1. Fluoroscopic Image Intensifier Image Production

2. Minification Gain Example Minification Gain = Input screen diameter2
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
Minification Gain = Input screen diameter2
Output screen diameter2
^ is the symbol for squared if using MS Excel
Typical output screen diameter is
1” (2.5cm)
What is the minification gain if the input screen diameter is 12” and the output screen diameter is 1”?
Example
Minification Gain = 122 12
= 144 1
= 144

3. Flux Gain
Measurement of increase in light photons due to conversion efficiency of the output screen
Flux gain deals only with the gain accomplished by the electron to light conversion at the output screen
This 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
If the output phosphor produces 50 light photons for each electron that strikes it, the flux gain would be 50.
Example

4. Total Brightness Gain = Minification gain x Flux 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
What is the total brightness gain from an II tube with a minification gain of 36 and a flux gain of 60?
Example
Total Brightness Gain = 36 x 60
= 2,160

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

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. input screen image cannot be viewed by output screen
Magnification Tubes
These tubes often termed:
Multi-
Dual-
Triple- or
Quad-field intensifiers
Magnification image intensifiers are capable of 1.5 – 4.0 levels of magnification
Resolution can be increased from
~ 4 lp/mm to 6 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 Example
Magnification = Diameter of image intensifier input screen
Diameter of input screen utilized during magnification
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
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. What is the magnification factor?
Example
Magnification = 9” / 6”
= magnification factor

10. Magnification Effects
Minification gain reduced due to fewer photoelectrons being incident at the output phosphor as a result of magnification
(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
Patient Dose Rate

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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