X-RAY TUBE

1. X-RAY TUBE

2. THE TUBE CONSISTS OF CATHODE AND ANODE ENCLOSED WITHIN THE GLASS ENVELOPE (PYREX GLASS) OR METAL ENVELOPE ENCASED IN A PROTECTIVE HOUSING (LEAD+ METAL SHIELDING)

7. TUBE OPERATION

8. THE CATHODE IS A COMPLEX DEVICE AND CAN BE REFFERED TO AS THE CATHODE ASSEMBLY. THIS ASSEMBLY CONSISTS OF THE FILAMENTS, FOCUSING CUP, AND ASSOCIATED WIRING.

9. THE WIRE IS ABOUT 0.1 - 0.2mm THICK AND 7-15 mm LONG

10. THE FILAMENT IS A SMALL COIL OF THIN THORIATED TUNGSTEN WIRE.1%-2% OF THORIUM INCREASES EFFICIENCY OF THERMIONIC EMISSION.

11. TUNGSTEN IS A MATERIAL OF CHOICE BECAUSE OF ITS HIGH MELTING POINT 3410 C. RHENIUM ( 3170C) AND MOLYBDENUM (2,620 C) CAN ALSO BE USED.

12. TUNGSTEN Z # 74 MELTING POINT- 3,410 DEG. CELSIUS

13. THORIUM Z # 90

14. DUAL FILAMENT

15. FILAMENT

16. SCHEMATIC OF DUAL FILAMENT

17. MOST DIAGNOSTIC TUBES HAVE DUAL FILAMENT WHICH MEANS:LARGE AND SMALL FOCAL SPOT

18. A TUNGSTEN FILAMENT WILL NOT EXHIBIT SIGNIFICANT THERMIONIC EMISSION BELOW 2,200 C

20. NOT ALL OF THE ELECTRONS THERMIONICALLY EMITTED FROM THE FILAMENT ARE ATTRACTED TO ANODE. SMALL % WILL EVAPORATE AND CAUSE THE TUBE ARCING. AS A RESULT OF THIS, THE TUBE BREAKS DOWN.

21. ANOTHER MAJOR CAUSE OF TUBE FAILURE IS THE BRAKING OF THE FILAMENT ITSELF. FILAMENTS BECOME INCREASINGLY THIN AS VAPORIZATION TAKES PLACE. WHEN ABOUT 10% OF THE DIAMETER HAS VAPORIZED, FILAMENT BECOMES SUBJECT TO BREAKING.

22. AN AVERAGE DIAGNOSTIC X-RAY TUBE LIFE IS ONLY ABOUT 6-9 HOURS (10,000-20,000 EXPOSURE) AT NORMAL FILAMENT HEATING LEVEL. ROUTINELY DELAYED EXPOSURES WHILE THE FILAMENT IS ENDURING MAX. CURRENT SHORTEN TUBE LIFE BY 50-60% ( DOWN TO 5,000-6,000 EXPOSURES)

23. THE FOCUSING CUP IS THE SHALLOW DEPRESSION IN THE CATHODE ASSEMBLY DESIGNED TO HOUSE THE FILAMENT

24. MOST X-RAY TUBES HAVE THE FOCUSING CUP AT THE SAME NEGATIVE POTENTIAL AS THE FILAMENT

25. IT IS ALSO POSSIBLE TO USE HIGHER NEGATIVE POTENTIAL ON THE CUP TO EVEN FURTHER DECREASE THE SIZE OF ELECTRON BEAM. THIS TYPE OF FOCUSING CUP IS CALLED BIASED

26. AS MORE AND MORE ELECTRONS BUILD UP IN THE AREA OF THE FILAMENT, THEIR NEGATIVE CHARGES BEGINTO OPPOSE THE EMISSION OF ADDITIONAL ELECTRONS. THIS PHENOMENON IS CALLED THE SPACE CHARGE EFFECT AND LIMITS X-RAY TUBES TO MAXIMUM mA ranges of 1,000-1,200 mA

27. FOCUSING CUP

28. THE ANODE IS THE +++++SIDE OF THE X-RAY TUBE

29. FUNCTIONS OF ANODE: • TARGET FOR PROJECTILE ELECTRONS • CONDUCTOR OF HIGH VOLTAGE FROM THE CATHODE BACK TO X-RAY GENERATOR. • PRIMARY THERMAL CONDUCTOR

30. THE ENTIRE ANODE IS COMPLEX DEVICE AND IS REFFERED TO AS ANODE ASSEMBLY. IT CONSISTS OF:1. ANODE 2. STATOR 3. ROTOR

31. ANODE ASSEMBLY

32. ANODES:

33. ANODE +++++ TUNGSTEN TARGET

35. ANODE ANGLES: 5 – 15°

36. ANODE ANGLES:

37. LINE FOCUS PRINCIPLE

38. TUNGSTEN IS THE MATERIAL OF CHOICE FOR THE TARGET OF GENERAL USE X-RAY TUBES. REASONS ARE: • HIGH ATOMIC NUMBER ( Z#) 74. HIGH Z# INCREASED EFFICIENCY OF X-RAY PRODUCTION. • HIGH MELTING POINT 3410 C • HIGH THERMAL CONDUCTIVITY

39. SPECIALTY X-RAY TUBES FOR MAMMO. HAVE MOLYBDENUM & RHODIUM TARGETS BECAUSE OF THEIR LOW K-SHELL CHARACTERISTIC X-RAY ENERGY

40. DURING NORMAL USE FOCAL TRACK REACHES TEMP. BETWEEN 1,000-2000 C

41. BECAUSE OF TUNGSTEN HIGH MELTING POINT, IT CAN WITHSTAND NORMAL OPERATING TEMPS.RHENIUM PROVIDES MECHANICAL STRENGTH & THERMAL ELASTICITY IN ROTATING ANODES

42. INDUCTION MOTOR ROTATES THE ANODE

43. INDUCTION MOTOR ROTOR STATOR

45. ROTATION SPEED OF ANODES • REGULAR TUBES 3,000-4,000 RPM • HIGH EFFICIENCY 10,000-12,000 RPM

46. EFFECT OF THE FAILURE OF THE INDUCTION MOTOR

47. WHEN FIRST ACTIVATING AN X-RAY UNIT USE AN ANODE WARM UP PROCEDURE

48. FAILURE TO FOLLOW THE WARM-UP PROCEDURE CAN CAUSE THE WHOLE ANODE TO CRACK.

49. MANY NEWER ANODES ARE STRESS RELIEVED • THEY DISSIPATE HEAT MORE EFFICIENTLY • THEY DO NOT REQUIRE ELABORATE WARM-UP PROCEDURE

50. PITTING OF THE ANODE FROM EXTENDED USE