Post on 21-Jan-2016
X-ray tube
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)
TUBE OPERATION
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.
THE FILAMENT IS A SMALL COIL OF THIN THORIATED TUNGSTEN WIRE. 1%-2% OF
THORIUM INCREASES EFFICIENCY OF THERMIONIC EMISSION. THE WIRE IS
ABOUT 0.1 - 0.2mm THICK AND 7-15 mm LONG. TUNGSTEN IS A MATERIAL OF
CHOICE BECAUSE OF ITS HIGH MELTING POINT-3410 C . RHENIUM
( 3170C) AND MOLYBDENUM (2,620 C) CAN ALSO BE USED.
TUNGSTEN
Z # 74
MELTING POINT- 3,410 DEG. CELSIUS
THORIUM
Z # 90
DUAL FILAMENT
FILAMENT
SCHEMATIC OF DUAL FILAMENT
SELECTION OF COLD FILAMENT TUBES
MOST DIAGNOSTIC TUBES HAVE DUAL FILAMENT
A TUNGSTEN FILAMENT WILL NOT EXHIBIT SIGNIFICANT
THERMIONIC EMISSION BELOW
2,200 C
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.
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.
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)
THE FOCUSING CUP IS THE SHALLOW DEPRESSION IN THE
CATHODE ASSEMBLY DESIGNED TO HOUSE THE FILAMENT
MOST X-RAY TUBES HAVE THE FOCUSING CUP AT THE SAME
NEGATIVE POTENTIAL AS THE FILAMENT
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
AS MORE AND MORE ELECTRONS BUILD UP IN THE AREA OF THE FILAMENT, THEIR NEGATIVE
CHARGES BEGIN TO 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
FOCUSING CUP
THE ANODE IS THE +++++
SIDE OF THE X-RAY TUBE
FUNCTIONS OF ANODE:
• TARGET FOR PROJECTILE ELECTRONS• CONDUCTOR OF HIGH VOLTAGE FROM
THE CATHODE BACK TO X-RAY GENERATOR.
• PRIMARY THERMAL CONDUCTOR
THE ENTIRE ANODE IS COMPLEX DEVICE AND IS REFFERED TO AS ANODE ASSEMBLY. IT CONSISTS
OF:1. ANODE 2. STATOR 3. ROTOR
ANODE ASSEMBLY
TWO TYPES OF ANODES
STATIONARY
ROTATING
ANODES:
ANODES:
ANODE +++++
TUNGSTENTARGET
ANODE ANGLES: 5 – 15°
ANODE ANGLES:
LINE FOCUS PRINCIPLE
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
SPECIALTY X-RAY TUBES FOR MAMMO. HAVE MOLYBDENUM & RHODIUM TARGETS BECAUSE OF
THEIR LOW K-SHELL CHARACTERISTIC X-RAY ENERGY
DURING NORMAL USE FOCAL TRACK REACHES TEMP. BETWEEN
1,000-2000 C
BECAUSE OF TUNGSTEN HIGH MELTING POINT, IT CAN
WITHSTAND NORMAL OPERATING TEMPS.RHENIUM PROVIDES MECHANICAL STRENGTH &
THERMAL ELASTICITY IN ROTATING ANODES
INDUCTION MOTOR ROTATES THE ANODE
INDUCTION MOTOR
ROTORSTATOR
ROTATION SPEED OF ANODES
• REGULAR TUBES 3,000-4,000 RPM
• HIGH PERFORMANCE 10,000-12,000 RPM
EFFECT OF THE FAILURE OF THE INDUCTION MOTOR
WHEN FIRST ACTIVATING AN X-RAY UNIT USE AN ANODE WARM
UP PROCEDURE.
FAILURE TO FOLLOW THE WARM-UP PROCEDURE CAN CAUSE THE
WHOLE ANODE TO CRACK.
MANY NEWER ANODES ARE
STRESS RELIEVED
• THEY DISSIPATE HEAT MORE EFFICIENTLY
• THEY DO NOT REQUIRE ELABORATE WARM-UP PROCEDURE
PITTING OF THE ANODE FROM EXTENDED USE
X-RAY PRODUCTION PROCESS IS VERY INEFFICIENT IN THE
DIAGNOSTIC RANGE. >99% OF ELECTRONS KINETIC ENERGY IS
CONVERTED TO HEAT & ONLY <1% RESULTS IN X-RAYS IN
MEDIUM RANGE KVP.
ELECTRONS TRAVEL FROM CATHODE TO ANODE WITH APPROX. ½ SPEED OF
LIGHT IN MEDIUM RANGE KVP ( 70-80KVP)
X-RAY BEAM FILTRATION
X-RAY BEAM IS FILTERED TO INCREASE ITS QUALITY AND
DECREASE THE PATIENT DOSE
FILTRATION TYPES
• INHERENT
• ADDED
INHERENT FILTERS ARE: TUBE WINDOW, OIL, HOUSING
PORT. APPROX. 0.5 mm OF Al equiv.
ADDED FILTERS ARE: Al PLATE, COLLIMATOR MIRROR, PLASTIC COVER. APPROX. 1-2 mm
Al equiv.
INHERENT
ADDED
TOTAL FILTRATION= INHERENT + ADDED
AT LEAST 2.5 mm AL equiv. FOR TUBES OPERATING
ABOVE 70 kVp
LEAKAGE RADIATION
RADIATION COMING THROUGH THE HOUSING. NO MORE THAN
100mR/ Hr AT 1m
X-RAY TUBE AND COLLIMATOR
COLLIMATION
• PBL
• CONES
• CYLINDERS
• DIAPHRAGMS
CONE
CYLINDER
DIAPHRAGM