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