1. Resident Physics Lectures
Fluoroscopic Imaging (year 1)
George David
Associate Professor of Radiology
Medical College of Georgia

2. Early Fluoroscopy Radiologist directly viewed fluorescent screen
screen covered with lead glass for protection
Low light levels
10-30 minute dark adaptation required by wearing red goggles

3. Human eye light receptors
rods (scotopic vision)
respond to very low light levels
night vision
peripheral vision
poor visual acuity
poor gray shade detection
cones (photopic vision)
high direct vision acuity
blind at low illumination levels

4. Image Intensifiers
evacuated glass envelope
vacuum tube
          
                              

5. Image Intensifier Input Output x-rays ==> light X-rays
Small viewing screen

6. Image Intensifier Components
input phosphor
x-rays to light
photocathode
light to electrons
electrostatic focusing lens
steer those electrons
accelerating anode
speed up those electrons
output phosphor
electrons to light

7. Input Phosphor Cesium iodide (CsI) x-rays ==> light
CsI crystal needles perpendicular to substrate
minimizes lateral light diffusion or scattering
improves resolution
typical image tube resolution
3 - 5 line pairs / mm

8. Input Phosphor K-Edge of phosphors absorbs ~ 2/3 of incident beam
CS ==> (36 keV)
I ==> 33.2 keV
well suited to average fluoro beam energy
30 to 40 keV
absorbs ~ 2/3 of incident beam
absorption
energy

9. light photons ==> electrons
Photocathode
light photons ==> electrons
attached to input phosphor
minimizes light diffusion
Photo-emissive metal
light causes emission of photoelectrons
# photoelectrons emitted proportional to incident light from input phosphor

10. Electrostatic Focusing Lens
lenses + -
Several electrodes plated to inside glass envelope
+ voltage applied to electrodes
voltages determine magnification mode
focuses each point of input phosphor to a point on output phosphor
inverts & reverses image
Side-side
Top=bottom
Black-white

11. Accelerating Anode in neck of image tube + 25 - 35 kV charge
accelerates electrons
faster electrons produce more light when they strike output phosphor

12. electrons ==> light
Output Phosphor
Small viewable fluorescent screen
inch diameter
converts electron’s kinetic energy to light
~ 50 fold increase in # light photons over input phosphor
Output
Phosphor
electrons ==> light

13. Output Phosphor thin aluminum layer on back of output phosphor
prevents screen’s light from re-entering tube & reaching input phosphor
Output
Phosphor - X
Aluminum

14. Output phosphor viewing
direct
uses lenses & mirrors
television
high quality closed circuit television chain

15. Image Tube Parameters Brightness Gain Conversion Factor
ratio of II brightness to a “standard” screen
Conversion Factor
light output per radiation rate input
Deterioration over time
10% decline in brightness / year typical
must increase patient exposure to get same light intensity

16. II Gain (Intensification Factor)
Output phosphor brightness “standard” screen brightness
typically ~ 10,000

17. II Gain (Intensification Factor)
Brightness gain = minification gain X flux gain
Minification gain
making image smaller also makes it brighter
Flux gain
acceleration of electrons toward output phosphor
Electrons +

18. Minification Minification gain
12” mag mode
9” mag mode
Minification gain
Area of (effective) input phosphor Area of output phosphor
Smaller image
Less magnification More minification Larger gain
Larger image More magnification Less minification Smaller gain

19. Minification Gain
image brighter because output screen smaller than input screen
changes with magnification mode (9”, 6”, etc)
changes by about 2X for each mag mode
typically 81 for 9” mode (output phosphor about 1” diam)
36 for 6” mode
16 for 4” mode
Highest magnification
Lowest Minification gain
Lowest magnification
Highest Minification gain 6”
12” 9”

20. Auto-Brightness
Generator senses image brightness and modulates beam intensity to hold brightness constant
Lowest magnification
Highest Minification gain Lowest dose
Highest magnification
Lowest Minification gain Highest dose
12” 6” 9”

21. Flux Gain Caused by high + voltage of anode
acceleration of electrons in tube
Does not change with magnification mode
typical value ~ 50

22. Contrast
Ratio of brightness at center of image with & without blocking center
typically 10:1 to 20:1
Light Meter
Lead

23. Image Tube Contrast degrades over time depends on collimation
tube properties
input phosphor photon absorption
output phosphor electron absorption
light backscatter from output phosphor to input phosphor
Light Meter
Lead

24. Other II Characteristics
Lag
persistence of illumination after irradiation
insignificant for modern tubes
Distortion
Image intensifier has rounded bottom
Electron steering better in center than in periphery
unequal magnification
straight lines appear bent
pincushion effect

25. Vignetting loss of brightness in image periphery caused by
periphery displayed over larger area of input screen
decreases brightness
poorer periphery focus

26. Multi-Field Image Tubes
Dual, 3X, 4X field sizes common
Image focused by adjusting voltage on focusing electrodes (electronic lenses)
By law, collimators must cone in during mag operation
X-ray field should match imaged field
9 “ 6”

27. Magnification Advantages Disadvantages Magnifies anatomy
improves spatial resolution
Disadvantages
smaller field of view
increased radiation intensity (but less tissue exposed)
decreased minification gain
9 “ 6”

28. Digital Fluoroscopic Receptors
Flat Panel Digital Technology
Now used for angiography / cardiology and some portable C-arms
No spatial distortion