the optical signal, the signal is erased
In the television-picture tube, as the electron beam creates the television optical signal, it immediately fades
Hence the term ‘fluorescent screen’

10. Image Quality - Contrast
Contrast levels of a TV monitor can be adjusted on the TV monitor itself
Contrast should be set as follows
The darkest object in the scene is just below the black level on the monitor
The brightest objects of interest do not completely saturate or ‘white out’ details of the image
It is appropriate to adjust the contrast and brightness control to maximize the visibility of the object even at the expense of increased noise

11. 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 are controlled by the automatic brightness control (ABC)
Usually the ABC will stabilize the image brightness and the x-ray exposure factors
The 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

12. Video CRT Monitor – Viewing Conditions
Viewing Conditions: viewing conditions change with viewing distance
This allows the raster pattern to blend from the viewers perspective
525 line pattern on a 9” monitor has a minimum viewing distance of 37”
525 line pattern on a 17” monitor has a 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 the brightness and contrast of the image at the monitor itself – not the “imaging chain”

13. Image Quality – Horizontal Resolution
Bandwidth or Bandpass refers to the total number of cycles per second available for display by the television camera and monitor electronics
This number will set and overall limit the resolving power (capability) of the TV camera
It is a product of scan lines, frame rate, and frequency rate
Horizontal resolution is the ability to resolve the image dots on each scan line
Frequency bandwidth is the 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

14. Image Quality – Vertical Resolution
Vertical resolving power is the ability of a TV system to resolve objects spaced apart in the vertical direction (to resolve horizontal lines)
More lines = better resolution
Vertical resolution varies with the size of the object, as well as the diameter dimension of the input phosphor
It is essentially, the vertical reproduction of the image as seen from the output phosphor by the pick up tube
Vertical resolution can be measured by the following formula
Vertical resolution (lp/mm) = number of horizontal lines across the object
2 x diameter of object in millimeters

15. Image Quality – Vertical Resolution
512 scan lines on target
1024 scan lines on target
Vertical resolving power is the ability of a TV system to resolve objects spaced apart in the vertical direction (to resolve horizontal lines)
More lines = better resolution

16. Vertical Resolution - Kell Factor
The Kell factor is a component of vertical resolution
It is defined as the ratio between the vertical resolution and the number of scan lines in the TV system
The ratio between the actual vertical resolution of a TV monitor (as specified in TV lines) and the number of horizontal scan lines is called the Kell factor
Calculated by the following formula
Kell factor = vertical resolution
number of scan lines

17. Vertical Resolution - Kell Factor
Imaged phantom on left side has an increased Kell Factor
and consequently better resolution

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

19. Next up: Image recording systems