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 ( 3170C) 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 XRAY 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 +++++
TUNGSTEN
TARGET
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
ROTOR
STATOR
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
Anode Heel Effect
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.
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 Radiation
X-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