Presentation on theme: "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."— Presentation transcript:
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)
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 mm 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
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
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 TUNGSTEN TARGET
ANODE ANGLES: 5 – 15°
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, 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.
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 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