Resident Physics Lectures

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 x-rays ==> light • Input • X-rays • Output • 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 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 to input phosphor • minimizes light diffusion • Photo-emissive 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 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. 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 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 • 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. 12” mag mode 9” mag mode Minification • Minification gain Area of (effective) input phosphor----------------------------------------- Area of output phosphor Larger imageMore magnificationLess minificationSmallergain Smaller image Less magnificationMore minificationLargergain

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 gainLowest dose Highest magnification Lowest Minification gainHighest dose 6” 12” 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. Light Meter Lead Contrast • Ratio of brightness at center of image with & without blocking center • typically 10:1 to 20:1

23. Light Meter Lead 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

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. 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

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

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