Resident Physics Lectures

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

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 x-rays ==> light • 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 absorption • 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 energy

9. Photocathode light photons ==> electrons • 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. lenses + + + + - - 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 Output Phosphor electrons ==> light • 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 Output Phosphor X Aluminum

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 TV Fiber Optic Bundle

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 A/B • 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 B A

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 Highest magnification Lowest Minification gain Lowest magnification Highest Minification gain

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

24. Light Meter Lead 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. 9 “ 6” 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. 9 “ 6” 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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