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

THE FILAMENT IS A SMALL COIL OF THIN THORIATED TUNGSTEN WIRE.

1%-2% OF THORIUM INCREASES EFFICIENCY OF THERMIONIC

EMISSION.

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

MOST DIAGNOSTIC TUBES HAVE DUAL FILAMENT

WHICH MEANS:

LARGE AND SMALL FOCAL SPOT

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

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 EFFICIENCY 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 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: ALUMINIUM PLATE,

COLLIMATOR MIRROR, PLASTIC COVER. APPROX. 1-2 mm Al

EQUIVALENT.

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

One unfortunate consequence of the line-focus principle is that the radiation intensity on the cathode side of the x-ray field is greater than that on the anode side. Electrons interact with target atoms at various depths into the target.The x-rays that constitute the useful beam emitted toward the anode side must traverse a greater thickness of target material than the x-rays emitted toward the cathode direction. The intensity of x-rays that are emitted through the “heel” of the target is reduced because they have a longer path through the target, and therefore increased absorption. This is the heel effect.

Anode Heel Effect

The difference in radiation intensity across the useful beam of an x-ray field can vary by as much as 45%. The central ray of the useful beam is the imaginary line generated by the centermost x-ray in the beam. If the radiation intensity along the central ray is designated as 100%, then the intensity on the cathode side may be as high as 120%, and that on the anode side may be as low as 75%.

The heel effect is important when one is imaging anatomical structures that differ greatly in thickness or mass density. In general, positioning the cathode side of the x-ray tube over the thicker part of the anatomy provides more uniform radiation exposure of the image receptor. The cathode and anode directions are usually indicated on the protective housing, sometimes near the cable connectors.

Off Focus RadiationX-ray tubes are designed so that projectile electrons from the cathode interact with the target only at the focal spot. However, some of the electrons bounce off the focal spot and then land on other areas of the target, causing x-rays to be produced from outside of the focal spot). These x-rays are called off-focus radiation

Off focus radiation is undesirable because it extends the size of the focal spot. The additional x-ray beam area increases skin dose modestly but unnecessarily. Off focus radiation can significantly reduce image contrast.

Finally, off focus radiation can image patient tissue that was intended to be excluded by the variable-aperture collimators. Examples of such undesirable images are the ears in a skull examination, the soft tissue beyond the cervical spine, and the lung beyond the borders of the thoracic spine