Resident Physics Lectures

1. Resident Physics Lectures Fluoroscopic Imaging

2. The Bad Ol� Days Radiology 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 sensitive to blue-green wavelengths 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 Glass tube 2 to 4 mm thick curved bottom lead lined protects operator from stray radiation lined with �mu� metal protects image tube from defocusing stray magnetic fields

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) 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 CS ==> (36 keV) I ==> 33.2 keV well suited to average fluoro beam energy 30 to 40 keV absorbs ~ 2/3 of incident beam

9. Photocathode attached directly to input phosphor minimizes light diffusion photoemissive metal light causes emission of photoelectrons # photoelectrons emitted proportional to incident light from input phosphor

10. Electrostatic Focusing Lens Several electrodes plated to inside of glass envelope + voltage applied to electrodes each electrode at different voltage voltages determine magnification mode focuses each point of input phosphor to a point on output phosphor inverts & reverses image

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. Output Phosphor Small viewable fluorescent screen 0.5 - 1 inch diameter converts electron�s kinetic energy to light ~ 50 fold increase in # light photons over input phosphor

13. Output Phosphor thin aluminum layer on back of output phosphor prevents screen�s light from going back through tube and reaching input phosphor

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

15. Image Intensifier - TV Coupling Using a Lens lens coupling Fiber Optic

16. Fiber Optic II - TV Coupling cannot record image directly from image tube All recording done from TV

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

18. II Gain (Intensification Factor) Output phosphor brightness-------------------------------------- �standard� screen brightness typically ~ 10,000

19. 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

20. Minification Minification Factor = Diameter of (effective) input phosphor---------------------------------------------- Diameter of output phosphor Minification Factor Changes with Magnification Mode! Effective input phosphor diameter decreases with magnification so does image tube gain

21. Minification Minification Factor = Diameter of (effective) input phosphor--------------------------------------------- Diameter of output phosphor Area Reduction Factor gain proportional to area reduction factor equals [minification factor] 2 OR Area of (effective) input phosphor----------------------------------------- Area of output phosphor

22. 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

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

24. Contrast Ratio of brightness at center of image with & without blocking center Typically 10:1 to 20:1 Degrades over time

25. Other II Characteristics Lag persistence of illumination after irradiation insignificant for modern tubes Distortion electron steering better in center than in periphery unequal magnification straight lines appear bent pincushion effect

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

27. 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

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

29. Large Field of View II�s Applications digital imaging angiography digital spots 12�, 14�, 16� available Construction metal often used instead of glass for strength Advantage large field of view Disadvantage expensive bulky / heavy

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