1. Fluoroscopic Image Intensifier Image production
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
PPT created by: Jed Miles, BSRS, RT(R), CRT-CA

2. Minification Gain
Occurs as a result of the same number of electrons produced at the large input screen being compressed into an area of the smaller output screen
Typical output screen diameter is 1” (2.5cm)
Calculated as follows:
Minification gain = input screen diameter2
output screen diameter2
Example: What is the minification gain if the input screen diameter = 12” and the output screen diameter = 1”
= 122 / 12
= 144 / 1
= 144
^ is the symbol for squared if using MS Excel

3. Flux Gain
Measurement of increase in light photons due to conversion efficiency of the output screen
Example: if the output phosphor produces 50 light photons for each electron that strikes it, the flux gain would be 50
Flux gain deals only with the gain accomplished by the electron to light conversion at the output screen
Will cause a decrease in image quality exactly like an intensifying screen causing decreased resolution in diagnostic images due to the penumbral effect of individual phosphor crystals

4. Total Brightness Gain
Measurement of the multiplication of the light photons at the output phosphor compared to the incident radiation at the input phosphor
Total brightness gain = Minification gain x Flux gain
Example: what is the total brightness gain from an II tube with a minification gain of 36 and a flux gain of 60?
brightness gain = minification gain x flux gain
= 36 x 60
= 2,160

5. Another Brightness Gain Measure
Ratio of light intensity of output phosphor to input exposure rate
Conversion factor is calculated as follows:
Conversion factor = intensity of output phosphor / mR/sec
Candelas (cd): measurement of output phosphor light intensity - which is a unit of luminous intensity
Therefore: Conversion factor = cd/m or (cd/m2 / mR/sec)
mR/sec
Typical values of modern image intensifier systems are
cd/m2
This represents a gain of 8,000 – 25,000 times
80-250

6. Brightness Gain Deterioration
As much as 10% a year due to aging of input and output phosphors
Diagnostic intensifying screens also age in the same manner
Brightness gain can be evaluated by monitoring radiation dose required to obtain diagnostic image using a standardized fluoroscopic phantom
When required dose increases to a significant amount, the image intensifier tube should be replaced

7. Magnification
Increased voltage to electrostatic lenses results in increased electron deflection
Causes focal point to move or shift toward input screen
Results in reduced primary field of view being magnified prior to the striking the output screen
Creates “analog electronic magnification” so to speak
Electron flow of reduced primary field of view being magnified via a focal point shift prior to striking the output screen

8. Magnification Tubes
These tubes often termed multi-, dual-, triple-, or quad-field intensifiers
Magnification image intensifiers are capable of – 4.0 levels of magnification
Resolution can be increased from ~ 4 lp/mm to lp/mm
When operating in magnification mode, electron image created at outer edge or periphery of input screen image cannot be viewed by output screen
Therefore, primary photon beam should be collimated to useful viewing field of intensifier when operating in a magnification mode
Minimal magnification used for most studies

9. Magnification Factor
Image intensification tubes described according to diameter of area of input screen utilized
Magnification is in direct proportion to ratio of input screen to output screen diameters
Example: A 23/15cm dual-focus tube has a 9” (23cm) input screen when operating normally and uses a 6” (15cm) area when operating in a magnified mode
Magnification factor calculated as follows:
Magnification = diameter of image intensifier input screen
diameter of input screen utilized during magnification
= 9”/6”
= 1.5 magnification factor

10. Magnification Effects
As a result of magnification, minification gain reduced due to fewer photoelectrons being incident at the output phosphor
Results in a dimmer image
To maintain brightness, tube mA must be increased to produce more x-rays.
Results in more x-rays per unit area present at input phosphor and consequently increased patient dose
Magnification modes will result in increased:
Spatial resolution
Contrast resolution
Patient dose rate

11. What’s Next?
Please close this PowerPoint presentation, and then click Assignment 1 to continue.